Final Report Summary - GRAIN 2 (GReener Aeronautics International Networking-2)
Executive Summary:
As mentioned in the Executive Summary of the Strategic Research & Innovation Agenda, Aviation has an important role to play in reducing greenhouse gas emissions as well as noise and local air quality issues. The continuous increase of air passenger transport generates an increasing use of hydrocarbon fuel with excessive emission of CO2 and NOX (greenhouse gases, pollutants and noise). It is well known that commercial aircraft operations impact the atmosphere by the emissions of greenhouse gases and greenhouse gas precursors, and also through the formation of contrails and cirrus clouds. In 2011, during the Aerodays in Madrid, the EC launched the future of Aeronautics in the ACARE Flight Path 2050 Vision for the Aircraft report containing the ambitious goals on the environmental impact with 90% reduction in NOx emissions, 75% reduction in CO2 emissions per passenger kilometer, and the reduction of the noise by 65%, all relative to year 2000.
To achieve the ACARE Strategic Research & Innovation Agenda green aeronautics technologies will play a more and more dominant role in mastering the challenge on “Protecting the environment and the energy supply”. GRAIN2 Supported Action, based on the same collaborative and win-win spirit introduced in former EU-China GRAIN project, will provide inputs and roadmaps for the development of large scale simulation strategies for greener technologies to meet the above future requirements on emissions, fuel consumption and noise. To reach these targets, green technologies efforts will have to be collected and prospected in three major lines: Air vehicle, Air Transport System and Sustainable Energies. Three folds to be investigated as future greening technologies:
1) Greening the aircraft and the aero engine: innovative methods and tools for optimized aircraft and aeroengines using best fuel efficiency, optimized propulsion/airframe system allowing the prediction of effects on exhaust emissions of new engines technologies and fuel, Multidisciplinary/Multiphysics Modeling, Simulation, Optimization and Control, new multifunction materials, including environmentally green materials and smart structures;
2) Greening the operational environment: utilization of environmentally friendly chemicals, accurate knowledge of the engine exhaust emissions, in particular applied to low or better free emission taxing, new ATM concepts;
3) Reducing the carbon foot print of aviation via sustainable alternative fuels: development of biofuels for greenhouse gas emission reduction; increase the knowledge of acceptance conditions at engine aircraft level, optimization of the aircraft/fuel tandem.
Project Context and Objectives:
GRAIN2 is an international networking project co-funded by the 7thFramework Program (FP07) and by the China Ministry of Industry and Information Technologies Industry Corporation (MIIT). The main objective of GRAIN2 is to focus its greening activities following the Flight Path 2050 Vision for Aircraft en route to the very ambitious challenge “Protecting the environment and the energy supply” in three major following lines: i) greening the air vehicle, ii) greening the Air transport System and iii) Reducing the carbon foot print of aviation via sustainable alternative fuels.
To achieve carefully these objectives four Key Greener Technologies (KGT) are considered:
• KGT1: Propulsion related green technologies (including NOx and CO2reduction, contrails, mission modeling, alternative fuels, ...)
• KGT2: Airframe Flight Physics (including Drag reduction, noise reduction, HPC innovative architecture, numerical simulation, ...)
• KGT3: Environmental friendly materials and structures (including Smart structures and materials, bio-sourced materials, composite technologies, metal alloys, surface coatings, structural health monitoring, ...)
• KGT4: CNS (Communications Navigation and Surveillance)/ATM for greener air transport
A dissemination and communication platform for Collecting and Prospecting Green Applications has also been developed in order to give traversal support to all KGTs.
GRAIN2 will identify innovative R&D methods, tools and HPC environments (supercomputers and GPGPUs) in the different KGTs according to the needs of major aeronautical industries to deeper understand the mechanism of engine exhaust emissions, to improve fuel efficiency and environmental performance, to lower noise for landing gear and high lift surfaces, to introduce new materials with multiple functions, to help significantly the development of biofuels for greenhouse gas emission reduction, etc.
These objectives will be met by the GRAIN2 supporting joint Europe-China networking actions by a series of open dissemination events including single or multi KGT groups, R&D prospecting activities like Open Forums, Workshop, Short Course involving physical modeling, simulation, optimization and control experts from Europe and China in the different KGT areas.
The targeted International Greener Networking critical outcome of GRAIN2 is a green leverage according to the Flight Path 2050 Vision for Aircraft preparing towards future HORIZON 2020 EU-China Coordinated Calls.
The objectives of the project, as described previously, are global along the project. The consortium is devoting all the effort to deepen on the mutual collaboration though the technical discussions. These technical discussions are feeding the organized events and meetings. It means, that in addition to the global objectives of the project some more specific objectives can be listed:
- Organization of the events and meetings, to launch the technical discussions and establish a path for the technical share among partners. And mainly, the organization of the Open Forum and final meeting of the project, where the conclusions have been wrapped up.
- Invite external stakeholders and experts to participate and contribute to the definition of the best topics to collaborate and define research priorities.
- Establish a first analysis of the situation of the selected topics, analysing the state of the art but also looking for an agreement about which topics are more suitable to be developed and which capabilities can contribute Europe and China with.
- Define the key technologies, which are of common interest, and can be the basis to foster the international cooperation.
1.1 KGTs Objectives and overview
The GRAIN 2 project structure features four key technology streams (Key Green Technologies, KGT) that were identified as the most promising ones for making aviation more environmentally friendly. These technologies are:
• KGT1 - Propulsion related technologies (including NOx and CO2 reduction, contrails, mission modeling, alternative fuels, ...)
• KGT2 - Airframe Flight Physics (including Drag reduction, noise reduction, HPC innovative architecture, numerical simulation, ...)
• KGT3 - Environmental friendly materials and structures (including Smart structures and materials, bio-sourced materials, composite technologies, metal alloys, surface coatings, structural health monitoring (SHM), ...)
• KGT4 - CNS (Communications Navigation and Surveillance)/ATM for greener air transport
The particular objectives and overview of the different KGTs are described below:
1.1.1 KGT1
KGT1 focusses on the identification of new emerging RTD areas of mutual interest on the different green aspects of propulsion technologies (KGT1) including NOx and CO2 reduction, contrail, emission modeling, alternative energies, etc.
This work package will be decomposed in two parts:
1. The first one, propulsion related green technologies, will be basically oriented to different aspects of traditional engine technologies like noise, fuel consumption, etc.
2. The second one, new energies for aviation, will be oriented to different aspects of sustainable innovative energies for aviation application.
KGT1-2 has been set-up in the frame of the Europe and international engagement in term of emission and reduction. Within Europe, dependence on crude oil is reduced by drop-in liquid fuels from other sources at a competitive cost. This has been facilitated by a coherent research strategy, regulatory enablers and streamlined certification and approval processes and the establishment of sustainable supply chains.
The progressive introduction of fuel cells and battery powered vehicles for ground operations at airports has made an important contribution to reducing the carbon footprint of the aviation sector. Electrical and hybrid-electrical engines have entered the aviation market.
The whole life cycle impact of vehicles, equipment and systems has also been addressed.
The availability of alternative aviation fuel not only has a huge environmental impact but it is vital that manufacturers both in Europe and China try to maintain a position within world competition for the sustainable growth of aviation transport for improving energy independence, lessening global-warming effects, and mitigating the economic uncertainty of crude oil prices. Within Europe, dependence on crude oil is reduced by drop-in liquid fuels from other sources at a competitive cost. This has been facilitated by a coherent research strategy, regulatory enablers and streamlined certification and approval processes and the establishment of sustainable supply chains.
In order to implement and develop this existing approach, the KGT1-2 New energy for aviation proposes to address, in the frame of EU-China cooperation different aspects of sustainable innovative energies for aviation application:
1. Innovative fuels for aeronautics
• Drop-in (sustainable feedstock) and new pathways (Methane, Sugar to Alcohol, etc.)
The aim is to bridge the gap between R&D and industry on innovative fuels for aeronautics; to identify and evaluate possible alternative fuels in compliance with sustainable feedstock; modeling of alternative fuel system design on the characteristics and properties of alternative fuels; to identify and access new pathways for alternative fuels at competitive cost;
2. Electric energy
• Energy storage, fuel cells
3. Engine performance
• Modelling
The aim is to investigate alternative fuel performance in compliance with keeping the same safety level as kerosene in the whole aero-engine system and considering the environmental and economic performance of selected alternative fuels based on the aircraft safety. To analysis alternative fuel combustion performance in a highly coupled integrated engine modeling of alternative fuel safety margin using SoS (System of Systems) in compliance with keeping the same safety level as kerosene in the whole aero-engine system; to assess the emission characteristics of alternative fuels. All these technologies have to be assessed in order to validate their sustainability in terms of environmental, economic and societal impact. This approach has to be closely linked to the transversal KGT5 and rely on the developed tools to achieve this validation (Life Cycle Analysis).
1.1.2 KGT2
KGT2 focus on the identification of new emerging RTD areas of mutual interest on the different green technologies of Airframe Flight Physics including drag reduction, noise reduction, HPC innovative architecture, numerical simulation, etc.
This workpackage is decomposed in two parts:
1. Drag and CO2 reduction
2. Noise reduction
In line with the overall objectives of GRAIN2 and on the basis of the achievements made in the previous GRAIN project, one of the main objectives in KGT2 is to make one step forward in the exploration of emerging greener technologies of future aircraft, as well as of the tools and methodologies invoked to realize and implement the identified technologies for improved airframe designs. In close interaction with other KGTs, another main purpose is to establish a collaborative platform and network including competent partners and, further to identify and select state-of-the-art technologies, tools and methods of mutual interest in Europe and in China for future collaboration of in-depth technical investigation.
1.1.3 KGT3
KGT3 focusses on the environmental friendly materials and structures, including smart structures and materials, bio-sourced materials, composite technologies, metal alloys, surface coatings, structural health monitoring, and intelligent health management, et.. The main focus is on light-weight materials and recyclable materials but also all those technologies which help to reduce the total weight of an aircraft and technologies which enable the reduction of the usage and maintenance cost (both of time and resources). The overall goal is to reduce the future footprint of aircraft after their lifespan. Within the work package material technologies like composite materials developments and applications, new manufacturing processes, and designs that help to reduce the pollution produced by the involved processes and technologies to enable the recyclability of components are analyzed.
1.1.4 KGT4
The high number of new greenfield airports in China that are expected to become operational within the span of this decade bring a fast changing and growing air transport network in China. In addition to that there are the new regulations in China around available airspace for business and general aviation. This creates a vast set of opportunities and challenges for the environmental footprint of the global air traffic system.
Europe has, in the last few decades, gathered a lot of experience around the concepts of Flexible Use of Airspace, Network Management, and greener CNS/ATM systems. Expectations are thus high that significant benefits can be derived from joint R&D in CNS/ATM solutions. WP4 will derive a list of those common R&D interests in CNS/ATM solutions.
A more specific objective is to improve the knowledge and capability of how CNS/ATM systems and technology can be used to reduce environmental footprint. WP4 will mainly focus in new ATM concepts and operational paradigms, along with the associated innovation in CNS systems, procedures and infrastructure. In this context, this workpackage will deal with 4D trajectory management and its role to move in the future an airspace centric ATM to a trajectory centric ATM. Thus, environmentally optimised aircraft procedures will be tackled along with systems, concepts and technologies aiming at increasing the automation levels in ATM.
This KGT4 is closely coordinate with Eurocontrol, SESAR and CleanSky frameworks.
Project Results:
After more than 10 years of collaboration between EU and China a lot of opportunities have been identified. These are based on the high research capabilities on both sides and the large infrastructures available for industry and research.
Also some big challenges have been identified which are originated by the growing air traffic in China and its integration at an international level, the development of new aviation products and the extension of some green aeronautical technologies to other transport modalities.
The different GRAIN2 KGTs have already identified RTD topics for future collaboration. Here below we list the synthesis of these topics. Additional details can be seen in the following sections.
KGT1: Propulsion related green technologies
Propulsion
1. Low Pressure Turbine (LPT) aerodynamics and heat transfer
2. Multidisciplinary design and optimization of turbomachinery components.
New Energy for Aviation
1. Safer and more efficient certification of Aviation Alternative Fuel (AAF)
Two key subjects are proposed for aero-thermodynamical research in aircraft engines
KGT1-1: Propulsion related green technologies-PROPULSION
Low Pressure Turbines
The demand for higher bypass ratios in modern commercial jet engines requires the low-pressure turbines at the exit of the engine:
• to provide an increasingly larger power output allowing a direct drive of large fans without penalizing the global efficiency.
• At the same time, their design asks for a reduction of the turbine blade count and/or
• the implementation of new, lighter materials of in order to save weight and/or secure lower manufacturing and operating costs.
This eventually implies an increase of the aerodynamic load on each blade, towards the development of very high or even ultra-high lift airfoils. These are characterized by
• a high velocity peak on the suction side,
• followed by a significant diffusion.
• The resulting strong adverse pressure gradient along this surface may possibly induce a separation of the boundary layer, particularly at low Reynolds number, This may finally cause a significant loss in lift and a consequent drop in engine efficiency.
For large commercial turbofans, a loss in LP turbine efficiency of the order of 2 % may occur between sea-level take-off conditions and high-altitude cruise flight (where the Reynolds number drops by almost one order of magnitude). Consequently, deep analysis, understanding and prediction capabilities are of utmost importance to secure an efficient engine design.
This implies the need for both
• accurate time-resolved measurements (including higher order turbulence characteristics) at engine similar conditions (Mach and Reynolds number, turbulence, upstream/downstream unsteadyness)
• high fidelity computational methods, going far beyond the classical RANS approaches, not very satisfactory at the lowest Reynolds numbers.
Today’s experimental and computational approaches probably provide an acceptable mean flow description and global (integrated) performance (loss) parameters, but many of them fail in the description of higher order statistical characteristics. It therefore remains a major area for further basic and applied research in this particular area of the turbine.
Based on several research programs conducted in US and Europe over the last decade, the so-called ‘geared-fan’ engine architecture appears very promising as the future engine architecture. The principle is to choose for a small size, fast low pressure turbine (reducing therefore its volume and weight) and to introduce a gearbox between the low rpm - large fan and the high rpm - small LP turbine.
• Reynolds number issues are still present,
• Mach number issues (transonic flow, shocks) appear as well.
• Moreover, considering the higher rotational speed of the turbine, structural issues will also develop.
This calls for a multidisciplinary design strategy. Competences need again to be developed both from the experimental and the numerical side.
Multi-disciplinary optimization
Not so long ago, the design of a component was dominated by the experience gained during previous designs and simple correlations. Prototypes were built and tested, and improvements were proposed based on the newly acquired knowledge.
Nowadays, Computational Fluid Dynamics tools are in continuous development for aerodynamics, heat transfer, structural mechanics, aero-acoustics, vibration ... issues. They start to be introduced (unfortunately not all of them at the same time) into the design process as an alternative to the time consuming and very expensive tests in experimental facilities. Designs are immediately tested by CFD, requiring (for each new individual design) a few hours of computational time. The iterative nature of the design, however, has remained in most of the cases ... and classical CFD remains costly (especially in time). At the end, experiments are still made on the final design for a final validation of the product and of the CFD predictions. This information is an important feedback to verify and validate the models as they can reveal their shortcomings and can be used to improve them.
However, new challenges arise. The shortening of the design cycle, to adapt to rapidly changing market demands, and the increase of the performance are nowadays the main challenges in component design. Many different designs need to be evaluated and compared. Improvements are obtained by carefully examining the effect of each parameter on the performance of the design. Choices such as the material of a part and its shape can be evaluated in terms of e.g. strength, efficiency, manufacturability, overall production cost, etc.
However, the large amount of parameters that can be independently modified complicates the task, and increases the classical CPU time. In order to get much faster responses, the last years have seen a large development of so called optimization algorithms, coupled to ‘Meta-Models’ (also known as complex ‘interpolations’). These algorithms, such as e.g. genetic algorithms (the optimization) coupled to e.g. a neural network (the Meta-Model or the ‘interpolator’) allow to explore the design space with a speed which is 3 or 4 orders of magnitude higher than a classical CFD approach. The challenge is of course to define the correct objective functions and constraints and to correctly ‘train’ the Meta-Model to provide accurate predictions. The main challenge here is to apply this approach in a multi-disciplinary environment. This challenge also calls for the availability of multi-disciplinary accurate measurements and/or high fidelity CFD methods in order to support the training of the various Meta-Models.
KGT1-2: Propulsion related green technologies-NEW ENERGY FOR AVIATION
Safer and more efficient certification of Aviation Alternative Fuel (AAF)
The availability of alternative aviation fuel not only has a huge environmental impact but it is vital that manufacturers both in Europe and China try to maintain a position within world competition for the sustainable growth of aviation transport for improving energy independence, lessening global-warming effects, and mitigating the economic uncertainty of crude oil prices. “Drop-in” fuels are defined to substitute for kerosene without change of aero-engine and aircraft as well as infrastructure. Accordingly, the current standard specification for alternative fuel (ASTM 7566-13) have been deduced from the jet fuel specification properties (ASTM 1655) based on as same as kerosene, which results in high cost in fuel refinement. Since the difference between alternative fuels and kerosene in components, only testing of physical and chemical properties cannot confirm the security of alternative fuel in aero-engine. Therefore, the certification process of alternative fuel includes the testing of physics and chemistry, the testing of fit-for-purpose, hot section testing, component testing, and system test according to ASTM D4054, which results in fuel-cost in certification testing. Hence, three challenges including high cost in fuel refinement, fuel-cost in certification testing, lack available testing condition and the acceptance criteria in recent standard are expected to be overcome.
With the development of this research project, CAAC and EASA will develop the certification of alternative fuels in compliance with airworthiness standards, and promote the formation of the Sino-EU alternative aviation fuel certification standards.
• Low fuel cost in certification process
• Low cost in biofuel refining
• Novel drop-in & non drop-in fuel certification process and management.
