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Highly-accurate/reliable WT test demonstration of low-noise innovative MLG configuration

Final Report Summary - ARTIC (Highly-accurate/reliable WT test demonstration of low-noise innovative MLG configuration)

Executive Summary:
The ARTIC project has been developed in response to the requirements of the European Clean Sky Joint Technology Initiative to assess low noise technologies applied to main landing gear architectures. ARTIC has put together a consortium led by Trinity College Dublin, a well-known European research centre NLR and SME partner INASCO. This group has well demonstrated competencies in wind tunnel model design and manufacturing, experimental aeroacoustics, noise measurements and data analysis.

The objective of the activity under this CfP is the test of a full-scale main landing gear (MLG), fuselage mounted and sized for a high-wing regional aircraft configuration. A novel low noise technology will be utilised to achieve a noise reduction over the base line configuration. Extensive aero-acoustic tests were planned in one of Europe’s leading wind tunnels, DNW’s Large Low-speed Facility. This is the only aero-acoustic wind tunnel in Europe with a test section of sufficient size to complete this investigation.

The key characteristics associated with the landing gear noise problem are:
• Contributes to approximately 30% of the overall noise emission of the aircraft during take-off and approach phases
• The noise signature is broadband in nature covering frequencies from approximately 90Hz to 4KHz
• The annoyance level associated with noise within this frequency range is high for exposed communities
• Landing gear consists of numerous structures, surfaces and components which are generally not optimised from an aero-acoustic point of view
• Turbulence from non-aerodynamic components of the landing gear is a direct noise source
• The wake of the landing gear structures can interact with other airframe components and generate an indirect noise source

In the past full-scale models of landing gears have rarely been tested due to the large test facilities required. Most experimental airframe noise research has been performed using small-scale models. This leads to great difficulty when using model-scale results for full-scale noise predictions due to the lack of details in the geometrical modelling. One of the significant planned contributions of ARTIC was that a full representation of the landing gear detail and associated structures (e.g. bay cavity, bay doors, belly fuselage etc.) was to be included and addressed at full scale. The output designs of the low noise down selection made at the GRA level have been received by ARTIC and implemented on the designed wind tunnel model.

The ARTIC project successfully completed its design phase. A number of numerical codes were also developed in preparation for the wind tunnel test analysis. However, despite a significant extension, the SME partner INASCO was unable to complete the manufacture of the model. ARTIC therefore ended without a wind tunnel test campaign.

Project Context and Objectives:
This report details the full status of the project on completion of the final reporting period. An overview of the work packages and activity within each work package is given below. The following sections will provide detailed progress and activity in each work package.

WP1 – Main Landing Gear Design
This work package received detailed CAD of the main landing gear design as in input from GRA. Design work was undertaken to develop a full scale model featuring details of gear struts, wheels, cargo bay and doors, fairings and a portion of fuselage. Additionally the noise reduction technology to be tested was also specified by the GRA ITD member. During the design process PDR and CDR meetings were held, with input from the GRA ITD member, for the acceptance of the model designs.

The model design is modular with the ability to include the selected noise reduction technology and allow for testing of the sealed fuselage, single landing gear leg and dual leg configurations. The model includes rotation about the mid plane to allow for testing of the “acoustic mirror” concept using the wind tunnel floor. These features will minimise change of configuration time and therefore maximise use of the wind tunnel test campaign. The model designs were used as input to perform structural static and dynamic and aero-elastic analyses by means of FE methods. INASCO completed a considerable FE design loop task to reduce the model weight to the target maximum of 6 tonnes specified by the WT. This task required input information on the flow loading for which TCD conducted CFD simulations of the model at various yaw angles to provide estimates. The eventual design required modification of the wind tunnel interface to include a central support pylon and two additional pylons at the front and rear end caps of the model. The wind tunnel subcontractor completed design work on the support pylons and a floating floor in the wind tunnel at the height of the model.

WP2 – Main Landing Gear Manufacture
In this work package the designs finalised in WP1 were to be used to manufacture a full scale modular main landing gear model, the associated noise reduction technology and enable the acoustic mirror test concept. The GRA member and wind tunnel subcontractor provided manufacturing tolerances and target model weight as inputs into the work package.

The deliverables in this work package were not achieved. The modular parts comprising the test article, along with all necessary fittings and accessories, were partially manufactured according to the approved WP1 drawings. In order to speed up the manufacturing, a parallel production sequence of parts and additional subcontractors were considered.