KGT2: Airframe Flight Physics
Noise reduction
1. Advanced experimental and numerical methodologies for analysing noise generation and propagation.
2. Innovative noise control/reduction technologies
3. Low-noise concepts and installation effects
Drag and CO2 reduction
1. Advanced simulation of high Reynolds number, realistic setting controlled flows on large/innovative HPC architectures.
2. Advanced turbulent models for modelling detached flows and laminar-turbulent transition.
3. Multidisciplinary optimization technology
4. Instability analysis and unsteadiness control in aircraft design.
KGT2-1: Airframe Flight Physics-Noise reduction
Computational methodologies and experimental technologies for effective analysis of aero-acoustic noise generation and propagation
For next-generation quieter aircraft design and making steps toward the realization of the ambitious goals set by ACARE Flightpath 2050, one of the essential aspects is to comprehensively understand and accurately address noise generation and radiation by means of experimental measurements and computational simulations. In spite of remarkable effort and progress made over the years, particularly, in EU framework programs, it is recognized that current experimental technologies and numerical methodologies are not sufficiently complying with the need of effective industrial use for accurate analysis of aircraft noise emission. For massive industrial applications in design process and for problem diagnosis and mitigation, reducing aircraft development time and costs has in many cases been attained at a price of reduced reliability and accuracy of solutions.
Comprehensive aero-acoustic analysis rests on an accurate exploration on near-field unsteady aerodynamic flow physics and far-field noise transmission. This requires a correlated setup, in either experimental measurements or numerical simulations, of systematic studies on turbulent aerodynamic flows (as noise-generating sources) and far-field noise propagation (in relation to aircraft noise emission and its impact on environment). A lack of reliable correlation between noise generation and propagation forms a significant source of inaccuracy in many current aero-acoustic measurements and simulations.
For accurate CFD/CAA (Computational Fluid Dynamics/Computational Aero-Acoustics) analysis, furthermore, the computational resources may become excessively demanding, or even prohibitive, for applications of industrial relevance to formulate aerodynamic noise generation in correlation with far-field noise propagation. High-Performance Computing (HPC) architectures, in conjunction with robust numerical algorithms and advanced flow-physics modelling approaches, have been emerging as large-scale computational methodologies exploitable to make steps forward for facilitating the use of CFD/CAA as potential industrial tools implemented in the process of developing quieter aircraft.
The proposed RTD topic, standing on the basis of emerging computational methods and experimental technologies that have shown promising capabilities (yet often in academic research), aims at new development and further improvement to enhance the TRL beyond the current state of the art on two primary aspects: robust CFD/CAA methodologies and effective experimental technologies for addressing industry-relevant aero-acoustic problems.
Aircraft noise-control technologies and integration strategies of control systems
Over almost two decades, the urgency to reduce the airframe and engine noise components has become increasingly pressing, and the most straightforward approach to obtain noise reduction without affecting the other key attributes in aircraft design has been to apply proven technologies and refine existing designs. But it has since become more obvious that the ambitious ACARE targets set for 2050 won’t be reached with existing technologies. Given the noise generation mechanisms cannot be drastically reduced from further incremental refinement of the current designs, the step for a breakthrough change has to come from innovative approaches based on fundamental research and detailed investigations of novel concepts.
To achieve the ambitious goals set by Flightpath 2050, it is necessary to develop the most effective technologies for modifying the near-field noise generation mechanisms and leading to far-field noise reduction. This objective has sustained continuous efforts over the recent years, aiming at the development of innovative methods for the control of noise generation and propagation, giving birth to new innovative methodologies and technologies targeting airframe and engine noise reduction.
In spite of significant contributions made in a large amount of previous and ongoing effort on effective noise-control technologies, nonetheless, the gap remains large from current academic laboratory research to actual industrial deployment. Indeed, quantifying the benefits of a new design, noise control or abatement procedure in terms of decibels is a very challenging task. Two key factors are at least partly responsible for this difficulty. Firstly, the non-linearity of fluid dynamics makes it very complicated and effort-consuming to predict the transient behaviour of a flow field, especially for the very high Reynolds number at stake in aeronautics. Unfortunately, knowledge on the transient features of turbulence are an essential ingredient of the aero-acoustic prediction chain, because the mechanisms causing noise in the far-field result from subtle imbalances in amplitudes and phases between reciprocating motions in the medium. Secondly, the past efforts conducted to reduce the noise emitted by the various components of an airplane have resulted in a fair balance between their respective contributions integrated over a complete flight. Different components are clearly dominating the noise spectrum at take-off, and others are responsible for most annoyance during landing. In order to address in a timely and cost-effective way the problem of aircraft noise using noise-control technologies, it is essential to devise integration strategies aiming at developing and deploying best common denominator technologies, in terms of applicability to the different elements
By means of numerical simulations and/or experimental testing, the proposed RIA topic aims at development of emerging innovative noise-control concepts and, going beyond the proof of concepts reported in academic research, enhancing further the TRL with relevant validation of identified promising noise-reduction technologies, which are supported further by the development of integration strategies of control systems installed on different aircraft components.
Low-noise configurations and airframe-aeroengine installation supported by multidisciplinary optimization
Aviation has a significant impact on the environment and the EU citizens due to emissions (noises, pollutant matter and contrails) to which the population is exposed. On the other hand, aviation generates around 2% of EU GDP and accounts directly and indirectly for 3.7 million jobs. This has led to the implementation of a number of ambitious goals set by the sector at horizon 2050 in the Strategic Research and Innovation Agenda (SRIA) of the Advisory Council for Aviation Research and Innovation in Europe (ACARE). For noise emissions, the target to meet by the 2020 horizon is to reduce perceived external noise by 50%, while the new horizon defined for 2050 (described in the Flightpath 2050 document) is targeting a reduction by 65%. In support of this objective, it has been acknowledged that the technological objectives will not be reached through an evolutionary approach only. Breakthrough innovations are needed, i.e. new solutions which rely on a disruption with respect to current approaches.
In the process of developing future new aircraft concepts, obviously, the step must be progressed by a close coupling with research on innovative low-noise configurations of airframe components and power systems, as well as on low-noise aircraft installation. The proposed research and innovation actions target new low-noise technologies and concepts that are not currently used in aeronautics or that have not yet being put in combination in the aviation sector. By means of numerical simulations and experimental measurements, innovative low-noise high-lift configurations, landing-gear systems and airframe-aeroengine installation, in which multidisciplinary optimization will play a significant role in supporting the design of new low-noise concepts and will be undergone further development to improve its applicability for robust aero-acoustic design.
The proposed RIA topic aims at demonstrating the validity of the low-noise technologies and concepts resting on sound technical and scientific approaches. The aerodynamic and aero-acoustic performance should be jointly assessed quantitatively against the relevant criteria such as economic viability, time efficiency, potential to cope with evolutions of regulations, environmental friendliness and energy sustainability etc. The proposed topic will also assess at the end the potential of the low-noise technologies and concepts developed at further technology readiness levels and possible barriers that may prevent such developments.
KGT2-2: Airframe Flight Physics-DRAG AND CO2 reduction
Advanced simulation of high Reynolds number range, realistic setting controlled flows on large/innovative HPC architectures
Substantial resources have been invested over the years into Computational Fluid Dynamics (CFD), particularly in the development of efficient and robust numerical algorithms for solution of the governing equations, grid generation and adaptation, powerful post-processing tools, etc. Not surprisingly, these investments (many of them made in the framework of European programs) have resulted in a remarkable progress in CFD capabilities. This contributed significantly to improve the flow physics understanding, reduce aircraft development time and cost minimizing the reliance on wind-tunnel and flight tests. However, experience shows that industrial numerical simulation tools are presently suffering two main drawbacks that prevent their full industrial deployment for massive applications in design process: excessive long computational times for problems of industrial relevance, and reduced reliability and accuracy of the solutions at flight extremes. This includes the accurate simulation of turbulent drag reduction through the application of active/passive flow-control techniques to manipulate the drag produced by the flow structures in turbulent boundary layers.
If current simulation capabilities, including LES and RANS/LES models, can be made to work with drag reduction techniques (passive/active flow control), the simulation of full aircraft at high Re would certainly reveal many indirect effects of reducing drag locally and which can be very useful for aircraft design process.
Research in flow control needs to be done subject to more complex conditions: realistic geometries, strong curvature, adverse pressure gradient, free-stream turbulence, etc that characterizes the practical aircraft setting. The efforts should be made to substantially up the Reynolds number range of problems to be addressed. This would mean to go increasingly for extremely high-quality LES and possibly LES-RANS. The latter requires very careful discrimination and much judgment on the penalties incurred. With any shift towards more complex conditions and LES model, the use of high-order methods is ever more important.
Realistic actuators and sensors that provide the prospect of realistic boundary conditions that feed into the simulation tools are of high importance. Simulators need to understand what actuators are realistic in a practical setting (frequency, density of actuators, size, reliability....). Important part of the computational research needs then be devoted to how these actuators can be represented realistically in a computational setting. A proper and accurate set of experimental data necessary for the validation process is also of vital importance.
New numerical methods and new hardware architectures are emerging as possible solutions of these problems: High order methods in complex geometries could be the solution to obtain the desired accuracy. At the same time, the capabilities of leading-edge emerging HPC architectures are still not fully exploited by simulation tools for large aeronautical problems. These do not take advantage of the immense new capabilities of new hardware architectures, such as streaming processors or many-core platforms. Likewise, current high order methods are mainly limited to academic environment, it is necessary to mature their application in complex geometries and high Reynolds problems.
In order to tackle these problems. We propose to go beyond the state of the art in the following main research lines: 1) Advanced techniques for efficient implementation of solvers on heterogeneous and highly parallel High-Performance Computing (HPC) architectures. 2) Innovative and efficient numerical algorithms for large scale computing in aeronautics, based on advanced parallel architectures. In particular High-order methods in complex geometries and high Reynolds numbers.
Advanced turbulent models for Computational Fluid Dynamics
Computational Fluid Dynamics (CFD) techniques have become essential in the aircraft design loop and are regularly used by most industrial aircraft manufacturers. Namely, these numerical techniques are used to define aircraft aerodynamic surfaces and provide valuable information to the structural and loads departments enabling a cost efficient aircraft design with reduced fuel consumption. Furthermore, the use of CFD is saving time and resources, limiting the number of costly wind tunnel experiments and flight tests. In some occasions, numerical simulations are the only available tool to evaluate extreme aircraft conditions and performance such as stalled conditions, shock boundary layer interaction or buffet. Although CFD techniques have been shown to be mature enough for benign design flow conditions, the accuracy of the numerical solutions near the design envelope limits, or when new aircraft configurations are considered, are still limited.
Two key elements are identified as the main bottlenecks for the full acceptance of CFD results by industry near the limits of the design envelope.
• Firstly, turbulence modelling has seen major enhancements during the last decades but its accuracy remains limited for detached flows.
• Secondly, laminar-turbulent transition modelling is one of the major bottlenecks for CFD and essential to accurately compute aircraft performance and loads.
Detached flows and turbulence modelling
The following future activities are suggested to address future needs in the accurate simulation of complex aerodynamic flows:
• Further development of improved RANS approaches, focussing on reliable separation prediction for complex 3D flows and correct prediction of turbulence levels in the early separated shear layer (responsible for errors in reattachment prediction). RANS will remain the only affordable approach in high-Re boundary layers for decades to come – also in the framework of hybrid RANS-LES methods, the accuracy of which can be limited by RANS errors.
• Industrialisation of hybrid RANS-LES models, which consists of several aspects:
o Consolidation of the latest state of the art improved methods
o Further improvement of Grey Area behaviour without loss of generality and robustness
o “Smarter” models requiring less user input and decisions:
▪ More “flexible” zonal/embedded approaches
▪ Automatic detection of RANS and LES regions
▪ Automatic and adaptive meshing for hybrid RANS-LES methods
o Addressing the expertise gap between the developer and user communities via clear best practice guidelines, method applicability recommendations and training events
• Development and implementation of models for flow control devices, in order to capture the effects of e.g. vortex generators, synthetic jets, plasma actuators, turbulent screens, riblets in a cost-effective manner. Extension of RANS models to include such effects will enable the assessment of such advanced concepts at aircraft level as well as opening the door to efficient (adjoint) optimisation of flow control device parameters and placement.
Additionally to improvements in cruise flight drag, significant benefits can also be expected from a better knowledge and modelling of boundary layer flow during transient phases of flight. High-lift configurations need reliable predictions of boundary layers with regions of strong adverse pressure gradient which are close to separation and sometimes even locally separated. Separation limits the maximum lift that a wing can generate at low speeds, and thus limits the achievable speed reduction required to ensure safe take-off and landing conditions. Prediction of separation is highly sensitive to simulation tool deficiencies and quickly becomes unreliable rendering aerodynamic optimization and drag reduction efforts futile.
Laminar-turbulent transition
The physical mechanisms by which a laminar flow transitions to a turbulent regime are relatively well understood. In boundary layer cases, transition can be (in most scenarios) explained by modal linear instability, which considers spatial linear growth of Tollmien–Schlichting (TS) waves. However, in some cases a temporal transient growth of perturbation (i.e. non modal instability) can explain some “bypassed transition” mechanism to turbulence. Other physical mechanisms for transition are 3D cross-flow transition, or purely inviscid instabilities which can be explained by inflection points in the velocity gradients. Techniques to compute modal and non-modal instability analysis are varied. One of the most widespread methods (and accepted in industry) is the e^N method, which accounts for the TS type of transition. However, there is a lack of generalised models that are able to predict laminar-turbulent transition. Namely, CFD methods to predict transition do not in general incorporate the linear instability knowledge (neither modal nor non-modal). It is therefore necessary to unify the linear stability information, and incorporate this into mature CFD techniques, to create new models that can explain transition to turbulence for a variety of flow scenarios such as in the presence of adverse pressure gradients, bubbles or detached flows.
In summary, most advanced Computational Fluid Dynamics tools are able to accurately predict the vast majority of flow problems if the appropriate computational model (e.g. geometrical simplifications, turbulence model) and enough resolution (e.g. mesh of sufficient refinement and quality or increased formal order of the method) are selected for simulation. Computational inaccuracies may be categorised as modelling errors (e.g. separated flows and transition to turbulence) or purely numerical errors. To mitigate these two types of errors requires better models and higher resolution.
Consequently, more reliable and accurate models, such as turbulence models for detached flows and models to predict laminar-turbulent transition are required which will ensure a significant widening of applicability of CFD solvers, particularly in off-design and maximum load situations.
Multidisciplinary optimization technology for innovative aircraft configurations including local flow control applications
The improvement of the optimization process in the aerodynamic design of aircraft configurations is becoming of key factor to reduce development and operational costs and decrease the time-to-market for new aircrafts. Indeed, despite the increased computational power, the greater sophistication and perfection of physical and computational models and an ever more automated industrialization process, it is undeniable that the timescale for realization of new aircraft (from paper to the market) has not substantially reduced. In this framework, innovative optimization techniques can play a major role, as effective and intelligent exploration of the space of all possible configurations is an essential requirement to reach the goal of time-to-market decrease in aerospace industry.
The expected increase of air traffic for the next fifteen years has established several environmental targets for the next generation of aircraft: 50% reduction in carbon dioxide emissions, fuel consumption, perceived noise and development time, as well as 80% reduction in emissions of nitrogen oxides. The achievement of these targets makes necessary to explore other unconventional/innovative configurations. In this search for optimized innovative configurations one also can add to this design optimization processes the possibilities of taking on board flow control devices early in the design stages. The typical application of flow control devices are presently used with the purpose of mitigating some undesired effect, namely drag or flow detachment. That means that flow control is generally a patch that is applied on and already designed aircraft, thus, hampering the exploitation of its full potential, that could only be achieved after a complete integration of the flow control into the aerodynamic design process.
However, in practical optimization problems, in which hundreds of design variables are considered, the use of current RANS industrial solvers is unfeasible because of their high computational cost. Alternative mathematical tools are necessary in this context; different techniques as surrogate modeling, control theory together with code optimization and proper parallelization methods can decrease the cost of the optimization aerodynamic design process.
Other contribution is related to the consideration of the design process as a global optimization problem; in this regard, the use of evolutionary algorithms (EAs), due to their ability to obtain global optima, in combination with metamodels to substitute expensive CFD simulations, makes possible to perform a preliminary exploration of the design space at a relative low cost.
There are several examples of aerodynamic optimization using an Evolutionary Programming algorithm hybridized with a Support Vector regression algorithm (SVMr) as a metamodel. Specific issues as precision, dataset training size, geometry parameterization sensitivity and design of experiments techniques are still an issue and the potential of the proposed approach to achieve innovative shapes that would not be achieved with traditional methods is very attractive.
We may then faced with the question of how to be sure we have found out the maximum potential of the outcome of the preliminary designs - if results aren't as good as hoped, there will be doubts such as "did we put the actuators in the wrong place?. Should the amplitude/frequency be adjusted? Very quickly, even considering mono-disciplinary optimisation, we have a very large parameter space. Adjoint optimisation of flow control systems therefore becomes a very attractive prospect: Sensitivities can be computed for thousands of parameters for the same computational expense as one parameter, making it possible to consider delegating actuator placement and configuration to an optimisation process. Work in this direction is developed by Uni-Kaiserslautern and have successfully computed the adjoint sensitivities for synthetic jet AFC to a full complexity high-lift / fuselage configuration.
Another crucial point is the introduction of robust or reliability based optimization techniques in the design loop. Indeed, industrial optimization processes must take account of the stochastic nature of the system and processes to design or re-design and consider the variability of some of the parameters that describe them. Thus it is necessary to characterize the system that is being studied from various points of view related to the treatment of uncertainty. In particular, it is necessary to consider the sensitivity of the system to the uncertain parameters and assess its reliability. Then, having established the ability to characterize the system from this point of view, it is necessary to build an optimization loop that can improve its reliability or that is capable of providing a robust optimum, or that could withstand acceptably random perturbations of design parameters or operating conditions.