WP3 – Wind tunnel test campaign
The wind tunnel test campaign was planned for the DNW facility at Marknesse in the Netherlands. The LLF wind tunnel is a world leading industrial wind tunnel for low speed testing. Full use will be made of existing in house microphone arrays and associated DAQ systems. This wind tunnel has been used in the past for full scale aero-acoustic testing of aircraft landing gear. This work package developed the test matrix to maximise use of the test slot. There was also considerable preparatory work in terms of designing the wind tunnel microphone arrays for the flyover and beamforming measures. Array design and optimisation studies were performed by DNW and NLR in order to maximise the usefulness of the microphone array measurements.

WP4 – Aero-acoustic measurement analysis
In this work package the data from the test campaign was to be analysed and the characteristics of the noise emission established. As well as industry standard measures such as 1/3 octave band spectra, OASPL or EPNL this work package will also include the application of advanced beam forming and source separation techniques to gain an insight into the mechanisms of the noise reduction achieved by the tested technology.

Preparatory work, both experimental and numerical, was conducted by TCD for the ARTIC wind tunnel tests as part of WP4. This work focused on the bay cavity noise emission and the performance of the mesh screen low noise technology. Without numerical modelling these features of the experimental results would be difficult to interpret from far field sensors alone.

Cavity noise, due to the design of the bay of the landing gear, is one of the dominant noise sources of landing gears. Cavities such as landing gear ones are susceptible to resonance based on feedback between the internal cavity pressure and the shear layer over the cavity opening. TCD used analytic and numerical codes based on the Wave Expansion Method to investigate the cavity modes. During the ALLEGRA CFP two experimental wind tunnel test campaigns were conducted. The first included a full scale nose landing gear (NLG) and associated bay, the second a half scale main landing gear (MLG) and associated bay. The fact that the models included the bay cavity within the fuselage section, allows for an investigation of the contribution of the bay cavity modes to the overall noise emission of the models. Only the NLG test campaign included local sensors inside the bay cavity. These local sensors were used to validate the WEM numerical simulations. This provided added benefit from the historic ALLEGRA dataset within the ARTIC project.

The results demonstrate that the developed WEM code offers good potential to investigate the cavity modes excited in landing gear bays. This is vital in properly understanding and interpreting experimental data of complex noise sources such as landing gears where multiple possible tonal sources exist in the same frequency ranges for example the bay, the wheels and the main strut.

The fairing materials selected for use in ARTIC were chosen as a result of successful experimental testing in the ALLEGRA project at half scale. The ALLEGRA project did not investigate an optimised perforate but did investigate multiple materials in the half scale wind tunnel testing. Therefore ARTIC aimed to increase the understanding of these materials in order to better interpret the performance of the low noise technology.

Various methods and techniques exist for the computation of flow through porous surfaces. These methods exist in three broad categories, namely; The Direct Numerical Method (DNS); The Permeable Boundary Condition Method, and; The Macroscopic Flow model, which uses a Volume-Averaged Equation within the porous media zone. Out of these three methods, the macroscopic flow model Volume-Averaged Method presents the least computationally expensive approach, and thus becomes preferable for less expensive analysis. Therefore, the approach adopted in the preparation for the ARTIC wind tunnel tests was based on the Macroscopic Volume Averaged Flow method. This code was developed in order to make predictions of the ARTIC mesh fairing technology. This required an experimental assessment of the ARTIC mesh in order to identify its properties.

Experiments were therefore conducted on a variety of mesh samples in a closed test section wind tunnel facility at TCD in order to generate model fit curves of flow loss resistance K as a function of porosity for wire screens, and in so doing, compare the experimentally acquired flow loss properties to the empirically derived values, where the effectiveness and suitability of each are discussed in D4.1. Successful CAA predictions of mesh fairings were achieved and also reported in this deliverable.

The developed beam forming methodologies of TCD and NLR were to be applied to the ARTIC test data. Investigations were made using the historic ALLEGRA dataset for insight into the likely challenges of the ARTIC test campaign.

WP5 – Coordination, Management and JTI Integration
The project has been managed within this work package to ensure the technical objectives are delivered and to ensure on-going communication among the partners. The first task of this WP was to establish a data transfer protocol to ensure compatibility between all the partners in relation to the exchange of data and results. These transfer procedures were used across the other WPs. A project website was established to enable easy transfer of data and information between partners. A master project schedule document was used to track all progress against project tasks and deliverables. Dissemination aspects of the work and regular liaison with the JTI-GRA were also an integral part of this work package.