Integration of Local flow control for cruise drag reduction in future transport Aircraft
Latest research in flow control has shown important benefits in both cruise and low speed configuration by avoiding flow detachment, reattaching the flow, re-laminarizing the boundary layer, etc.
The typical application of flow control devices is on an already designed geometry, with the purpose of mitigating some undesired effect, namely drag or flow detachment. That means that flow control is generally a patch that is applied on and already designed aircraft, thus, hampering the exploitation of its full potential, that could only be achieved after a complete integration of the flow control into the aerodynamic design process.
First steps of flow control applications integrated into the design process are currently ongoing with the application of hybrid laminar flow control on the wing leading edge; thus requiring a small redesign of the airfoil profile for already manufactured wings. But a big step is still required in the design process in order to take onboard the possibilities of the flow control devices from the early design stages, and not as a mitigation action.
Universities and research centers have made a huge effort to develop new flow control devices with progressively less and less power consumption and an increasing accuracy in their simulation models. Nevertheless, there is still a breach that needs to be traversed: we need to learn how to capitalize on the full potential of the wide range of available flow control devices, and this exploitation does not yield from an isolated optimization but from the complete integration of the system.
The expertise in flow control devices is currently spread among academia, research centers and industry. Each of us masters a different flow control device, the typical aerodynamic design process or even the preliminary design process involving all the disciplines. Thus we have a quite comprehensive toolbox of flow control techniques, from which some of them have already been successfully applied for solving a specific problem (eg. boundary layer suction to extend laminar regime on the wing). Nevertheless, the range of applicability and the potential benefit of many other devices remain still relatively unknown. It is widely accepted that we could better design the aero shapes if we introduce the flow control techniques in the early stages (eg. increased efficiency leading to smaller surfaces, less interference drag...)
The process of the technology readiness level evolution demands some specific steps before the possible industrialization: from a concept proof stage we need to develop pilot cases. Those pilot cases should be attractive and representative enough. The application of the already developed knowledge on a real problem will give us the opportunity to attain higher maturity levels. “We do not know how to use a hammer only after reading its user manual, but after hitting some nails”.
The path to such an ambitious activity is surrounded by traps that might attract the attention and divert our efforts. Thus a clear scope management will be required to keep the focus of the activity within the area of biggest interest, namely the application of the different flow control techniques for solving a specific problem.
Any multidisciplinary optimization that couples aerodynamic forces with the structure, although extremely interesting, cannot be considered within this activity by two main reasons: firstly, aero-structural coupling technology is already relatively mature and it is being applied by the industry, universities and research organizations, and secondly, this wider scope would weaken the focus of the activity on the integration of the flow control devices into the aerodynamic design. The first step in the process is to couple two bricks: aerodynamic design and flow control devices, before thinking about a higher complexity application with other disciplines involved.
KGT3: Environmental friendly materials and structures
1. Environmentally friendly materials
2. Functional integration
3. Structural Health Monitoring
KGT3: Environmental friendly materials and structures
Research topics related to materials
Current aircraft designs are already a result of the transition from metal structures towards lightweight composite structures mainly made from carbon fiber composites (CFRP). While these materials allow a lighter construction and therefore improve the ratio of aircraft weight to passenger/cargo weight there are several challenges regarding the environmental impact of these materials:
• The production of CFRP materials requires a lot of energy, which reduced the benefit of saving energy during its lifetime. Therefore alternative material sourced like bio-sourced materials should be investigated.
• The production process of composite structures still features a large amount of manual work. There is still a large potential to improve and automate all manufacturing steps in order to make them more efficient and reduce the required energy.
• Composite materials are difficult to recycle because they may lose their superior properties during the recycling process. Considering the amount of energy put into the production and manufacturing of these materials the recycling techniques should be improved in order to created secondary materials with a minimized loss in performance.
The following research topics have been identified as to most promising ones in the context of environmentally friendly materials and production.
Bio-sourced fibers
Much of the energy required for producing conventional composite materials is put into the production of the fibers. This includes both the production process and the origin of the raw materials, since they are mainly based on fossil resources. Bio-sourced fibers can be an alternative, but require specific research and development to reach competitive performance levels and production processes suitable for an industrial use.
With respect to bio-sourced fibers the following aspects are foremost important for future research:
• Fatigue behavior and durability – Bio-sourced fibers cannot be tailored to mechanical performance to the same degree as conventional fibers. They have a different structure (often hollow) and show a much higher spread in parameters due to their natural origin. Since this has a direct impact on the mechanical performance of the material the fatigue behavior and durability should be investigated. This also includes the development of numerical simulations that take into account the special properties of these materials.
• Cost-effective and environmentally friendly production of long-fiber semi-finished products with adapted properties – While conventional fibers can be produced in a continuous process the length of natural fibers is limited. For an industrial application however it is essential to provide semi-finished materials with a long-fiber character and defined properties. Future research should first focus on the general feasibility of producing such materials and later on developing suitable production processes for larger scales.
In addition to the aspects mentioned here future research activities should also be built on the developments and findings of the EU project Eco-Compass.
Bio-sourced resins and additives
Although the matrix material of fiber composites holds the fibers in place and distributes the loads to the fibers its mechanical performance is not always as critical as that of the fibers. Therefore a feasible solution for improving the environmental footprint of composite materials may be replacing conventional petrol-based matrix materials with bio-sourced materials while maintaining the high mechanical performance using conventional fibers.
Research and innovation efforts, during upcoming years, should be focused on obtaining 100% bio-based polymeric matrices that could meet the high requirements of aeronautical industry, encompassing resins , fillers and curing agents.
As second step, greener synthetic routes should be explored in order to reduce the carbon footprint of the associated processes, together with energy consumption decrease and minimization of reaction byproducts.
Besides, bio-mimetic self-healing thermoset composite will probably undergo the highest growth within the different materials used when it comes to aircrafts, providing the ability of self-repairing small damages caused by impacts or erosion. Thereby, the lifespan of aircrafts will be greatly increased, maintenance costs will be reduced. Additionally, indirect environmental impact is expected since the energy consumption and gas emission associated to part manufacturing will be reduce as a result of their durability thereof.
Nevertheless, there is a bigger challenge to be achieved in order to guarantee eco-sustainability of aeronautical industry; composites recycling after aircrafts end-of-life, where polymer matrices composition will play a key role in future. Chemical recycling technologies, comprising depolymerization and purification processes to obtain recycled raw materials and recycled long fibers should be seriously investigated during next years.
As in bio-sourced fibers case, future research activities should also be built on the developments and findings of the EU project Eco-Compass.
Thermoplastic composites
Thermoplastics composites have the advantage over thermoset materials of higher impact resistance. They also offer the possibility of employing fast forming processes, such as press forming. The combination of these types of processing with automation of the manufacturing process offers the possibility to improve the overall manufacturing process.
There is a long way to be toured in terms of eco-friendliness in thermoplastic composites, however, developments carried out in EU project Eco-Compass regarding fibers and bio-sources could represent a considerable shortcut to achieve the thermoplastic target of 100% bio-based materials. Nevertheless, high performance demanded in current aeronautical thermoplastic composites, such as PEEK or PPS based ones, represents a real challenge to be achieved by bio-based materials that use to be based in polyester backbones that cannot accomplish mandatory requirements, such as high maximum service temperatures.
Besides its bio-based character, fire resistance should be enhanced, by means of replacing halogen based flame retardants by intrinsic covalently linked halogen-free additives. New fire resistant thermoplastics should impart desired properties without penalizing mechanical and thermal properties.
Fiber reinforced thermoplastics present several advantages in respect with thermosets. Thermoplastic matrices offer the ability to be reprocessed by thermal transformation conventional processes, such as injection molding or thermoforming, avoiding the current thermoset composite recycling technologies that consist of difficult and high energy consumption chemical methodologies. Notwithstanding, a previous conditioning milling steps has to be done to obtain short fiber reinforced thermoplastics that can be midway between high performance and engineering polymer, and hence, can be used as raw materials to obtain non-structural high added value parts of aircrafts.
Chemical surface treatment
In contrast to conventional fibers bio-sourced fibers often do not have the same non-mechanical properties. They are not as fire-resistant, pick up moisture more easily and may need fungus protection. Due to the natural variations of the fibers they may also need a surface preparation to assure proper and even bonding to the matrix material. It is therefore essential to develop suitable surface treatments to handle these properties. The key aspects to consider here are the environmental compliance of the chemical agents and processes involved and their impact on the materials properties, e.g. additional weight due to required additives.
Low-cost and automated manufacturing
Current processes involved in manufacturing complex composite parts still involve a large amount of manual work. Processes like fibre placement have already been developed to industrial application, but are often limited to simple geometries and specific raw materials (pre-impregnated fibers). In order to automate the remaining manual tasks, technologies like robot-based pick-and-place systems, which can also handle dry fibers and complex geometries should be developed to a stage where they can contribute to reducing costs and improving the quality of smaller series.
Other approaches for improving manufacturing processes should focus on using simpler manufacturing methods, like oven curing or automated resin infusion. With a focus on saving energy during production and avoiding scrap parts process monitoring and control is a very promising field of research. Using monitoring methods, such as ultrasonic cure monitoring, the manufacturing process can be adapted to what is actually happening with the part to be produced. This approach can replace the current practice of running processes that are longer than necessary (and therefore consume more energy) to avoid scrap parts.
Life-cycle assessment on innovative composite materials
Since the main purpose of using bio-sourced materials instead of conventional materials is the improvement of the environmental impact, a proper life-cycle analysis should always be part of the research activities described above.
This of course requires verified models for quantifying energy consumption, COx-output etc. during the production and lifetime of these materials. A complete life-cycle assessment should therefore be developed for the most promising materials and the processes involved in their production.
Research topics related to functional integration
Alternative materials to currently used composites may be much more environmentally friendly during the initial production process and have a better environmental footprint during their lifetime. However, bringing these materials into future aircraft may not be successful if only mechanical properties are considered, since the development of modern composite materials is highly focused on superior mechanical performance. A promising approach to let alternative materials compete with these conventional materials is the integration of additional functionality and the enhancement of inherent properties of these materials. The aim of this approach is to not only replace current materials, but to have an alternative material with integrated functionality that can replace a conventional material and additional external measures and systems, which provide the functionality in a conventional way. The functional integration may include:
• Tailored thermal and electrical conductivity (e.g. as a replacement for additional insulation)
• Energy storage integration (replacing external batteries)
• Enhanced damping properties (replacing measures for acoustic insulation and simplifying mechanical compliance)
The following research topics have been identified as the most promising ones in the context of functional integration.
Energy storage
A great advantage of composite materials is their flexible lay-up in which additional functionality can be integrated easily. Especially as soon as electrical systems replace hydraulic or pneumatic systems the demand for electrical power storage systems also grows. Due to the fact that common storage systems, such as batteries, are very heavy and rather bulky, a constructive solution would be their integration in or their combination with structural components, such as the frame or unloaded parts, such as attachments.
There are several areas of interest for the research in the fields of structural energy storage:
• Structural Integration of available systems into a composite with sufficient electrical conductivity and the ability of replacement and repair;
• Development of semi-finished storage systems with a comparable high energy density and/or power density for better structural integration;
• Improvement of composite materials for their multifunctional use as components of an energy storage device;
• The integration process and design has to be evaluated in terms of lightweight construction considerations.
• The structural energy storage system should be as fault-tolerant as possible in terms of cash-issues. Furthermore safety-issues such as toxicity, flammability and environmental compatibility have to be taken into account.
Research topics related to Structural Health Monitoring
Independent of the materials being used it is essential to assure a proper mechanical function of aerospace structures during their entire lifetime. In this context there are two aspects, which can greatly reduce the efficiency of the materials during design and operation
• The spectrum of the mechanical loads which is to be expected during given operating conditions is understood rather well. However there are many uncertainties that must be considered (e.g. manufacturing tolerances). These uncertainties lead to large safety factors and may make a structure much heavier that it actually needs to be.
• The current aircraft operation and design is based on strictly scheduled maintenance plans. Since an in-depth inspection leads a grounding of the aircraft and therefore reduces availability, inspection intervals are increasingly extended. This in turn requires a structure with an undetected damage to last until the next inspection interval, making it much heavier that actually necessary.
These issues can be addressed by introducing a structural health monitoring (SHM). This continuous monitoring is already common practice for systems like engines and electrical circuitry, but is practically completely absent for structures. The goals of introducing structural health monitoring are
• Finding damages as early as possible in order to take appropriate actions while the damage is small. This should allow for a simple repair instead of replacing large parts after undetected damage growth.
• Detecting severe impacts and their location during operation on the ground and in flight, for which composite airframe structures are much more vulnerable. This allows for early and complete detection of possible damaging impacts that may otherwise remain undetected.
• Extending the life-time of existing structures by monitoring critical areas as an economically more efficient solution than conventional inspection at short intervals.
• Reducing structural weight by increasing the capabilities of detecting damages between scheduled inspections and therefore being able to reduce unnecessarily high safety factors.
• Demand-driven maintenance instead of strictly scheduled maintenance in order to make the aircraft and fleet operation more efficient.
The following research topics have been identified as the most promising ones in the context of structural health monitoring:
SHM using Lamb waves
In contrast to other SHM methods like fiber-optic sensing or comparative vacuum measurements the use of Lamb waves, which can be thought of as integrated ultrasound inspection, allows a monitoring of the area around the sensors and not just at the sensor location. This potentially allows the coverage of large areas with a relatively small number of sensors.
Using a network of sensors distributed across the structure the propagation of guided acoustic waves (particularly Lamb waves) is evaluated. The presence of damage will lead to reflections of the waves and conversions between different wave modes. This can be detected as a change in the sensor signals.
Due to the measurement principle the time delay of the signal change with respect to the excitation of the wave is known at each sensor. Using different techniques like triangulation or reconstruction methods known from conventional ultrasonic testing the location of the damage can be determined. While this has been shown successfully for simple structures one common problem is, that the methods are often not applicable to complex structures. This is mainly due to variations in mechanical properties, which are directly linked to the way the waves propagate. In complex structures the stiffness may be different at each location and in each direction.
Therefore a major focus of future research related to SHM using lamb waves should be the improvement of suitable methods for determining damage locations in complex structures. This should not only be achieved by using a very dense sensor network and ignoring much of what is known about the structure, but by actually considering the properties of the structure and their impact on the wave propagation.
Another important aspect that is worth being focused on in future research initiatives is the generation of reference signals. As described above, signal changes are being detected and evaluated. This requires that suitable reference data is available. Since the wave propagation is influenced by many environmental aspects like temperature, humidity and structural loading, reference signals are required for all these conditions. It is not possible to determine all these signals by measurement. Therefore effective simulation methods should be developed in order to generate reference signals based on the given conditions of an aircraft.
SHM using fiber optic Bragg Gratings
Optical fibers containing a (potentially large) number of strain sensing Bragg Gratings offer a number of appealing advantages for application in aircraft structures, such as light weight, tolerance for harsh environments, long term stability, completely passive operation and no interference with other signals. The optical fibres can be embedded in the (composite) structure or surface mounted. The latter has the big advantage that a sensor can be installed at any time during manufacturing and operational life and that a broken sensor can be replaced.
Besides measuring the local strain, these sensors can also measure the temperature or even the pressure. Apart from damage detection the same or part of the sensor network can be applied for load monitoring and impact detection. This allows for a multiple purpose sensor network.
The current application of damage detection by means of FBGs lies in skin/stringer debonds and cracks in metallic joints. Another highly interesting application is the monitoring of composite repairs, which can circumvent the current problems related to the bond quality and durability of secondary bonding by monitoring the repair for disbands. This may save weight and also remove the need for so-called “chicken rivets”.
The new generation of data acquisition systems, developed within Europe, to read out the FBGs are based on optical chip designs and are very small and efficient. The sample rates are high enough for lower frequency (20 kHz) damage detection, but need to be further improved for impact detection (100 kHz) and Lamb wave detection (MHz). In the latter case FBG sensors can be combined with Lamb wave sensors described in the previous section.
SHM Technology
This rather general aspect covers the entire technological portfolio, which is required for a continuous monitoring of structures.
Many different methods and physical concepts for structural health monitoring are available. The research activities addressed here should therefore focus on answering the question of which methods are most suitable for monitoring a given structure within a defined framework, including the sensitivity to different damages and damage locations. This information is especially important for aircraft designers in order to give them an early indication on what improvement can be expected from SHM and how they can consider SHM in an early design stage.
General research aspects, which are relevant for using SHM systems and still needs to be addressed for all sensing technologies are the correlation of acquired signals to damage severity, which is important for whether and when more detailed inspection is required and the in-flight operation of SHM systems, where SHM signals have to be reliably separated from flight load influences.
Advanced sensor technology
The sensors being used for structural health monitoring can vary from simple and cheap piezo-ceramic discs to grouped and pre-packaged multi-sensor elements or autonomous sensor modules with on-board energy harvesting, data acquisition and communication.
In this context future research activities related to sensor technology should focus on the following aspects:
• How much functionality should be integrated into a single sensing element? – The aim of this research is to find an optimum compromise between a centrally organized network, where all functions are concentrated in a single device and a totally decentralized network where all functionality is concentrated in the sensing elements.
• Multi-physical sensing – Often more than one physical information is needed. This could be for example omnidirectional strain, multi-axial strain, temperature, acceleration or humidity. Instead of gathering all this information with independent systems it makes more sense to use a single sensor for multiple sensing purposes where possible or to group different sensors for reducing wiring and weight. A single multi-physical sensor may be heavier and more expensive than a conventional sensor, but could determine information that otherwise could only be obtained with multiple simple sensors. Wave modes of Lamb waves are a good example for this, because a single multi-axial (including out-of-plane) sensor might allow the separation of different wave modes, which would otherwise require at least three simple sensors. Another aspect that is worth being addressed here are sensors with a varying directivity.