The main results of the project have been published in the scientific journals and presented at conferences. The consortium has an excellent track record for publication in the premium journals relating to aero-acoustics, aerodynamics and manufacturing which was further enhanced by the ARTIC project.

Project Results:
The advancement of European society is dependent on safe, efficient, and environmentally-friendly technologies. It is an on-going challenge for European industry to meet customer and legislative requirements, satisfy societal demands and sustain competitiveness in the global arena. In growing industrialised regions of the world a special focus has to be placed on less resource-intensive and knowledge-based, enabling technologies. The need for new technologies to meet not only customer demands from the aerospace section but also the important global needs of a reduction in CO2, NOx and noise emission is a driving factor for economic growth.

Development of novel aircraft concepts requires a complex compromise between contradictory requirements in safety, exhaust emissions, noise, performance and price. Exterior noise of aircraft will, most likely, be subject to further regulation in the future and therefore require additional technological advances for airframe, wing and engine design. To overcome the challenges of providing ultra-light, energy-efficient aircraft with acceptable exterior and interior noise levels, concepts based on smart materials and structures are currently being investigated in the European JTI Clean Sky and Clean Sky 2.

The ARTIC project has contributed to this process through the delivery of new design tools capable of meeting the needs of the European aviation industry. In addition to contributing to European objectives the ARTIC project has positively impacted graduate research programmes at the university partner. This partner is already a leading provider of the training and experience necessary for European aerospace graduates.

While the manufacturing work package of ARTIC failed to produce a model, and therefore no wind tunnel test could be conducted, there were still significant scientific outputs from the project.

In order to achieve a more complete understanding of the ARTIC landing gear noise emission and performance of the low noise technologies TCD began development of in-house codes which focused in the bay cavity noise and the performance of the mesh fairing. These two elements of the noise emission are perhaps the most challenging to interpret from the far field measurements of the DNW wind tunnel.

Cavity noise, due to the design of the bay of the landing gear, is one of the dominant noise sources of landing gears. Cavities such as landing gear ones are susceptible to resonance based on feedback between the internal cavity pressure and the shear layer over the cavity opening.

The Wave Expansion Method (WEM) has received much attention due to the fact that it is a highly efficient numerical procedure. Many applications have been examined, its efficiency being of particular interest to large-scale sound propagation problems . TCD has in the past made extensive use of this numerical methodology for the investigation of multi-modal propagation of acoustic waves in ducts. This is a highly efficient full-domain discretisation method, which requires as few as two-to-three mesh points per wavelength. An inhomogeneous potential flow may be easily included in the method. A modal decomposition technique is used to provide detailed information about the modal content of the sound field. The methodology is highly applicable to the calculation of bay cavity modes in the MLG bay.

In order to better understand the propagation of the noise waves inside the bay and the importance of the bay cavity tones on the overall far field noise, an equivalent domain to the landing gear cavity was meshed and tested using a series of numerical simulations. In order to simulate an oscillation in the shear layer, the numerical monopole volume source was located in the centre of the bay cavity opening. The complex pressure was solved in the domain as a function of frequency and the amplitude was plotted over the mesh in order to obtain the pressure field in the cavity.

The numerical results were found to match quite well the experimental results of historic datasets of landing gear noise available to TCD. The numerical study performed on the bay matched the theoretical results and the experimental results, highlighting the contribution of the bay noise to the total emissions. The results demonstrated that the developed WEM code offers good potential to investigate the cavity modes excited in landing gear bays. This is vital in properly understanding and interpreting experimental data of complex noise sources such as landing gears where multiple possible tonal sources exist in the same frequency ranges for example the bay, the wheels and the main strut.

Due to the fact that turbulence intensity and velocity distribution behind a perforated material are dependent on the geometry of the perforate, it may be possible to design and optimise a perforated material for optimum acoustic performance. The ARTIC low noise technology includes two different types of perforated material namely a perforate plate and a mesh. The fairing materials selected for use in ARTIC were chosen as a result of successful experimental testing in the ALLEGRA project at half scale. The ALLEGRA project did not investigate an optimised perforate but did investigate multiple materials in the half scale wind tunnel testing. Therefore ARTIC aimed to increase the understanding of these materials in order to better interpret the performance of the low noise technology.

In order to better understand the mesh technology selected for ARTIC a numerical investigation of the technology was undertaken. This was a challenging prospect since the resolution of the physical struts of the mesh is at the limit was what is currently possible with CFD codes. Numerical simulations of flow through 2D and 3D mesh screens are carried out in addition to implementing a numerical model where such wire screens are imposed as porous zones and modelled within the flow domain by using a Volume Averaged Method within the CFD solver, which introduces sinks into the Navier-Stokes Equations.

Attempts to numerically simulate or resolve the very fine details of larger and more intricate applications of wire screens towards landing gear noise reduction effects has been a rather difficult and highly computational demanding task for CFD/CAA experts and engineers till date, due to the very fine grid resolution implication and computational costs required to actually resolve or simulate the physical wire screen as a screen shield within a CFD flow domain for model or full scale landing gear geometries.


Therefore, within this research framework, focus is also directed on implementing an alternative approach, where numerically modelling the effect of wire screens is proposed and adopted by introducing a virtual porous zone within the CFD flow domain. This porous zone can be selected so as to possess similar geometric characteristics, such as length and thickness of a physical wire screen.

To account for these characteristics, the pressure drop characteristic is introduced by using a Volume Averaged Method within the CFD solver, which introduces sinks into the Naiver-Stokes Equations. Empirical flow loss properties of wire screens are utilized within the sink, and experiments are further conducted in a closed test section wind tunnel facility in a bid to generate model fit curves of flow loss resistance as a function of porosity for wire screens, and to compare its flow loss properties to empirically derived values, where the effectiveness and suitability of each approach are discussed. For the empirical flow loss properties, Idelchik's relation is used to model the flow loss resistance within the porous wire screen zone. Velocity modification and turbulence alteration characteristics will be introduced by further injecting turbulence characteristics acquired both experimentally and numerically, while self-noise characteristics is subsequently introduced.

The numerical methodology developed here has been shown to accurately predict the performance of mesh fairings over a wide range of porosities. The experimental testing at TCD demonstrated good agreement between the meshes tested and Idelchik's empirical model for the flow loss coefficient.

The use of this flow loss coefficient when modelling an equivalent porous zone within a numerical flow domain significantly reduces the computation cost of the CFD simulations. This approach eliminates the need to simulate the small scale features of the mesh itself. The developed methodology is highly suitable for comparison with the ARTIC wind tunnel test data and has the potential to shed further light on the physics behind the mesh fairing technology.

Potential Impact:
The main results of the project have been published in the scientific journals and presented at conferences. The consortium has an excellent track record for publication in the premium journals relating to aero-acoustics, aerodynamics and manufacturing. The consortium has contributed to the GRA annual review for each year of the project.

The work within this project has generated research publications within the fields of experimental aero-acoustics, noise source identification, aerodynamics and noise transmission. This has produced a number of international conference level publications and the AIAA/CEAS conferences on aero-acoustics, InterNoise, Greener Aviation and the International Congress on Sound and Vibration have been used as events for dissemination during the project.

The work has produced journal level publications suitable for the highest level research publications. TCD has utilised its role as a national contact point within the X-Noise thematic network to promote and disseminate the outputs of this research project to the European aero-acoustics community. TCD has organised and hosted previous CEAS workshops on the aero-acoustics of jets and actively contributes to the yearly workshops. TCD will also host the 2017 X-Noise workshop following the close of the project.

The outputs of this project have integrated with world leading graduate level education at the university partner TCD. This has enabled graduate level researchers to develop skills targeted at the needs of the European aerospace community. In addition, the project has served to sustain the momentum of a number of significant national and international programmes which have been focused on the study of the aircraft noise problem. These programmes have led to the further development of specific expertise in advanced experimental techniques, noise source identification and sound propagation which can further contribute to the advancement of European aerospace technologies. In particular the interaction of researchers has enabled the development of new tools for source separation, identification and modelling for the aerospace industry.

Over the course of the project the research group at TCD has focused considerable effort on the landing gear noise problem. The 3 journal publications and 14 international conference papers have been produced by the TCD research group either as a direct result of ARTIC or through parallel supporting activities.

List of Websites:
Project Coordinator:
Dr Gareth J. Bennett BA, BAI, MSc, PhD, Senior Member AIAA, CEng MIEI
Associate Professor
Parsons Building
Department of Mechanical and Manufacturing Engineering
School of Engineering
Faculty of Engineering, Mathematics and Science
Trinity College Dublin, the University of Dublin
Dublin 2
D02 PN40
Ireland
T +353(0)1896-1383/3878
F +353(0)16795554
gareth.bennett@tcd.ie
http://people.tcd.ie/bennettg