• SHM method synergies – In order to monitor a complex structure more than one SHM method may be required. In order to maintain an overall small number of sensors synergies between different SHM methods should be investigated. E.g. for a combination of SHM using Lamb waves and fiber optic strain sensing the fiber optic sensors may also be used for detecting the Lamb waves.
Structural integration of sensors
A very challenging and critical aspect for adding an SHM system to a structure is the proper integration or application of the sensors and the required wiring. The SHM system may not in any way reduce the structural performance. It must be robust enough to theoretically last longer than the structure itself. This requires that the sensing elements themselves are sufficiently robust and that the entire system is properly protected against environmental influences, including mechanical loading, humidity etc.
A reliable structural integration does not just relate to how the sensors are embedded in or attached to the structure, but also how the installation process is integrated into the overall manufacturing process. The state of the art regarding sensor integration has recently been demonstrated by the DLR in the EU project SARISTU where several hundred sensors were successfully integrated into a door-surrounding structure.
For future research activities the sensor installation should be automated as much as possible and the wiring of large sensor networks should be reduced in order to save weight while maintaining a good signal quality and an efficient operation of the sensor network.
Sensor network layout
So far there are no rules and regulations for how to layout a sensor network in order to reliably monitor a structure. However, the way the sensors are distributed may greatly influence the performance of a sensor network. An optimized sensor network would be capable of completely and reliably monitoring a defined area with a small number of sensors.
The challenge is not only to use a small number of sensors, but also to avoid blind spots in the network and assure that all areas of a structure are covered by a defined number of sensors. This also requires taking account of manufacturing limitations and structural restrictions for sensor installation.
The positioning of virtual sensors in a numeric representation of the structure can help to predict sensor signals based on measured wave propagation (e.g. high-resolution laser vibrometry) or simulation data taking into account specific sensor properties. Using such a virtual representation of a sensor network, the layout can be optimized to meet a required structural coverage and probability of damage detection. The proposed research activities in this context include the simulation of Lamb wave propagation in complex structures and optimization algorithms for sensor networks.
Application of SHM technology in aviation
Currently, certification of SHM technology for existing (aging) aircraft is challenging. Moreover, the current new aircraft design process does not account for benefits that SHM techniques can have. One reason for this is because, unlike a non-destructive inspection (NDI), SHM has difficulty to prove its performance in terms of probability of detection. The implementation of the SHM technologies to the existing and new aircrafts strongly depends on their performance (e.g. probability of detection) under various damage location and size in a cost effective way.
Determination of the POD (reliability) for an SHM sensor system is very important for successful application in aircraft structures, but is much more demanding than for traditional NDI methods where this can be deduced from various test details representative of the structure to be inspected, containing damages of various sizes sufficiently mimicking operational conditions. These have to be inspected by several inspectors yielding the final data set. In case of a SHM system the same principle can be applied. However, the human factor is no longer present. Missed cracks are only due to the capability of the system in finding a damage of a certain size at a certain location. A damage located away from a sensor will in general be harder to detect than a nearby damage. An SHM system consists of a network of sensors and signal processing capability designed for a specific structure. To determine the detection capability experimentally, a representative part of the structure now has to be manufactured instead of a representative detail in case of NDI system. Experimental validation of the detection capability of a SHM system is therefore very expensive. On the other hand in the absence of the uncertain human factor, which is hard to model, the detectability can to a large extent be computed. Such a probabilistic model assisted approach, in which the damage detection is simulated together with the uncertainties (e.g. signal noise), can alleviate the costs significantly and only requires a limited amount of experimental data for validation of the numerical analyses.
Condition-based maintenance
The life cycle of a component or system is comprised globally of the following phases:
1. design, production and installation
2. implementation and operation usage
3. degradation as a result of usage
4. recovery, repair and maintenance
The original equipment manufacturer sets the maintenance schedule based on estimates of intended usage and assumptions about future process parameters. To guarantee the safe operation, a conservative approach is applied and heavy usage and severe degradation is assumed in the design phase. As a result, in general, the actual usage is lighter than assumed and the design life may be considered conservative. On the other hand, new (unknown) usage scenarios and degradation processes may not have been taken into account, which may lead to non-conservatism in the design.
The existing aircraft sensors and the sensors of the SHM system provide information on the health of the aircraft structure. Condition Based Maintenance requires knowledge about the momentary and future condition of a component or system. This knowledge then serves as the input to optimize maintenance and asset management procedures, such as the adjustment of maintenance schedules, maintenance techniques, logistical processes, design, inventory management or operational usage. The economic consequences of these efforts are taken into consideration to determine the return of investment of actions taken. Another important aspect for CBM is the (local, domain specific) regulations and certification- and safety requirements.
KGT4: CNS/ATM for greener air transport
1. Greener Flight Trajectory Operations
2. Collaborative Network Management
Greener Flight Trajectory Operations
“Moving from Airspace to Trajectory Based Operations” entails the systematic and continuous sharing of aircraft 4 D Trajectories between various participants in the ATM process all through the flight phases from flight planning through flight execution to ensure that all partners have a common and continuously updated view of a flight and have access to the most up to date data available to perform their respective tasks. It enables the dynamic adjustment of ATM assets like airborne/ground capabilities as well as airspace characteristics to balance predicted demand with unpredicted distortions to the business/mission trajectories kept to a minimum. Whenever possible, the necessary strategic and tactical interventions are considered all through the flight phases seeing the trajectory from an en-route to en-route in order to capture anything from gate to gate trajectory level as well as at the partners capability levels, taking into consideration the wider impact on the air and ground part of the trajectories concerned as well as for the airports and for the overall Network performance.
This is based on the operational and technology scope definition of the trajectory based operations concept and trajectory management framework, its content, performance and access across all flight phases and associated concept and technology developments. The SESAR Trajectory Management Framework (TMF) based on the Trajectory based operations concept specifies the structure needed to achieve the safe and efficient creation, amendment and distribution of the Reference Business/Mission Trajectory (RBT/RMT) including the RBT/RMT information content & quality, the Actors involved, and the Services associated with trajectory information (e.g. creation, proposed revision and update processes).
Justification / Need for collaboration
The ICAO TBO and SWIM ConOps are reaching maturity as well as relevant SESAR solutions. In addition related SESAR 2020 Very Large scale Demonstrations are planned for execution in the H2020 timeframe. However, it is foreseen that intercontinental inbound flights will not be capable of supporting TBO and therefore significantly impact the performance of the Single European Sky network. In addition, the air traffic management system of the intercontinental destination may not be able to fully support the European TBO based flight requirements. Therewith it will impact the operational performance of that flight. This will thus affect the ability of the airline to meet the Single Sky environmental and cost related goals.
The SESAR 2020 programme does not support projects with both European and Chinese partners. It confirms however the relevance and benefits of having take place such intercontinental flight studies and validations projects, since they will provide essential insights into the impact of such intercontinental flights to the Single European Sky performance goals. In addition it will also bring necessary insights in requirement for the intercontinental partners to support TBO flights in their region.
Expected impact
Submitted projects are to study and evaluate in the context of TBO, especially for the increasing number of Europe-China flights, as many of the underneath topics as possible.
• The needed Operational and Technical Capability levels for the Air Traffic Management system in intercontinental region with the view to a full harmonisation and seamless interoperability throughout the network in terms of operational procedures and technology standards.
• Identify and validate (bring down to numbers) the impact on emissions, noise, fuel burn of TBO based ATM flights by none-TBO ATM systems (both on the ground as airborne).
• Identify key enabling technologies (MCMF GNSS, LDACS, SatCom ...) that need synchronized deployment and synergetic collaboration, and the related requirements on compatibility, capacity, interoperability, and standards development. This can potentially be done in close collaboration with the SESAR Very Large scale Demonstration projects.
• Identify additional areas of optimisation of the available ATM capabilities and capacity, and validate their operational and environmental business and performance benefits (both for ground based and airborne systems).
• Identify areas of improvement in SWIM ground-ground and air-ground information, weather and flight plan data sharing (both for ground based and airborne systems).
Abbreviations:
AMAN Arrival Manager
ATM Air Traffic Management
GNSS Global Navigation Satellite System
LDACS L-band Digital Aeronautical Communication System
MCMF Multi-Constellation Multi-Frequency
TBO Trajectory Based Operations
RBT / RMT Reference Business/Mission Trajectory
SESAR Single European Sky ATM Research
SWIM System Wide Information Management
TMF Trajectory Management Framework
Collaborative Network Management
The transition towards Trajectory Based Operations requires the sharing of information both during the process of planning and execution of all combined flight operations. This requires all involved parties and stakeholders, including aircraft operator, ground handler, airports, ANSPs and network management to have continuous access to the latest information in relation to the execution and decision making around flights.
Improved network-wide flight planning systems are needed to support the planning and execution of Business Trajectory (BT)/Mission Trajectories (MT). This allows the involved stakeholder to act better based on shared relevant data, during normal operations as well as under non-nominal conditions such as adverse weather or contingencies.
In addition, for a more seamless and efficient execution of the 4D executed flight trajectory with optimum climb and descent profiles, air traffic management needs to be able to plan and manage the flow of traffic with high accuracy and no delays. To achieve the required more strategic arrival management of inbound flights, increased planning horizons of Arrival Management Systems (AMAN) are needed, which use accurate flight trajectory predictions.
Based on the predicted commercial traffic demand and planned operational air traffic known in the network, better coordination should be achieved between civil and military users for flexible allocation and use of airspace.
Justification / Need for collaboration
Whereas SESAR focuses on optimising overall network operation in Europe, the challenge also exists to extend this concept to intercontinental operations. In particular for flight operations with China, there is already a need identified for more flexible airspace access, improved air traffic flow management and collaborative decision. There is a clear need to align Chinese networked operations developments with those in the EU and US.
The SESAR2020 programme presently does not plan any collaboration projects between partners from Europe and China to demonstrate and evaluate collaborative network management in support of trajectory based intercontinental operations. Both in China and Europe, the introduction of advanced communication, navigation and surveillance technology, globally connected aircraft operators and ground based partners will allow improved sharing of flight data and aircraft status information between aircraft and ground infrastructure, including inflight updates of the most optimum trajectory.
On the ground, the operation at the busy airports in EU and CN will benefit from closer networked information sharing between airlines, airport, ATC and other service providers. The information sharing optimises airport movements, decision making and turnaround process, and also for long range intercontinental operations.
In essence, collaborative network management will provide the necessary enabling technology towards optimised long range trajectory based operations between CN and EU.
Expected impact
Joint studies and research projects on collaborative network management between Europe and China will provide benefits on both sides.
• Coordination between China and Europe in the development of the collaborative network management processes will ensure alignment of regional concepts;
• Increased flight efficiency through user preferred routing and trajectory optimisation will be a priority, since reducing fuel consumption will provide benefits in emissions and air quality;
• More dense air traffic and complex arrival flows can be handled more efficiently by increased time horizons in the arrival management processes for 4D trajectory operations;
• Inflight updates of the most optimum flight trajectory, taking into account latest weather forecasts and earlier planning of the arrival sequence;
• Airspace access may be obtained more efficiently by flexible allocation based on civil and military needs shared over the network;
• Airport processes and aircraft turnaround will be more efficient by a better collaborative decision making process, enabled by networked sharing of latest data;
• Sudden disturbances in the network in/between China and Europe can be handled more efficiently by collaborative decision making based on shared data.
Chinese and European airlines will benefit from the above items both on intercontinental and regional flights. In particular the evaluation of possibilities for collaborative network management for the benefit of intercontinental flights between the regional air traffic networks in China and Europe will provide valuable feedback in the development of the mutual ATM networks, systems and services.
Potential Impact:
The partners in the GRAIN2 project were selected to ensure transnationality and the maximum complementarily of skills towards the successful achievement of the project objectives. More specifically, the consortium groups partners from nine different EU countries Spain, United Kingdom, Germany, France, Belgium, Sweden, The Netherlands, Italy, Czech Republic and China. All partners were firmly committed to carry out the different project activities.
The scope and dimension in the GRAIN2 project is such that none of the partners could have undertaken a similar project only by themselves. Indeed, by participating in a consortium of this kind the industrial partners have benefitted from the expertise and resources from the different partner groups. Thus, the aeronautic industries has benefitted of the scientific knowledge and skills of the RTD organizations and universities. The RTD partners, in turn, have taken profit from the experience and practical know-how of the aeronautic industries in Europe and China. By participating in the GRAIN2 project the project partners have expanded their field of RTD activities in Europe and China, therefore helping to the advance of science in both regions by means of collecting and disseminating state of the art scientific and technical information in the field of multi-physics simulation and validation.
The following outputs have been generated in the GRAIN2 project.
Strategic outputs
The general strategic objectives of the project are three fold: 1) to identify areas of mutual RTD interest; 2) to develop concepts of collaboration in those Key Green Technological areas between the European and Chinese partners; 3) prepare specific RTD activities that are mature for joint proposal for FP7 and for HORIZON 2020.
These three aspects have been successfully achieved along the project lifespan, as demonstrated in the final deliverables, progress reports and mainly on the veents, where the technical discussions have expanded the objectives to more precise technical discussions and shares.
Scientific and technical outputs.
a) An identification of emerging RTD research topics coming from the different KGTs.
b) A state of the art document for each of the Key Green Technological area.
c) A web-based GRAIN Communication System for storage and dissemination of the collected data relevant to computational methods and experimental tests for multidisciplinary applications in aeronautics.
d) Definition of future joint RTD work on critical Key Green Technological areas.
Above outputs have been disseminated and exploited both internally and externally by the project partners.
External exploitation addresses mainly two target groups: the aeronautics industry and the scientific community. The dissemination actions includes the planned kick-off workshop and final workshops in Europe, the Course in China as well as presentations of the GRAIN2 outputs at specialized industrial meetings (such as the Aeronautic Days organized by the EC).
China-Europe Workshop: this event has gathered during three days (3) most of the Chinese aeronautical community on Integrated Computational and Experimental Multi-physics for new Challenges in Aeronautics. Audience gathered scientists, technologists and decision maker managers from Europe and P.R. of China involved in aeronautics. The format of the conference including invited lectures and a general Round Table has provided at a glance a first information on the areas of possible win-win cooperation and with a clear picture of the level of expertise of contributors in the themes of the Workshop very similar to GRAIN2 ones.
Course: this event has deployed State of the Art lectures delivered by international experts including China on topics of major interest to GRAIN2 for future collaboration on multi-physics applications in the next Framework Programme HORIZON 2020. The time schedule of the Short Course allowed detailed presentations of lecturers intertwined with long break discussions between them and the audience. A two hours final Round table with academy, industrial and governmental institutions discussed the impact of Key Green Technological applications on research and industry environments connecting cultures and technologies.
Forum: The greener Horizons Forum has featured keynote speakers from academy, industry and government who shared their perspectives on the new global challenges of eco-efficiency, impact of civil and military aircraft emissions, debates on future greener opportunities to develop new environmentally friendly biofuels, and emerging modelling and large scale computational greener trends targeting a low level of emission, drag, noise and carbon neutral materials research, and programs. The Open Greener Horizon Forum also featured panel discussions in which leaders from industry, government, and academia addressed current issues and trends in R&D aerospace technology to establish an efficiency standard for the design of new greener aircraft.
The individual exploitation plans of the different partner groups participating in the project are as follows: Research centers and universities in Europe and China has used the experience shared in the GRAIN2 project for development and validation of new mathematical and numerical methods concepts and software applicable to the solution of multidisciplinary problems in aeronautics as well as for new experimental methods and tests for validation purposes. The industrial companies, in the other hand, have also used the outputs of the GRAIN2 project as an essential ingredient for defining validation of multidisciplinary projects as well as for every day design of aircrafts. For all partners, the mutual knowledge of the RTD activities in Europe and China acquired during the GRAIN2 CSA is of interest for defining future joint RTD projects and actions with participation of European and Chinese organizations.
Turnover in the aeronautics sector exceeded €85 billions in 2004 and its balance of trade surplus –the difference between the products it sells to the world and the equivalent that are imported– was €28 billions. Increasing these figures in a form compatible with the new imperative of “More Affordable, Safer, Cleaner and Quieter” aircrafts can only be achieved by combining increasing RTD investment in undertanding the complexity of all multi-physics aspects involved in the design, manufacturing and operation of an aircraft.
The enhanced design of aircrafts structures and components accounting for the multidisciplinary couplings will invariably lead to better, safer and more competitive European aircrafts. A reduction in the design and production cycle leading to a reduced time-to-market is also expected. This naturally leads to the improved competitiveness of European aerospace industry currently under severe concurrence with respect to USA aeronautics technology, and indeed the increased competitiveness leads to an overall economic growth.
The outputs from the GRAIN2 CSA contributes to achieve above objectives in a number of ways. The different discussions in the GRAIN2 events are an indispensable source of information for both current design engineering and as reference for future RTD work towards the development of multidisciplinary design tools in aeronautics and aerospace industry. The data quality assessment methodology and the guidelines produced in the project have a similar value as a reference for improved design and also for establishing standard and rules for the efficient use of computational methods in multidisciplinary applications.
List of Websites:
www.cimne.com/grain2
Contact Details:
Prof. Gabriel Bugeda, bugeda@cimne.upc.edu Senior Researcher at CIMNE and full professor at UPC (Universitat Politècnica de Catalunya –BarcelonaTech). Phone: +34 93 401 64 94, Address; c/ Jordi Girona s/n, Campus Nord UPC, Edifici C1, 08034 Barcelona
Prof. Jacques Périaux, jperiaux@gmail.com Senior Researcher and visiting professor at CIMNE. Phone: +34 93 401 74 95, Address; c/ Jordi Girona s/n, Campus Nord UPC, Edifici C1, 08034 Barcelona
Dr. Jordi Pons-Prats, jpons@cimne.upc.edu Researcher and Aeronautics group leader at CIMNE. Phone +34 93 413 41 89, Address: c/ Esteve Terrades 5, Campus UPC PMT, Edifici C3, 08860 Castelldefels, Barcelona
As mentioned in the Executive Summary of the Strategic Research & Innovation Agenda, Aviation has an important role to play in reducing greenhouse gas emissions as well as noise and local air quality issues. The continuous increase of air passenger transport generates an increasing use of hydrocarbon fuel with excessive emission of CO2 and NOX (greenhouse gases, pollutants and noise). It is well known that commercial aircraft operations impact the atmosphere by the emissions of greenhouse gases and greenhouse gas precursors, and also through the formation of contrails and cirrus clouds. In 2011, during the Aerodays in Madrid, the EC launched the future of Aeronautics in the ACARE Flight Path 2050 Vision for the Aircraft report containing the ambitious goals on the environmental impact with 90% reduction in NOx emissions, 75% reduction in CO2 emissions per passenger kilometer, and the reduction of the noise by 65%, all relative to year 2000.
To achieve the ACARE Strategic Research & Innovation Agenda green aeronautics technologies will play a more and more dominant role in mastering the challenge on “Protecting the environment and the energy supply”. GRAIN2 Supported Action, based on the same collaborative and win-win spirit introduced in former EU-China GRAIN project, will provide inputs and roadmaps for the development of large scale simulation strategies for greener technologies to meet the above future requirements on emissions, fuel consumption and noise. To reach these targets, green technologies efforts will have to be collected and prospected in three major lines: Air vehicle, Air Transport System and Sustainable Energies. Three folds to be investigated as future greening technologies:
1) Greening the aircraft and the aero engine: innovative methods and tools for optimized aircraft and aeroengines using best fuel efficiency, optimized propulsion/airframe system allowing the prediction of effects on exhaust emissions of new engines technologies and fuel, Multidisciplinary/Multiphysics Modeling, Simulation, Optimization and Control, new multifunction materials, including environmentally green materials and smart structures;
2) Greening the operational environment: utilization of environmentally friendly chemicals, accurate knowledge of the engine exhaust emissions, in particular applied to low or better free emission taxing, new ATM concepts;
3) Reducing the carbon foot print of aviation via sustainable alternative fuels: development of biofuels for greenhouse gas emission reduction; increase the knowledge of acceptance conditions at engine aircraft level, optimization of the aircraft/fuel tandem.
Project Context and Objectives:
GRAIN2 is an international networking project co-funded by the 7thFramework Program (FP07) and by the China Ministry of Industry and Information Technologies Industry Corporation (MIIT). The main objective of GRAIN2 is to focus its greening activities following the Flight Path 2050 Vision for Aircraft en route to the very ambitious challenge “Protecting the environment and the energy supply” in three major following lines: i) greening the air vehicle, ii) greening the Air transport System and iii) Reducing the carbon foot print of aviation via sustainable alternative fuels.
To achieve carefully these objectives four Key Greener Technologies (KGT) are considered:
• KGT1: Propulsion related green technologies (including NOx and CO2reduction, contrails, mission modeling, alternative fuels, ...)
• KGT2: Airframe Flight Physics (including Drag reduction, noise reduction, HPC innovative architecture, numerical simulation, ...)
• KGT3: Environmental friendly materials and structures (including Smart structures and materials, bio-sourced materials, composite technologies, metal alloys, surface coatings, structural health monitoring, ...)
• KGT4: CNS (Communications Navigation and Surveillance)/ATM for greener air transport
A dissemination and communication platform for Collecting and Prospecting Green Applications has also been developed in order to give traversal support to all KGTs.
GRAIN2 will identify innovative R&D methods, tools and HPC environments (supercomputers and GPGPUs) in the different KGTs according to the needs of major aeronautical industries to deeper understand the mechanism of engine exhaust emissions, to improve fuel efficiency and environmental performance, to lower noise for landing gear and high lift surfaces, to introduce new materials with multiple functions, to help significantly the development of biofuels for greenhouse gas emission reduction, etc.
These objectives will be met by the GRAIN2 supporting joint Europe-China networking actions by a series of open dissemination events including single or multi KGT groups, R&D prospecting activities like Open Forums, Workshop, Short Course involving physical modeling, simulation, optimization and control experts from Europe and China in the different KGT areas.
The targeted International Greener Networking critical outcome of GRAIN2 is a green leverage according to the Flight Path 2050 Vision for Aircraft preparing towards future HORIZON 2020 EU-China Coordinated Calls.
The objectives of the project, as described previously, are global along the project. The consortium is devoting all the effort to deepen on the mutual collaboration though the technical discussions. These technical discussions are feeding the organized events and meetings. It means, that in addition to the global objectives of the project some more specific objectives can be listed:
- Organization of the events and meetings, to launch the technical discussions and establish a path for the technical share among partners. And mainly, the organization of the Open Forum and final meeting of the project, where the conclusions have been wrapped up.
- Invite external stakeholders and experts to participate and contribute to the definition of the best topics to collaborate and define research priorities.
- Establish a first analysis of the situation of the selected topics, analysing the state of the art but also looking for an agreement about which topics are more suitable to be developed and which capabilities can contribute Europe and China with.
- Define the key technologies, which are of common interest, and can be the basis to foster the international cooperation.
1.1 KGTs Objectives and overview
The GRAIN 2 project structure features four key technology streams (Key Green Technologies, KGT) that were identified as the most promising ones for making aviation more environmentally friendly. These technologies are:
• KGT1 - Propulsion related technologies (including NOx and CO2 reduction, contrails, mission modeling, alternative fuels, ...)
• KGT2 - Airframe Flight Physics (including Drag reduction, noise reduction, HPC innovative architecture, numerical simulation, ...)
• KGT3 - Environmental friendly materials and structures (including Smart structures and materials, bio-sourced materials, composite technologies, metal alloys, surface coatings, structural health monitoring (SHM), ...)
• KGT4 - CNS (Communications Navigation and Surveillance)/ATM for greener air transport
The particular objectives and overview of the different KGTs are described below:
1.1.1 KGT1
KGT1 focusses on the identification of new emerging RTD areas of mutual interest on the different green aspects of propulsion technologies (KGT1) including NOx and CO2 reduction, contrail, emission modeling, alternative energies, etc.
This work package will be decomposed in two parts:
1. The first one, propulsion related green technologies, will be basically oriented to different aspects of traditional engine technologies like noise, fuel consumption, etc.
2. The second one, new energies for aviation, will be oriented to different aspects of sustainable innovative energies for aviation application.
KGT1-2 has been set-up in the frame of the Europe and international engagement in term of emission and reduction. Within Europe, dependence on crude oil is reduced by drop-in liquid fuels from other sources at a competitive cost. This has been facilitated by a coherent research strategy, regulatory enablers and streamlined certification and approval processes and the establishment of sustainable supply chains.
The progressive introduction of fuel cells and battery powered vehicles for ground operations at airports has made an important contribution to reducing the carbon footprint of the aviation sector. Electrical and hybrid-electrical engines have entered the aviation market.
The whole life cycle impact of vehicles, equipment and systems has also been addressed.
The availability of alternative aviation fuel not only has a huge environmental impact but it is vital that manufacturers both in Europe and China try to maintain a position within world competition for the sustainable growth of aviation transport for improving energy independence, lessening global-warming effects, and mitigating the economic uncertainty of crude oil prices. Within Europe, dependence on crude oil is reduced by drop-in liquid fuels from other sources at a competitive cost. This has been facilitated by a coherent research strategy, regulatory enablers and streamlined certification and approval processes and the establishment of sustainable supply chains.
In order to implement and develop this existing approach, the KGT1-2 New energy for aviation proposes to address, in the frame of EU-China cooperation different aspects of sustainable innovative energies for aviation application:
1. Innovative fuels for aeronautics
• Drop-in (sustainable feedstock) and new pathways (Methane, Sugar to Alcohol, etc.)
The aim is to bridge the gap between R&D and industry on innovative fuels for aeronautics; to identify and evaluate possible alternative fuels in compliance with sustainable feedstock; modeling of alternative fuel system design on the characteristics and properties of alternative fuels; to identify and access new pathways for alternative fuels at competitive cost;
2. Electric energy
• Energy storage, fuel cells
3. Engine performance
• Modelling
The aim is to investigate alternative fuel performance in compliance with keeping the same safety level as kerosene in the whole aero-engine system and considering the environmental and economic performance of selected alternative fuels based on the aircraft safety. To analysis alternative fuel combustion performance in a highly coupled integrated engine modeling of alternative fuel safety margin using SoS (System of Systems) in compliance with keeping the same safety level as kerosene in the whole aero-engine system; to assess the emission characteristics of alternative fuels. All these technologies have to be assessed in order to validate their sustainability in terms of environmental, economic and societal impact. This approach has to be closely linked to the transversal KGT5 and rely on the developed tools to achieve this validation (Life Cycle Analysis).
1.1.2 KGT2
KGT2 focus on the identification of new emerging RTD areas of mutual interest on the different green technologies of Airframe Flight Physics including drag reduction, noise reduction, HPC innovative architecture, numerical simulation, etc.
This workpackage is decomposed in two parts:
1. Drag and CO2 reduction
2. Noise reduction
In line with the overall objectives of GRAIN2 and on the basis of the achievements made in the previous GRAIN project, one of the main objectives in KGT2 is to make one step forward in the exploration of emerging greener technologies of future aircraft, as well as of the tools and methodologies invoked to realize and implement the identified technologies for improved airframe designs. In close interaction with other KGTs, another main purpose is to establish a collaborative platform and network including competent partners and, further to identify and select state-of-the-art technologies, tools and methods of mutual interest in Europe and in China for future collaboration of in-depth technical investigation.
1.1.3 KGT3
KGT3 focusses on the environmental friendly materials and structures, including smart structures and materials, bio-sourced materials, composite technologies, metal alloys, surface coatings, structural health monitoring, and intelligent health management, et.. The main focus is on light-weight materials and recyclable materials but also all those technologies which help to reduce the total weight of an aircraft and technologies which enable the reduction of the usage and maintenance cost (both of time and resources). The overall goal is to reduce the future footprint of aircraft after their lifespan. Within the work package material technologies like composite materials developments and applications, new manufacturing processes, and designs that help to reduce the pollution produced by the involved processes and technologies to enable the recyclability of components are analyzed.
1.1.4 KGT4
The high number of new greenfield airports in China that are expected to become operational within the span of this decade bring a fast changing and growing air transport network in China. In addition to that there are the new regulations in China around available airspace for business and general aviation. This creates a vast set of opportunities and challenges for the environmental footprint of the global air traffic system.
Europe has, in the last few decades, gathered a lot of experience around the concepts of Flexible Use of Airspace, Network Management, and greener CNS/ATM systems. Expectations are thus high that significant benefits can be derived from joint R&D in CNS/ATM solutions. WP4 will derive a list of those common R&D interests in CNS/ATM solutions.
A more specific objective is to improve the knowledge and capability of how CNS/ATM systems and technology can be used to reduce environmental footprint. WP4 will mainly focus in new ATM concepts and operational paradigms, along with the associated innovation in CNS systems, procedures and infrastructure. In this context, this workpackage will deal with 4D trajectory management and its role to move in the future an airspace centric ATM to a trajectory centric ATM. Thus, environmentally optimised aircraft procedures will be tackled along with systems, concepts and technologies aiming at increasing the automation levels in ATM.
This KGT4 is closely coordinate with Eurocontrol, SESAR and CleanSky frameworks.
Project Results:
After more than 10 years of collaboration between EU and China a lot of opportunities have been identified. These are based on the high research capabilities on both sides and the large infrastructures available for industry and research.
Also some big challenges have been identified which are originated by the growing air traffic in China and its integration at an international level, the development of new aviation products and the extension of some green aeronautical technologies to other transport modalities.
The different GRAIN2 KGTs have already identified RTD topics for future collaboration. Here below we list the synthesis of these topics. Additional details can be seen in the following sections.
KGT1: Propulsion related green technologies
Propulsion
1. Low Pressure Turbine (LPT) aerodynamics and heat transfer
2. Multidisciplinary design and optimization of turbomachinery components.
New Energy for Aviation
1. Safer and more efficient certification of Aviation Alternative Fuel (AAF)
Two key subjects are proposed for aero-thermodynamical research in aircraft engines
KGT1-1: Propulsion related green technologies-PROPULSION
Low Pressure Turbines
The demand for higher bypass ratios in modern commercial jet engines requires the low-pressure turbines at the exit of the engine:
• to provide an increasingly larger power output allowing a direct drive of large fans without penalizing the global efficiency.
• At the same time, their design asks for a reduction of the turbine blade count and/or
• the implementation of new, lighter materials of in order to save weight and/or secure lower manufacturing and operating costs.
This eventually implies an increase of the aerodynamic load on each blade, towards the development of very high or even ultra-high lift airfoils. These are characterized by
• a high velocity peak on the suction side,
• followed by a significant diffusion.
• The resulting strong adverse pressure gradient along this surface may possibly induce a separation of the boundary layer, particularly at low Reynolds number, This may finally cause a significant loss in lift and a consequent drop in engine efficiency.
For large commercial turbofans, a loss in LP turbine efficiency of the order of 2 % may occur between sea-level take-off conditions and high-altitude cruise flight (where the Reynolds number drops by almost one order of magnitude). Consequently, deep analysis, understanding and prediction capabilities are of utmost importance to secure an efficient engine design.
This implies the need for both
• accurate time-resolved measurements (including higher order turbulence characteristics) at engine similar conditions (Mach and Reynolds number, turbulence, upstream/downstream unsteadyness)
• high fidelity computational methods, going far beyond the classical RANS approaches, not very satisfactory at the lowest Reynolds numbers.
Today’s experimental and computational approaches probably provide an acceptable mean flow description and global (integrated) performance (loss) parameters, but many of them fail in the description of higher order statistical characteristics. It therefore remains a major area for further basic and applied research in this particular area of the turbine.
Based on several research programs conducted in US and Europe over the last decade, the so-called ‘geared-fan’ engine architecture appears very promising as the future engine architecture. The principle is to choose for a small size, fast low pressure turbine (reducing therefore its volume and weight) and to introduce a gearbox between the low rpm - large fan and the high rpm - small LP turbine.
• Reynolds number issues are still present,
• Mach number issues (transonic flow, shocks) appear as well.
• Moreover, considering the higher rotational speed of the turbine, structural issues will also develop.
This calls for a multidisciplinary design strategy. Competences need again to be developed both from the experimental and the numerical side.
Multi-disciplinary optimization
Not so long ago, the design of a component was dominated by the experience gained during previous designs and simple correlations. Prototypes were built and tested, and improvements were proposed based on the newly acquired knowledge.
Nowadays, Computational Fluid Dynamics tools are in continuous development for aerodynamics, heat transfer, structural mechanics, aero-acoustics, vibration ... issues. They start to be introduced (unfortunately not all of them at the same time) into the design process as an alternative to the time consuming and very expensive tests in experimental facilities. Designs are immediately tested by CFD, requiring (for each new individual design) a few hours of computational time. The iterative nature of the design, however, has remained in most of the cases ... and classical CFD remains costly (especially in time). At the end, experiments are still made on the final design for a final validation of the product and of the CFD predictions. This information is an important feedback to verify and validate the models as they can reveal their shortcomings and can be used to improve them.
However, new challenges arise. The shortening of the design cycle, to adapt to rapidly changing market demands, and the increase of the performance are nowadays the main challenges in component design. Many different designs need to be evaluated and compared. Improvements are obtained by carefully examining the effect of each parameter on the performance of the design. Choices such as the material of a part and its shape can be evaluated in terms of e.g. strength, efficiency, manufacturability, overall production cost, etc.
However, the large amount of parameters that can be independently modified complicates the task, and increases the classical CPU time. In order to get much faster responses, the last years have seen a large development of so called optimization algorithms, coupled to ‘Meta-Models’ (also known as complex ‘interpolations’). These algorithms, such as e.g. genetic algorithms (the optimization) coupled to e.g. a neural network (the Meta-Model or the ‘interpolator’) allow to explore the design space with a speed which is 3 or 4 orders of magnitude higher than a classical CFD approach. The challenge is of course to define the correct objective functions and constraints and to correctly ‘train’ the Meta-Model to provide accurate predictions. The main challenge here is to apply this approach in a multi-disciplinary environment. This challenge also calls for the availability of multi-disciplinary accurate measurements and/or high fidelity CFD methods in order to support the training of the various Meta-Models.
KGT1-2: Propulsion related green technologies-NEW ENERGY FOR AVIATION
Safer and more efficient certification of Aviation Alternative Fuel (AAF)
The availability of alternative aviation fuel not only has a huge environmental impact but it is vital that manufacturers both in Europe and China try to maintain a position within world competition for the sustainable growth of aviation transport for improving energy independence, lessening global-warming effects, and mitigating the economic uncertainty of crude oil prices. “Drop-in” fuels are defined to substitute for kerosene without change of aero-engine and aircraft as well as infrastructure. Accordingly, the current standard specification for alternative fuel (ASTM 7566-13) have been deduced from the jet fuel specification properties (ASTM 1655) based on as same as kerosene, which results in high cost in fuel refinement. Since the difference between alternative fuels and kerosene in components, only testing of physical and chemical properties cannot confirm the security of alternative fuel in aero-engine. Therefore, the certification process of alternative fuel includes the testing of physics and chemistry, the testing of fit-for-purpose, hot section testing, component testing, and system test according to ASTM D4054, which results in fuel-cost in certification testing. Hence, three challenges including high cost in fuel refinement, fuel-cost in certification testing, lack available testing condition and the acceptance criteria in recent standard are expected to be overcome.
With the development of this research project, CAAC and EASA will develop the certification of alternative fuels in compliance with airworthiness standards, and promote the formation of the Sino-EU alternative aviation fuel certification standards.
• Low fuel cost in certification process
• Low cost in biofuel refining
• Novel drop-in & non drop-in fuel certification process and management.
KGT2: Airframe Flight Physics
Noise reduction
1. Advanced experimental and numerical methodologies for analysing noise generation and propagation.
2. Innovative noise control/reduction technologies
3. Low-noise concepts and installation effects
Drag and CO2 reduction
1. Advanced simulation of high Reynolds number, realistic setting controlled flows on large/innovative HPC architectures.
2. Advanced turbulent models for modelling detached flows and laminar-turbulent transition.
3. Multidisciplinary optimization technology
4. Instability analysis and unsteadiness control in aircraft design.
KGT2-1: Airframe Flight Physics-Noise reduction
Computational methodologies and experimental technologies for effective analysis of aero-acoustic noise generation and propagation
For next-generation quieter aircraft design and making steps toward the realization of the ambitious goals set by ACARE Flightpath 2050, one of the essential aspects is to comprehensively understand and accurately address noise generation and radiation by means of experimental measurements and computational simulations. In spite of remarkable effort and progress made over the years, particularly, in EU framework programs, it is recognized that current experimental technologies and numerical methodologies are not sufficiently complying with the need of effective industrial use for accurate analysis of aircraft noise emission. For massive industrial applications in design process and for problem diagnosis and mitigation, reducing aircraft development time and costs has in many cases been attained at a price of reduced reliability and accuracy of solutions.
Comprehensive aero-acoustic analysis rests on an accurate exploration on near-field unsteady aerodynamic flow physics and far-field noise transmission. This requires a correlated setup, in either experimental measurements or numerical simulations, of systematic studies on turbulent aerodynamic flows (as noise-generating sources) and far-field noise propagation (in relation to aircraft noise emission and its impact on environment). A lack of reliable correlation between noise generation and propagation forms a significant source of inaccuracy in many current aero-acoustic measurements and simulations.
For accurate CFD/CAA (Computational Fluid Dynamics/Computational Aero-Acoustics) analysis, furthermore, the computational resources may become excessively demanding, or even prohibitive, for applications of industrial relevance to formulate aerodynamic noise generation in correlation with far-field noise propagation. High-Performance Computing (HPC) architectures, in conjunction with robust numerical algorithms and advanced flow-physics modelling approaches, have been emerging as large-scale computational methodologies exploitable to make steps forward for facilitating the use of CFD/CAA as potential industrial tools implemented in the process of developing quieter aircraft.
The proposed RTD topic, standing on the basis of emerging computational methods and experimental technologies that have shown promising capabilities (yet often in academic research), aims at new development and further improvement to enhance the TRL beyond the current state of the art on two primary aspects: robust CFD/CAA methodologies and effective experimental technologies for addressing industry-relevant aero-acoustic problems.
Aircraft noise-control technologies and integration strategies of control systems
Over almost two decades, the urgency to reduce the airframe and engine noise components has become increasingly pressing, and the most straightforward approach to obtain noise reduction without affecting the other key attributes in aircraft design has been to apply proven technologies and refine existing designs. But it has since become more obvious that the ambitious ACARE targets set for 2050 won’t be reached with existing technologies. Given the noise generation mechanisms cannot be drastically reduced from further incremental refinement of the current designs, the step for a breakthrough change has to come from innovative approaches based on fundamental research and detailed investigations of novel concepts.
To achieve the ambitious goals set by Flightpath 2050, it is necessary to develop the most effective technologies for modifying the near-field noise generation mechanisms and leading to far-field noise reduction. This objective has sustained continuous efforts over the recent years, aiming at the development of innovative methods for the control of noise generation and propagation, giving birth to new innovative methodologies and technologies targeting airframe and engine noise reduction.
In spite of significant contributions made in a large amount of previous and ongoing effort on effective noise-control technologies, nonetheless, the gap remains large from current academic laboratory research to actual industrial deployment. Indeed, quantifying the benefits of a new design, noise control or abatement procedure in terms of decibels is a very challenging task. Two key factors are at least partly responsible for this difficulty. Firstly, the non-linearity of fluid dynamics makes it very complicated and effort-consuming to predict the transient behaviour of a flow field, especially for the very high Reynolds number at stake in aeronautics. Unfortunately, knowledge on the transient features of turbulence are an essential ingredient of the aero-acoustic prediction chain, because the mechanisms causing noise in the far-field result from subtle imbalances in amplitudes and phases between reciprocating motions in the medium. Secondly, the past efforts conducted to reduce the noise emitted by the various components of an airplane have resulted in a fair balance between their respective contributions integrated over a complete flight. Different components are clearly dominating the noise spectrum at take-off, and others are responsible for most annoyance during landing. In order to address in a timely and cost-effective way the problem of aircraft noise using noise-control technologies, it is essential to devise integration strategies aiming at developing and deploying best common denominator technologies, in terms of applicability to the different elements
By means of numerical simulations and/or experimental testing, the proposed RIA topic aims at development of emerging innovative noise-control concepts and, going beyond the proof of concepts reported in academic research, enhancing further the TRL with relevant validation of identified promising noise-reduction technologies, which are supported further by the development of integration strategies of control systems installed on different aircraft components.
Low-noise configurations and airframe-aeroengine installation supported by multidisciplinary optimization
Aviation has a significant impact on the environment and the EU citizens due to emissions (noises, pollutant matter and contrails) to which the population is exposed. On the other hand, aviation generates around 2% of EU GDP and accounts directly and indirectly for 3.7 million jobs. This has led to the implementation of a number of ambitious goals set by the sector at horizon 2050 in the Strategic Research and Innovation Agenda (SRIA) of the Advisory Council for Aviation Research and Innovation in Europe (ACARE). For noise emissions, the target to meet by the 2020 horizon is to reduce perceived external noise by 50%, while the new horizon defined for 2050 (described in the Flightpath 2050 document) is targeting a reduction by 65%. In support of this objective, it has been acknowledged that the technological objectives will not be reached through an evolutionary approach only. Breakthrough innovations are needed, i.e. new solutions which rely on a disruption with respect to current approaches.
In the process of developing future new aircraft concepts, obviously, the step must be progressed by a close coupling with research on innovative low-noise configurations of airframe components and power systems, as well as on low-noise aircraft installation. The proposed research and innovation actions target new low-noise technologies and concepts that are not currently used in aeronautics or that have not yet being put in combination in the aviation sector. By means of numerical simulations and experimental measurements, innovative low-noise high-lift configurations, landing-gear systems and airframe-aeroengine installation, in which multidisciplinary optimization will play a significant role in supporting the design of new low-noise concepts and will be undergone further development to improve its applicability for robust aero-acoustic design.
The proposed RIA topic aims at demonstrating the validity of the low-noise technologies and concepts resting on sound technical and scientific approaches. The aerodynamic and aero-acoustic performance should be jointly assessed quantitatively against the relevant criteria such as economic viability, time efficiency, potential to cope with evolutions of regulations, environmental friendliness and energy sustainability etc. The proposed topic will also assess at the end the potential of the low-noise technologies and concepts developed at further technology readiness levels and possible barriers that may prevent such developments.
KGT2-2: Airframe Flight Physics-DRAG AND CO2 reduction
Advanced simulation of high Reynolds number range, realistic setting controlled flows on large/innovative HPC architectures
Substantial resources have been invested over the years into Computational Fluid Dynamics (CFD), particularly in the development of efficient and robust numerical algorithms for solution of the governing equations, grid generation and adaptation, powerful post-processing tools, etc. Not surprisingly, these investments (many of them made in the framework of European programs) have resulted in a remarkable progress in CFD capabilities. This contributed significantly to improve the flow physics understanding, reduce aircraft development time and cost minimizing the reliance on wind-tunnel and flight tests. However, experience shows that industrial numerical simulation tools are presently suffering two main drawbacks that prevent their full industrial deployment for massive applications in design process: excessive long computational times for problems of industrial relevance, and reduced reliability and accuracy of the solutions at flight extremes. This includes the accurate simulation of turbulent drag reduction through the application of active/passive flow-control techniques to manipulate the drag produced by the flow structures in turbulent boundary layers.
If current simulation capabilities, including LES and RANS/LES models, can be made to work with drag reduction techniques (passive/active flow control), the simulation of full aircraft at high Re would certainly reveal many indirect effects of reducing drag locally and which can be very useful for aircraft design process.
Research in flow control needs to be done subject to more complex conditions: realistic geometries, strong curvature, adverse pressure gradient, free-stream turbulence, etc that characterizes the practical aircraft setting. The efforts should be made to substantially up the Reynolds number range of problems to be addressed. This would mean to go increasingly for extremely high-quality LES and possibly LES-RANS. The latter requires very careful discrimination and much judgment on the penalties incurred. With any shift towards more complex conditions and LES model, the use of high-order methods is ever more important.
Realistic actuators and sensors that provide the prospect of realistic boundary conditions that feed into the simulation tools are of high importance. Simulators need to understand what actuators are realistic in a practical setting (frequency, density of actuators, size, reliability....). Important part of the computational research needs then be devoted to how these actuators can be represented realistically in a computational setting. A proper and accurate set of experimental data necessary for the validation process is also of vital importance.
New numerical methods and new hardware architectures are emerging as possible solutions of these problems: High order methods in complex geometries could be the solution to obtain the desired accuracy. At the same time, the capabilities of leading-edge emerging HPC architectures are still not fully exploited by simulation tools for large aeronautical problems. These do not take advantage of the immense new capabilities of new hardware architectures, such as streaming processors or many-core platforms. Likewise, current high order methods are mainly limited to academic environment, it is necessary to mature their application in complex geometries and high Reynolds problems.
In order to tackle these problems. We propose to go beyond the state of the art in the following main research lines: 1) Advanced techniques for efficient implementation of solvers on heterogeneous and highly parallel High-Performance Computing (HPC) architectures. 2) Innovative and efficient numerical algorithms for large scale computing in aeronautics, based on advanced parallel architectures. In particular High-order methods in complex geometries and high Reynolds numbers.
Advanced turbulent models for Computational Fluid Dynamics
Computational Fluid Dynamics (CFD) techniques have become essential in the aircraft design loop and are regularly used by most industrial aircraft manufacturers. Namely, these numerical techniques are used to define aircraft aerodynamic surfaces and provide valuable information to the structural and loads departments enabling a cost efficient aircraft design with reduced fuel consumption. Furthermore, the use of CFD is saving time and resources, limiting the number of costly wind tunnel experiments and flight tests. In some occasions, numerical simulations are the only available tool to evaluate extreme aircraft conditions and performance such as stalled conditions, shock boundary layer interaction or buffet. Although CFD techniques have been shown to be mature enough for benign design flow conditions, the accuracy of the numerical solutions near the design envelope limits, or when new aircraft configurations are considered, are still limited.
Two key elements are identified as the main bottlenecks for the full acceptance of CFD results by industry near the limits of the design envelope.
• Firstly, turbulence modelling has seen major enhancements during the last decades but its accuracy remains limited for detached flows.
• Secondly, laminar-turbulent transition modelling is one of the major bottlenecks for CFD and essential to accurately compute aircraft performance and loads.
Detached flows and turbulence modelling
The following future activities are suggested to address future needs in the accurate simulation of complex aerodynamic flows:
• Further development of improved RANS approaches, focussing on reliable separation prediction for complex 3D flows and correct prediction of turbulence levels in the early separated shear layer (responsible for errors in reattachment prediction). RANS will remain the only affordable approach in high-Re boundary layers for decades to come – also in the framework of hybrid RANS-LES methods, the accuracy of which can be limited by RANS errors.
• Industrialisation of hybrid RANS-LES models, which consists of several aspects:
o Consolidation of the latest state of the art improved methods
o Further improvement of Grey Area behaviour without loss of generality and robustness
o “Smarter” models requiring less user input and decisions:
▪ More “flexible” zonal/embedded approaches
▪ Automatic detection of RANS and LES regions
▪ Automatic and adaptive meshing for hybrid RANS-LES methods
o Addressing the expertise gap between the developer and user communities via clear best practice guidelines, method applicability recommendations and training events
• Development and implementation of models for flow control devices, in order to capture the effects of e.g. vortex generators, synthetic jets, plasma actuators, turbulent screens, riblets in a cost-effective manner. Extension of RANS models to include such effects will enable the assessment of such advanced concepts at aircraft level as well as opening the door to efficient (adjoint) optimisation of flow control device parameters and placement.
Additionally to improvements in cruise flight drag, significant benefits can also be expected from a better knowledge and modelling of boundary layer flow during transient phases of flight. High-lift configurations need reliable predictions of boundary layers with regions of strong adverse pressure gradient which are close to separation and sometimes even locally separated. Separation limits the maximum lift that a wing can generate at low speeds, and thus limits the achievable speed reduction required to ensure safe take-off and landing conditions. Prediction of separation is highly sensitive to simulation tool deficiencies and quickly becomes unreliable rendering aerodynamic optimization and drag reduction efforts futile.
Laminar-turbulent transition
The physical mechanisms by which a laminar flow transitions to a turbulent regime are relatively well understood. In boundary layer cases, transition can be (in most scenarios) explained by modal linear instability, which considers spatial linear growth of Tollmien–Schlichting (TS) waves. However, in some cases a temporal transient growth of perturbation (i.e. non modal instability) can explain some “bypassed transition” mechanism to turbulence. Other physical mechanisms for transition are 3D cross-flow transition, or purely inviscid instabilities which can be explained by inflection points in the velocity gradients. Techniques to compute modal and non-modal instability analysis are varied. One of the most widespread methods (and accepted in industry) is the e^N method, which accounts for the TS type of transition. However, there is a lack of generalised models that are able to predict laminar-turbulent transition. Namely, CFD methods to predict transition do not in general incorporate the linear instability knowledge (neither modal nor non-modal). It is therefore necessary to unify the linear stability information, and incorporate this into mature CFD techniques, to create new models that can explain transition to turbulence for a variety of flow scenarios such as in the presence of adverse pressure gradients, bubbles or detached flows.
In summary, most advanced Computational Fluid Dynamics tools are able to accurately predict the vast majority of flow problems if the appropriate computational model (e.g. geometrical simplifications, turbulence model) and enough resolution (e.g. mesh of sufficient refinement and quality or increased formal order of the method) are selected for simulation. Computational inaccuracies may be categorised as modelling errors (e.g. separated flows and transition to turbulence) or purely numerical errors. To mitigate these two types of errors requires better models and higher resolution.
Consequently, more reliable and accurate models, such as turbulence models for detached flows and models to predict laminar-turbulent transition are required which will ensure a significant widening of applicability of CFD solvers, particularly in off-design and maximum load situations.
Multidisciplinary optimization technology for innovative aircraft configurations including local flow control applications
The improvement of the optimization process in the aerodynamic design of aircraft configurations is becoming of key factor to reduce development and operational costs and decrease the time-to-market for new aircrafts. Indeed, despite the increased computational power, the greater sophistication and perfection of physical and computational models and an ever more automated industrialization process, it is undeniable that the timescale for realization of new aircraft (from paper to the market) has not substantially reduced. In this framework, innovative optimization techniques can play a major role, as effective and intelligent exploration of the space of all possible configurations is an essential requirement to reach the goal of time-to-market decrease in aerospace industry.
The expected increase of air traffic for the next fifteen years has established several environmental targets for the next generation of aircraft: 50% reduction in carbon dioxide emissions, fuel consumption, perceived noise and development time, as well as 80% reduction in emissions of nitrogen oxides. The achievement of these targets makes necessary to explore other unconventional/innovative configurations. In this search for optimized innovative configurations one also can add to this design optimization processes the possibilities of taking on board flow control devices early in the design stages. The typical application of flow control devices are presently used with the purpose of mitigating some undesired effect, namely drag or flow detachment. That means that flow control is generally a patch that is applied on and already designed aircraft, thus, hampering the exploitation of its full potential, that could only be achieved after a complete integration of the flow control into the aerodynamic design process.
However, in practical optimization problems, in which hundreds of design variables are considered, the use of current RANS industrial solvers is unfeasible because of their high computational cost. Alternative mathematical tools are necessary in this context; different techniques as surrogate modeling, control theory together with code optimization and proper parallelization methods can decrease the cost of the optimization aerodynamic design process.
Other contribution is related to the consideration of the design process as a global optimization problem; in this regard, the use of evolutionary algorithms (EAs), due to their ability to obtain global optima, in combination with metamodels to substitute expensive CFD simulations, makes possible to perform a preliminary exploration of the design space at a relative low cost.
There are several examples of aerodynamic optimization using an Evolutionary Programming algorithm hybridized with a Support Vector regression algorithm (SVMr) as a metamodel. Specific issues as precision, dataset training size, geometry parameterization sensitivity and design of experiments techniques are still an issue and the potential of the proposed approach to achieve innovative shapes that would not be achieved with traditional methods is very attractive.
We may then faced with the question of how to be sure we have found out the maximum potential of the outcome of the preliminary designs - if results aren't as good as hoped, there will be doubts such as "did we put the actuators in the wrong place?. Should the amplitude/frequency be adjusted? Very quickly, even considering mono-disciplinary optimisation, we have a very large parameter space. Adjoint optimisation of flow control systems therefore becomes a very attractive prospect: Sensitivities can be computed for thousands of parameters for the same computational expense as one parameter, making it possible to consider delegating actuator placement and configuration to an optimisation process. Work in this direction is developed by Uni-Kaiserslautern and have successfully computed the adjoint sensitivities for synthetic jet AFC to a full complexity high-lift / fuselage configuration.
Another crucial point is the introduction of robust or reliability based optimization techniques in the design loop. Indeed, industrial optimization processes must take account of the stochastic nature of the system and processes to design or re-design and consider the variability of some of the parameters that describe them. Thus it is necessary to characterize the system that is being studied from various points of view related to the treatment of uncertainty. In particular, it is necessary to consider the sensitivity of the system to the uncertain parameters and assess its reliability. Then, having established the ability to characterize the system from this point of view, it is necessary to build an optimization loop that can improve its reliability or that is capable of providing a robust optimum, or that could withstand acceptably random perturbations of design parameters or operating conditions.
Integration of Local flow control for cruise drag reduction in future transport Aircraft
Latest research in flow control has shown important benefits in both cruise and low speed configuration by avoiding flow detachment, reattaching the flow, re-laminarizing the boundary layer, etc.
The typical application of flow control devices is on an already designed geometry, with the purpose of mitigating some undesired effect, namely drag or flow detachment. That means that flow control is generally a patch that is applied on and already designed aircraft, thus, hampering the exploitation of its full potential, that could only be achieved after a complete integration of the flow control into the aerodynamic design process.
First steps of flow control applications integrated into the design process are currently ongoing with the application of hybrid laminar flow control on the wing leading edge; thus requiring a small redesign of the airfoil profile for already manufactured wings. But a big step is still required in the design process in order to take onboard the possibilities of the flow control devices from the early design stages, and not as a mitigation action.
Universities and research centers have made a huge effort to develop new flow control devices with progressively less and less power consumption and an increasing accuracy in their simulation models. Nevertheless, there is still a breach that needs to be traversed: we need to learn how to capitalize on the full potential of the wide range of available flow control devices, and this exploitation does not yield from an isolated optimization but from the complete integration of the system.
The expertise in flow control devices is currently spread among academia, research centers and industry. Each of us masters a different flow control device, the typical aerodynamic design process or even the preliminary design process involving all the disciplines. Thus we have a quite comprehensive toolbox of flow control techniques, from which some of them have already been successfully applied for solving a specific problem (eg. boundary layer suction to extend laminar regime on the wing). Nevertheless, the range of applicability and the potential benefit of many other devices remain still relatively unknown. It is widely accepted that we could better design the aero shapes if we introduce the flow control techniques in the early stages (eg. increased efficiency leading to smaller surfaces, less interference drag...)
The process of the technology readiness level evolution demands some specific steps before the possible industrialization: from a concept proof stage we need to develop pilot cases. Those pilot cases should be attractive and representative enough. The application of the already developed knowledge on a real problem will give us the opportunity to attain higher maturity levels. “We do not know how to use a hammer only after reading its user manual, but after hitting some nails”.
The path to such an ambitious activity is surrounded by traps that might attract the attention and divert our efforts. Thus a clear scope management will be required to keep the focus of the activity within the area of biggest interest, namely the application of the different flow control techniques for solving a specific problem.
Any multidisciplinary optimization that couples aerodynamic forces with the structure, although extremely interesting, cannot be considered within this activity by two main reasons: firstly, aero-structural coupling technology is already relatively mature and it is being applied by the industry, universities and research organizations, and secondly, this wider scope would weaken the focus of the activity on the integration of the flow control devices into the aerodynamic design. The first step in the process is to couple two bricks: aerodynamic design and flow control devices, before thinking about a higher complexity application with other disciplines involved.
KGT3: Environmental friendly materials and structures
1. Environmentally friendly materials
2. Functional integration
3. Structural Health Monitoring
KGT3: Environmental friendly materials and structures
Research topics related to materials
Current aircraft designs are already a result of the transition from metal structures towards lightweight composite structures mainly made from carbon fiber composites (CFRP). While these materials allow a lighter construction and therefore improve the ratio of aircraft weight to passenger/cargo weight there are several challenges regarding the environmental impact of these materials:
• The production of CFRP materials requires a lot of energy, which reduced the benefit of saving energy during its lifetime. Therefore alternative material sourced like bio-sourced materials should be investigated.
• The production process of composite structures still features a large amount of manual work. There is still a large potential to improve and automate all manufacturing steps in order to make them more efficient and reduce the required energy.
• Composite materials are difficult to recycle because they may lose their superior properties during the recycling process. Considering the amount of energy put into the production and manufacturing of these materials the recycling techniques should be improved in order to created secondary materials with a minimized loss in performance.
The following research topics have been identified as to most promising ones in the context of environmentally friendly materials and production.
Bio-sourced fibers
Much of the energy required for producing conventional composite materials is put into the production of the fibers. This includes both the production process and the origin of the raw materials, since they are mainly based on fossil resources. Bio-sourced fibers can be an alternative, but require specific research and development to reach competitive performance levels and production processes suitable for an industrial use.
With respect to bio-sourced fibers the following aspects are foremost important for future research:
• Fatigue behavior and durability – Bio-sourced fibers cannot be tailored to mechanical performance to the same degree as conventional fibers. They have a different structure (often hollow) and show a much higher spread in parameters due to their natural origin. Since this has a direct impact on the mechanical performance of the material the fatigue behavior and durability should be investigated. This also includes the development of numerical simulations that take into account the special properties of these materials.
• Cost-effective and environmentally friendly production of long-fiber semi-finished products with adapted properties – While conventional fibers can be produced in a continuous process the length of natural fibers is limited. For an industrial application however it is essential to provide semi-finished materials with a long-fiber character and defined properties. Future research should first focus on the general feasibility of producing such materials and later on developing suitable production processes for larger scales.
In addition to the aspects mentioned here future research activities should also be built on the developments and findings of the EU project Eco-Compass.
Bio-sourced resins and additives
Although the matrix material of fiber composites holds the fibers in place and distributes the loads to the fibers its mechanical performance is not always as critical as that of the fibers. Therefore a feasible solution for improving the environmental footprint of composite materials may be replacing conventional petrol-based matrix materials with bio-sourced materials while maintaining the high mechanical performance using conventional fibers.
Research and innovation efforts, during upcoming years, should be focused on obtaining 100% bio-based polymeric matrices that could meet the high requirements of aeronautical industry, encompassing resins , fillers and curing agents.
As second step, greener synthetic routes should be explored in order to reduce the carbon footprint of the associated processes, together with energy consumption decrease and minimization of reaction byproducts.
Besides, bio-mimetic self-healing thermoset composite will probably undergo the highest growth within the different materials used when it comes to aircrafts, providing the ability of self-repairing small damages caused by impacts or erosion. Thereby, the lifespan of aircrafts will be greatly increased, maintenance costs will be reduced. Additionally, indirect environmental impact is expected since the energy consumption and gas emission associated to part manufacturing will be reduce as a result of their durability thereof.
Nevertheless, there is a bigger challenge to be achieved in order to guarantee eco-sustainability of aeronautical industry; composites recycling after aircrafts end-of-life, where polymer matrices composition will play a key role in future. Chemical recycling technologies, comprising depolymerization and purification processes to obtain recycled raw materials and recycled long fibers should be seriously investigated during next years.
As in bio-sourced fibers case, future research activities should also be built on the developments and findings of the EU project Eco-Compass.
Thermoplastic composites
Thermoplastics composites have the advantage over thermoset materials of higher impact resistance. They also offer the possibility of employing fast forming processes, such as press forming. The combination of these types of processing with automation of the manufacturing process offers the possibility to improve the overall manufacturing process.
There is a long way to be toured in terms of eco-friendliness in thermoplastic composites, however, developments carried out in EU project Eco-Compass regarding fibers and bio-sources could represent a considerable shortcut to achieve the thermoplastic target of 100% bio-based materials. Nevertheless, high performance demanded in current aeronautical thermoplastic composites, such as PEEK or PPS based ones, represents a real challenge to be achieved by bio-based materials that use to be based in polyester backbones that cannot accomplish mandatory requirements, such as high maximum service temperatures.
Besides its bio-based character, fire resistance should be enhanced, by means of replacing halogen based flame retardants by intrinsic covalently linked halogen-free additives. New fire resistant thermoplastics should impart desired properties without penalizing mechanical and thermal properties.
Fiber reinforced thermoplastics present several advantages in respect with thermosets. Thermoplastic matrices offer the ability to be reprocessed by thermal transformation conventional processes, such as injection molding or thermoforming, avoiding the current thermoset composite recycling technologies that consist of difficult and high energy consumption chemical methodologies. Notwithstanding, a previous conditioning milling steps has to be done to obtain short fiber reinforced thermoplastics that can be midway between high performance and engineering polymer, and hence, can be used as raw materials to obtain non-structural high added value parts of aircrafts.
Chemical surface treatment
In contrast to conventional fibers bio-sourced fibers often do not have the same non-mechanical properties. They are not as fire-resistant, pick up moisture more easily and may need fungus protection. Due to the natural variations of the fibers they may also need a surface preparation to assure proper and even bonding to the matrix material. It is therefore essential to develop suitable surface treatments to handle these properties. The key aspects to consider here are the environmental compliance of the chemical agents and processes involved and their impact on the materials properties, e.g. additional weight due to required additives.
Low-cost and automated manufacturing
Current processes involved in manufacturing complex composite parts still involve a large amount of manual work. Processes like fibre placement have already been developed to industrial application, but are often limited to simple geometries and specific raw materials (pre-impregnated fibers). In order to automate the remaining manual tasks, technologies like robot-based pick-and-place systems, which can also handle dry fibers and complex geometries should be developed to a stage where they can contribute to reducing costs and improving the quality of smaller series.
Other approaches for improving manufacturing processes should focus on using simpler manufacturing methods, like oven curing or automated resin infusion. With a focus on saving energy during production and avoiding scrap parts process monitoring and control is a very promising field of research. Using monitoring methods, such as ultrasonic cure monitoring, the manufacturing process can be adapted to what is actually happening with the part to be produced. This approach can replace the current practice of running processes that are longer than necessary (and therefore consume more energy) to avoid scrap parts.
Life-cycle assessment on innovative composite materials
Since the main purpose of using bio-sourced materials instead of conventional materials is the improvement of the environmental impact, a proper life-cycle analysis should always be part of the research activities described above.
This of course requires verified models for quantifying energy consumption, COx-output etc. during the production and lifetime of these materials. A complete life-cycle assessment should therefore be developed for the most promising materials and the processes involved in their production.
Research topics related to functional integration
Alternative materials to currently used composites may be much more environmentally friendly during the initial production process and have a better environmental footprint during their lifetime. However, bringing these materials into future aircraft may not be successful if only mechanical properties are considered, since the development of modern composite materials is highly focused on superior mechanical performance. A promising approach to let alternative materials compete with these conventional materials is the integration of additional functionality and the enhancement of inherent properties of these materials. The aim of this approach is to not only replace current materials, but to have an alternative material with integrated functionality that can replace a conventional material and additional external measures and systems, which provide the functionality in a conventional way. The functional integration may include:
• Tailored thermal and electrical conductivity (e.g. as a replacement for additional insulation)
• Energy storage integration (replacing external batteries)
• Enhanced damping properties (replacing measures for acoustic insulation and simplifying mechanical compliance)
The following research topics have been identified as the most promising ones in the context of functional integration.
Energy storage
A great advantage of composite materials is their flexible lay-up in which additional functionality can be integrated easily. Especially as soon as electrical systems replace hydraulic or pneumatic systems the demand for electrical power storage systems also grows. Due to the fact that common storage systems, such as batteries, are very heavy and rather bulky, a constructive solution would be their integration in or their combination with structural components, such as the frame or unloaded parts, such as attachments.
There are several areas of interest for the research in the fields of structural energy storage:
• Structural Integration of available systems into a composite with sufficient electrical conductivity and the ability of replacement and repair;
• Development of semi-finished storage systems with a comparable high energy density and/or power density for better structural integration;
• Improvement of composite materials for their multifunctional use as components of an energy storage device;
• The integration process and design has to be evaluated in terms of lightweight construction considerations.
• The structural energy storage system should be as fault-tolerant as possible in terms of cash-issues. Furthermore safety-issues such as toxicity, flammability and environmental compatibility have to be taken into account.
Research topics related to Structural Health Monitoring
Independent of the materials being used it is essential to assure a proper mechanical function of aerospace structures during their entire lifetime. In this context there are two aspects, which can greatly reduce the efficiency of the materials during design and operation
• The spectrum of the mechanical loads which is to be expected during given operating conditions is understood rather well. However there are many uncertainties that must be considered (e.g. manufacturing tolerances). These uncertainties lead to large safety factors and may make a structure much heavier that it actually needs to be.
• The current aircraft operation and design is based on strictly scheduled maintenance plans. Since an in-depth inspection leads a grounding of the aircraft and therefore reduces availability, inspection intervals are increasingly extended. This in turn requires a structure with an undetected damage to last until the next inspection interval, making it much heavier that actually necessary.
These issues can be addressed by introducing a structural health monitoring (SHM). This continuous monitoring is already common practice for systems like engines and electrical circuitry, but is practically completely absent for structures. The goals of introducing structural health monitoring are
• Finding damages as early as possible in order to take appropriate actions while the damage is small. This should allow for a simple repair instead of replacing large parts after undetected damage growth.
• Detecting severe impacts and their location during operation on the ground and in flight, for which composite airframe structures are much more vulnerable. This allows for early and complete detection of possible damaging impacts that may otherwise remain undetected.
• Extending the life-time of existing structures by monitoring critical areas as an economically more efficient solution than conventional inspection at short intervals.
• Reducing structural weight by increasing the capabilities of detecting damages between scheduled inspections and therefore being able to reduce unnecessarily high safety factors.
• Demand-driven maintenance instead of strictly scheduled maintenance in order to make the aircraft and fleet operation more efficient.
The following research topics have been identified as the most promising ones in the context of structural health monitoring:
SHM using Lamb waves
In contrast to other SHM methods like fiber-optic sensing or comparative vacuum measurements the use of Lamb waves, which can be thought of as integrated ultrasound inspection, allows a monitoring of the area around the sensors and not just at the sensor location. This potentially allows the coverage of large areas with a relatively small number of sensors.
Using a network of sensors distributed across the structure the propagation of guided acoustic waves (particularly Lamb waves) is evaluated. The presence of damage will lead to reflections of the waves and conversions between different wave modes. This can be detected as a change in the sensor signals.
Due to the measurement principle the time delay of the signal change with respect to the excitation of the wave is known at each sensor. Using different techniques like triangulation or reconstruction methods known from conventional ultrasonic testing the location of the damage can be determined. While this has been shown successfully for simple structures one common problem is, that the methods are often not applicable to complex structures. This is mainly due to variations in mechanical properties, which are directly linked to the way the waves propagate. In complex structures the stiffness may be different at each location and in each direction.
Therefore a major focus of future research related to SHM using lamb waves should be the improvement of suitable methods for determining damage locations in complex structures. This should not only be achieved by using a very dense sensor network and ignoring much of what is known about the structure, but by actually considering the properties of the structure and their impact on the wave propagation.
Another important aspect that is worth being focused on in future research initiatives is the generation of reference signals. As described above, signal changes are being detected and evaluated. This requires that suitable reference data is available. Since the wave propagation is influenced by many environmental aspects like temperature, humidity and structural loading, reference signals are required for all these conditions. It is not possible to determine all these signals by measurement. Therefore effective simulation methods should be developed in order to generate reference signals based on the given conditions of an aircraft.
SHM using fiber optic Bragg Gratings
Optical fibers containing a (potentially large) number of strain sensing Bragg Gratings offer a number of appealing advantages for application in aircraft structures, such as light weight, tolerance for harsh environments, long term stability, completely passive operation and no interference with other signals. The optical fibres can be embedded in the (composite) structure or surface mounted. The latter has the big advantage that a sensor can be installed at any time during manufacturing and operational life and that a broken sensor can be replaced.
Besides measuring the local strain, these sensors can also measure the temperature or even the pressure. Apart from damage detection the same or part of the sensor network can be applied for load monitoring and impact detection. This allows for a multiple purpose sensor network.
The current application of damage detection by means of FBGs lies in skin/stringer debonds and cracks in metallic joints. Another highly interesting application is the monitoring of composite repairs, which can circumvent the current problems related to the bond quality and durability of secondary bonding by monitoring the repair for disbands. This may save weight and also remove the need for so-called “chicken rivets”.
The new generation of data acquisition systems, developed within Europe, to read out the FBGs are based on optical chip designs and are very small and efficient. The sample rates are high enough for lower frequency (20 kHz) damage detection, but need to be further improved for impact detection (100 kHz) and Lamb wave detection (MHz). In the latter case FBG sensors can be combined with Lamb wave sensors described in the previous section.
SHM Technology
This rather general aspect covers the entire technological portfolio, which is required for a continuous monitoring of structures.
Many different methods and physical concepts for structural health monitoring are available. The research activities addressed here should therefore focus on answering the question of which methods are most suitable for monitoring a given structure within a defined framework, including the sensitivity to different damages and damage locations. This information is especially important for aircraft designers in order to give them an early indication on what improvement can be expected from SHM and how they can consider SHM in an early design stage.
General research aspects, which are relevant for using SHM systems and still needs to be addressed for all sensing technologies are the correlation of acquired signals to damage severity, which is important for whether and when more detailed inspection is required and the in-flight operation of SHM systems, where SHM signals have to be reliably separated from flight load influences.
Advanced sensor technology
The sensors being used for structural health monitoring can vary from simple and cheap piezo-ceramic discs to grouped and pre-packaged multi-sensor elements or autonomous sensor modules with on-board energy harvesting, data acquisition and communication.
In this context future research activities related to sensor technology should focus on the following aspects:
• How much functionality should be integrated into a single sensing element? – The aim of this research is to find an optimum compromise between a centrally organized network, where all functions are concentrated in a single device and a totally decentralized network where all functionality is concentrated in the sensing elements.
• Multi-physical sensing – Often more than one physical information is needed. This could be for example omnidirectional strain, multi-axial strain, temperature, acceleration or humidity. Instead of gathering all this information with independent systems it makes more sense to use a single sensor for multiple sensing purposes where possible or to group different sensors for reducing wiring and weight. A single multi-physical sensor may be heavier and more expensive than a conventional sensor, but could determine information that otherwise could only be obtained with multiple simple sensors. Wave modes of Lamb waves are a good example for this, because a single multi-axial (including out-of-plane) sensor might allow the separation of different wave modes, which would otherwise require at least three simple sensors. Another aspect that is worth being addressed here are sensors with a varying directivity.
• SHM method synergies – In order to monitor a complex structure more than one SHM method may be required. In order to maintain an overall small number of sensors synergies between different SHM methods should be investigated. E.g. for a combination of SHM using Lamb waves and fiber optic strain sensing the fiber optic sensors may also be used for detecting the Lamb waves.
Structural integration of sensors
A very challenging and critical aspect for adding an SHM system to a structure is the proper integration or application of the sensors and the required wiring. The SHM system may not in any way reduce the structural performance. It must be robust enough to theoretically last longer than the structure itself. This requires that the sensing elements themselves are sufficiently robust and that the entire system is properly protected against environmental influences, including mechanical loading, humidity etc.
A reliable structural integration does not just relate to how the sensors are embedded in or attached to the structure, but also how the installation process is integrated into the overall manufacturing process. The state of the art regarding sensor integration has recently been demonstrated by the DLR in the EU project SARISTU where several hundred sensors were successfully integrated into a door-surrounding structure.
For future research activities the sensor installation should be automated as much as possible and the wiring of large sensor networks should be reduced in order to save weight while maintaining a good signal quality and an efficient operation of the sensor network.
Sensor network layout
So far there are no rules and regulations for how to layout a sensor network in order to reliably monitor a structure. However, the way the sensors are distributed may greatly influence the performance of a sensor network. An optimized sensor network would be capable of completely and reliably monitoring a defined area with a small number of sensors.
The challenge is not only to use a small number of sensors, but also to avoid blind spots in the network and assure that all areas of a structure are covered by a defined number of sensors. This also requires taking account of manufacturing limitations and structural restrictions for sensor installation.
The positioning of virtual sensors in a numeric representation of the structure can help to predict sensor signals based on measured wave propagation (e.g. high-resolution laser vibrometry) or simulation data taking into account specific sensor properties. Using such a virtual representation of a sensor network, the layout can be optimized to meet a required structural coverage and probability of damage detection. The proposed research activities in this context include the simulation of Lamb wave propagation in complex structures and optimization algorithms for sensor networks.
Application of SHM technology in aviation
Currently, certification of SHM technology for existing (aging) aircraft is challenging. Moreover, the current new aircraft design process does not account for benefits that SHM techniques can have. One reason for this is because, unlike a non-destructive inspection (NDI), SHM has difficulty to prove its performance in terms of probability of detection. The implementation of the SHM technologies to the existing and new aircrafts strongly depends on their performance (e.g. probability of detection) under various damage location and size in a cost effective way.
Determination of the POD (reliability) for an SHM sensor system is very important for successful application in aircraft structures, but is much more demanding than for traditional NDI methods where this can be deduced from various test details representative of the structure to be inspected, containing damages of various sizes sufficiently mimicking operational conditions. These have to be inspected by several inspectors yielding the final data set. In case of a SHM system the same principle can be applied. However, the human factor is no longer present. Missed cracks are only due to the capability of the system in finding a damage of a certain size at a certain location. A damage located away from a sensor will in general be harder to detect than a nearby damage. An SHM system consists of a network of sensors and signal processing capability designed for a specific structure. To determine the detection capability experimentally, a representative part of the structure now has to be manufactured instead of a representative detail in case of NDI system. Experimental validation of the detection capability of a SHM system is therefore very expensive. On the other hand in the absence of the uncertain human factor, which is hard to model, the detectability can to a large extent be computed. Such a probabilistic model assisted approach, in which the damage detection is simulated together with the uncertainties (e.g. signal noise), can alleviate the costs significantly and only requires a limited amount of experimental data for validation of the numerical analyses.
Condition-based maintenance
The life cycle of a component or system is comprised globally of the following phases:
1. design, production and installation
2. implementation and operation usage
3. degradation as a result of usage
4. recovery, repair and maintenance
The original equipment manufacturer sets the maintenance schedule based on estimates of intended usage and assumptions about future process parameters. To guarantee the safe operation, a conservative approach is applied and heavy usage and severe degradation is assumed in the design phase. As a result, in general, the actual usage is lighter than assumed and the design life may be considered conservative. On the other hand, new (unknown) usage scenarios and degradation processes may not have been taken into account, which may lead to non-conservatism in the design.
The existing aircraft sensors and the sensors of the SHM system provide information on the health of the aircraft structure. Condition Based Maintenance requires knowledge about the momentary and future condition of a component or system. This knowledge then serves as the input to optimize maintenance and asset management procedures, such as the adjustment of maintenance schedules, maintenance techniques, logistical processes, design, inventory management or operational usage. The economic consequences of these efforts are taken into consideration to determine the return of investment of actions taken. Another important aspect for CBM is the (local, domain specific) regulations and certification- and safety requirements.
KGT4: CNS/ATM for greener air transport
1. Greener Flight Trajectory Operations
2. Collaborative Network Management
Greener Flight Trajectory Operations
“Moving from Airspace to Trajectory Based Operations” entails the systematic and continuous sharing of aircraft 4 D Trajectories between various participants in the ATM process all through the flight phases from flight planning through flight execution to ensure that all partners have a common and continuously updated view of a flight and have access to the most up to date data available to perform their respective tasks. It enables the dynamic adjustment of ATM assets like airborne/ground capabilities as well as airspace characteristics to balance predicted demand with unpredicted distortions to the business/mission trajectories kept to a minimum. Whenever possible, the necessary strategic and tactical interventions are considered all through the flight phases seeing the trajectory from an en-route to en-route in order to capture anything from gate to gate trajectory level as well as at the partners capability levels, taking into consideration the wider impact on the air and ground part of the trajectories concerned as well as for the airports and for the overall Network performance.
This is based on the operational and technology scope definition of the trajectory based operations concept and trajectory management framework, its content, performance and access across all flight phases and associated concept and technology developments. The SESAR Trajectory Management Framework (TMF) based on the Trajectory based operations concept specifies the structure needed to achieve the safe and efficient creation, amendment and distribution of the Reference Business/Mission Trajectory (RBT/RMT) including the RBT/RMT information content & quality, the Actors involved, and the Services associated with trajectory information (e.g. creation, proposed revision and update processes).
Justification / Need for collaboration
The ICAO TBO and SWIM ConOps are reaching maturity as well as relevant SESAR solutions. In addition related SESAR 2020 Very Large scale Demonstrations are planned for execution in the H2020 timeframe. However, it is foreseen that intercontinental inbound flights will not be capable of supporting TBO and therefore significantly impact the performance of the Single European Sky network. In addition, the air traffic management system of the intercontinental destination may not be able to fully support the European TBO based flight requirements. Therewith it will impact the operational performance of that flight. This will thus affect the ability of the airline to meet the Single Sky environmental and cost related goals.
The SESAR 2020 programme does not support projects with both European and Chinese partners. It confirms however the relevance and benefits of having take place such intercontinental flight studies and validations projects, since they will provide essential insights into the impact of such intercontinental flights to the Single European Sky performance goals. In addition it will also bring necessary insights in requirement for the intercontinental partners to support TBO flights in their region.
Expected impact
Submitted projects are to study and evaluate in the context of TBO, especially for the increasing number of Europe-China flights, as many of the underneath topics as possible.
• The needed Operational and Technical Capability levels for the Air Traffic Management system in intercontinental region with the view to a full harmonisation and seamless interoperability throughout the network in terms of operational procedures and technology standards.
• Identify and validate (bring down to numbers) the impact on emissions, noise, fuel burn of TBO based ATM flights by none-TBO ATM systems (both on the ground as airborne).
• Identify key enabling technologies (MCMF GNSS, LDACS, SatCom ...) that need synchronized deployment and synergetic collaboration, and the related requirements on compatibility, capacity, interoperability, and standards development. This can potentially be done in close collaboration with the SESAR Very Large scale Demonstration projects.
• Identify additional areas of optimisation of the available ATM capabilities and capacity, and validate their operational and environmental business and performance benefits (both for ground based and airborne systems).
• Identify areas of improvement in SWIM ground-ground and air-ground information, weather and flight plan data sharing (both for ground based and airborne systems).
Abbreviations:
AMAN Arrival Manager
ATM Air Traffic Management
GNSS Global Navigation Satellite System
LDACS L-band Digital Aeronautical Communication System
MCMF Multi-Constellation Multi-Frequency
TBO Trajectory Based Operations
RBT / RMT Reference Business/Mission Trajectory
SESAR Single European Sky ATM Research
SWIM System Wide Information Management
TMF Trajectory Management Framework
Collaborative Network Management
The transition towards Trajectory Based Operations requires the sharing of information both during the process of planning and execution of all combined flight operations. This requires all involved parties and stakeholders, including aircraft operator, ground handler, airports, ANSPs and network management to have continuous access to the latest information in relation to the execution and decision making around flights.
Improved network-wide flight planning systems are needed to support the planning and execution of Business Trajectory (BT)/Mission Trajectories (MT). This allows the involved stakeholder to act better based on shared relevant data, during normal operations as well as under non-nominal conditions such as adverse weather or contingencies.
In addition, for a more seamless and efficient execution of the 4D executed flight trajectory with optimum climb and descent profiles, air traffic management needs to be able to plan and manage the flow of traffic with high accuracy and no delays. To achieve the required more strategic arrival management of inbound flights, increased planning horizons of Arrival Management Systems (AMAN) are needed, which use accurate flight trajectory predictions.
Based on the predicted commercial traffic demand and planned operational air traffic known in the network, better coordination should be achieved between civil and military users for flexible allocation and use of airspace.
Justification / Need for collaboration
Whereas SESAR focuses on optimising overall network operation in Europe, the challenge also exists to extend this concept to intercontinental operations. In particular for flight operations with China, there is already a need identified for more flexible airspace access, improved air traffic flow management and collaborative decision. There is a clear need to align Chinese networked operations developments with those in the EU and US.
The SESAR2020 programme presently does not plan any collaboration projects between partners from Europe and China to demonstrate and evaluate collaborative network management in support of trajectory based intercontinental operations. Both in China and Europe, the introduction of advanced communication, navigation and surveillance technology, globally connected aircraft operators and ground based partners will allow improved sharing of flight data and aircraft status information between aircraft and ground infrastructure, including inflight updates of the most optimum trajectory.
On the ground, the operation at the busy airports in EU and CN will benefit from closer networked information sharing between airlines, airport, ATC and other service providers. The information sharing optimises airport movements, decision making and turnaround process, and also for long range intercontinental operations.
In essence, collaborative network management will provide the necessary enabling technology towards optimised long range trajectory based operations between CN and EU.
Expected impact
Joint studies and research projects on collaborative network management between Europe and China will provide benefits on both sides.
• Coordination between China and Europe in the development of the collaborative network management processes will ensure alignment of regional concepts;
• Increased flight efficiency through user preferred routing and trajectory optimisation will be a priority, since reducing fuel consumption will provide benefits in emissions and air quality;
• More dense air traffic and complex arrival flows can be handled more efficiently by increased time horizons in the arrival management processes for 4D trajectory operations;
• Inflight updates of the most optimum flight trajectory, taking into account latest weather forecasts and earlier planning of the arrival sequence;
• Airspace access may be obtained more efficiently by flexible allocation based on civil and military needs shared over the network;
• Airport processes and aircraft turnaround will be more efficient by a better collaborative decision making process, enabled by networked sharing of latest data;
• Sudden disturbances in the network in/between China and Europe can be handled more efficiently by collaborative decision making based on shared data.
Chinese and European airlines will benefit from the above items both on intercontinental and regional flights. In particular the evaluation of possibilities for collaborative network management for the benefit of intercontinental flights between the regional air traffic networks in China and Europe will provide valuable feedback in the development of the mutual ATM networks, systems and services.
Potential Impact:
The partners in the GRAIN2 project were selected to ensure transnationality and the maximum complementarily of skills towards the successful achievement of the project objectives. More specifically, the consortium groups partners from nine different EU countries Spain, United Kingdom, Germany, France, Belgium, Sweden, The Netherlands, Italy, Czech Republic and China. All partners were firmly committed to carry out the different project activities.
The scope and dimension in the GRAIN2 project is such that none of the partners could have undertaken a similar project only by themselves. Indeed, by participating in a consortium of this kind the industrial partners have benefitted from the expertise and resources from the different partner groups. Thus, the aeronautic industries has benefitted of the scientific knowledge and skills of the RTD organizations and universities. The RTD partners, in turn, have taken profit from the experience and practical know-how of the aeronautic industries in Europe and China. By participating in the GRAIN2 project the project partners have expanded their field of RTD activities in Europe and China, therefore helping to the advance of science in both regions by means of collecting and disseminating state of the art scientific and technical information in the field of multi-physics simulation and validation.
The following outputs have been generated in the GRAIN2 project.
Strategic outputs
The general strategic objectives of the project are three fold: 1) to identify areas of mutual RTD interest; 2) to develop concepts of collaboration in those Key Green Technological areas between the European and Chinese partners; 3) prepare specific RTD activities that are mature for joint proposal for FP7 and for HORIZON 2020.
These three aspects have been successfully achieved along the project lifespan, as demonstrated in the final deliverables, progress reports and mainly on the veents, where the technical discussions have expanded the objectives to more precise technical discussions and shares.
Scientific and technical outputs.
a) An identification of emerging RTD research topics coming from the different KGTs.
b) A state of the art document for each of the Key Green Technological area.
c) A web-based GRAIN Communication System for storage and dissemination of the collected data relevant to computational methods and experimental tests for multidisciplinary applications in aeronautics.
d) Definition of future joint RTD work on critical Key Green Technological areas.
Above outputs have been disseminated and exploited both internally and externally by the project partners.
External exploitation addresses mainly two target groups: the aeronautics industry and the scientific community. The dissemination actions includes the planned kick-off workshop and final workshops in Europe, the Course in China as well as presentations of the GRAIN2 outputs at specialized industrial meetings (such as the Aeronautic Days organized by the EC).
China-Europe Workshop: this event has gathered during three days (3) most of the Chinese aeronautical community on Integrated Computational and Experimental Multi-physics for new Challenges in Aeronautics. Audience gathered scientists, technologists and decision maker managers from Europe and P.R. of China involved in aeronautics. The format of the conference including invited lectures and a general Round Table has provided at a glance a first information on the areas of possible win-win cooperation and with a clear picture of the level of expertise of contributors in the themes of the Workshop very similar to GRAIN2 ones.
Course: this event has deployed State of the Art lectures delivered by international experts including China on topics of major interest to GRAIN2 for future collaboration on multi-physics applications in the next Framework Programme HORIZON 2020. The time schedule of the Short Course allowed detailed presentations of lecturers intertwined with long break discussions between them and the audience. A two hours final Round table with academy, industrial and governmental institutions discussed the impact of Key Green Technological applications on research and industry environments connecting cultures and technologies.
Forum: The greener Horizons Forum has featured keynote speakers from academy, industry and government who shared their perspectives on the new global challenges of eco-efficiency, impact of civil and military aircraft emissions, debates on future greener opportunities to develop new environmentally friendly biofuels, and emerging modelling and large scale computational greener trends targeting a low level of emission, drag, noise and carbon neutral materials research, and programs. The Open Greener Horizon Forum also featured panel discussions in which leaders from industry, government, and academia addressed current issues and trends in R&D aerospace technology to establish an efficiency standard for the design of new greener aircraft.
The individual exploitation plans of the different partner groups participating in the project are as follows: Research centers and universities in Europe and China has used the experience shared in the GRAIN2 project for development and validation of new mathematical and numerical methods concepts and software applicable to the solution of multidisciplinary problems in aeronautics as well as for new experimental methods and tests for validation purposes. The industrial companies, in the other hand, have also used the outputs of the GRAIN2 project as an essential ingredient for defining validation of multidisciplinary projects as well as for every day design of aircrafts. For all partners, the mutual knowledge of the RTD activities in Europe and China acquired during the GRAIN2 CSA is of interest for defining future joint RTD projects and actions with participation of European and Chinese organizations.
Turnover in the aeronautics sector exceeded €85 billions in 2004 and its balance of trade surplus –the difference between the products it sells to the world and the equivalent that are imported– was €28 billions. Increasing these figures in a form compatible with the new imperative of “More Affordable, Safer, Cleaner and Quieter” aircrafts can only be achieved by combining increasing RTD investment in undertanding the complexity of all multi-physics aspects involved in the design, manufacturing and operation of an aircraft.
The enhanced design of aircrafts structures and components accounting for the multidisciplinary couplings will invariably lead to better, safer and more competitive European aircrafts. A reduction in the design and production cycle leading to a reduced time-to-market is also expected. This naturally leads to the improved competitiveness of European aerospace industry currently under severe concurrence with respect to USA aeronautics technology, and indeed the increased competitiveness leads to an overall economic growth.
The outputs from the GRAIN2 CSA contributes to achieve above objectives in a number of ways. The different discussions in the GRAIN2 events are an indispensable source of information for both current design engineering and as reference for future RTD work towards the development of multidisciplinary design tools in aeronautics and aerospace industry. The data quality assessment methodology and the guidelines produced in the project have a similar value as a reference for improved design and also for establishing standard and rules for the efficient use of computational methods in multidisciplinary applications.
List of Websites:
www.cimne.com/grain2
Contact Details:
Prof. Gabriel Bugeda, bugeda@cimne.upc.edu Senior Researcher at CIMNE and full professor at UPC (Universitat Politècnica de Catalunya –BarcelonaTech). Phone: +34 93 401 64 94, Address; c/ Jordi Girona s/n, Campus Nord UPC, Edifici C1, 08034 Barcelona
Prof. Jacques Périaux, jperiaux@gmail.com Senior Researcher and visiting professor at CIMNE. Phone: +34 93 401 74 95, Address; c/ Jordi Girona s/n, Campus Nord UPC, Edifici C1, 08034 Barcelona
Dr. Jordi Pons-Prats, jpons@cimne.upc.edu Researcher and Aeronautics group leader at CIMNE. Phone +34 93 413 41 89, Address: c/ Esteve Terrades 5, Campus UPC PMT, Edifici C3, 08860 Castelldefels, Barcelona
