Final Report Summary - RECEPT (RECEPTivity and amplitude-based transition prediction)
Executive Summary:
The RECEPT project has delivered upstream aerodynamics research that will increase the accuracy of performance prediction for aircraft with laminar wings, allowing design of advanced and innovative aircraft. The achievements of the project have improved our understanding of transition in three-dimensional flows and have delivered new tools for laminar-turbulent transition prediction. The performed experimental and numerical investigations have enhanced our knowledge about the process of disturbance generation in three-dimensional boundary-layer flows caused by free-stream turbulence and surface roughness. The latter gives valuable information regarding the required surface quality for aircraft with laminar flow technology.
Dissemination of novel and high quality research work performed in the project has been insured through presentations at the international conferences and publication in peer-reviewed journals. Participation of industry will guarantee transfer of the new knowledge and improved method to real applications, e.g. those performed within the Joint Technology Initiative – Clean Sky.
The RECEPT consortium consisted of 12 organisations from 4 different member states (Sweden, Italy, France Germany) and one of International Cooperation Partner Countries, Russia. It contains 3 aircraft manufacturers (Airbus, SAAB, Piaggio), 5 research organisations (CIRA, DLR, FOI, ITAM, ONERA) and 4 universities (Kungliga Teknika Högskolan, Università di Genova, Università di Salerno, Universität Stuttgart).
The high quality research work performed in the RECEPT project will contribute to the need to improve the environmental impact of aircraft with regards to emissions, and to the drive to strengthen the competitiveness of European manufacturing industry.
Project Context and Objectives:
The major objective of the RECEPT project is the development of the capability to predict the in-flight performance of a future laminar flow aircraft through development of more accurate transition prediction tools.
The current procedure of development of new aircraft includes a number of wind tunnel tests (which are usually costly) in order to verify the performance predicted by CFD tools. There is at least one major difficulty with wind tunnel tests for an aircraft designed for Natural Laminar Flow (NLF). The transition of the flow from a laminar state to a turbulent one is highly affected by the level of perturbations in the incoming flow and surface quality. Today these parameters are not included in transition prediction tools. Major goal of RECEPT project is to improve the transition-prediction tools to account for these parameters.
To achieve these goals the RECEPT project will address the following issues
• Receptivity to small and large roughness elements as well as free-stream vortical disturbances;
• Three-dimensional stability analyses where the effects of flow three-dimensionality are strong (wing root area).
• The development of an amplitude based transition prediction method.
The RECEPT project aims at developing the next generation transition-prediction methods by adopting an amplitude-based method. The present work aims at removing the need for empirical relations and therefore making accurate prediction of the onset of transition both under wind tunnel and free-flight conditions possible.
The proposed method will consist of the following steps:
1. Estimation of the disturbance level and spectrum in the wind-tunnel and in free-flight,
2. Receptivity calculations giving the initial amplitudes of the most unstable disturbances,
3. Non-linear stability calculations to estimate the transition location based on the finite amplitude of the disturbances.
Incorporating substantially more physics into the transition prediction methods can be expected to generate a leap in performance and reliability of the methods. The last two proposed steps require new research, outlined in the present proposal.
The project is divided into 5 work packages. The first four deal with scientific aspects of the project and the last one with the administrative issues:
• WP1: This work package is devoted to experimental investigations of receptivity of three-dimensional boundary-layer flows to (i) external vortical perturbations, (ii) small and (iii) medium-size roughness elements. Two models will be manufactured and equipped. Experiments will be performed in a quiet wind tunnel (MTL at KTH) with controlled external perturbations and surface roughness quality.
• WP2: In this WP numerical approaches will be used to replicate the physics approached experimentally in WP1. Methods and numerical tools with different range of complexity and approximation level will be used to attack the problem. Direct numerical simulation will also be employed to provide specific detailed information.
• WP3: In this work package the development of an amplitude-based transition prediction method is investigated. The idea is, based on the results of WP1 and WP2, to take into account the initial amplitude of disturbances obtained from receptivity computations, in effect going from an amplification-based transition prediction criterion to an amplitude-based one.
• WP4: The objectives of this work package are twofold: (i) To apply the new receptivity and transition prediction tools, obtained in WP1-WP3, to geometries and flow parameters for realistic cases. (ii) To estimate the effects of flow three-dimensionality compared to infinite swept-wing approximation. In order to evaluate these effects, two different approaches to account for fully three-dimensional flows will be used.
• WP5: This work package covers the issues related to administration of the project, e.g. cost and time controlling, management of meetings and reports, communication with the European Commission and with the WP and Task leaders.
Project Results:
A series of highly detailed and accurate experiments have been performed with aim to create a database for development and validation of receptivity tools. To perform these experimental works:
• Based on the characteristics of the MTL wind tunnel and performed stability analysis, an airfoil has been chosen and modified. Based on this airfoil two different swept-wing models with high surface quality have been manufactured.
• To insure spanwise flow-homogeneity, contours of wind tunnel sidewalls for experiments at positive and negative angles of attack have been designed and manufactured.
• Necessary instrumentations for generation of controlled roughness elements and generation of free-stream vortical structures has been designed and manufactured.
• An accurate traversing system has been designed and manufactured.
• Detailed measurements have been completed and data were evaluated.
A large number of state-of-art high-resolution direct numerical simulations have been performed to investigate a number of different receptivity phenomena:
• Direct numerical simulations of large roughness elements in a three-dimensional boundary layer at high speed (Mach 0.65 and Reynolds number 6.9 millions) have been carried out to identify the critical size of roughness elements. Simulations show clear differences between the structures created behind sub- and super critical roughness elements.
• Several low speed (incompressible) cross-flow dominated flows corresponding to the experimental studies from literature have been simulated. These simulations include both receptivity to small surface roughness elements and transition control by means of micron-sized roughness elements.
• Interaction of freestream perturbations with the three-dimensional boundary-layer flow over wings at low and high speeds has been numerically investigated. These studies included interaction of freestream turbulence with different intensities and surface roughness. The results have been compared to available experimental data.
Development of efficient and robust receptivity tools and methods has been one of focus point of the project:
• A theory based on the random forcing of flow by thermal noise has been developed for three-dimensional flows.
• A new theory for computation of receptivity theory to surface roughness elements have been developed which accounts for the aspect ratio of the roughness. Its application to test cases analyzed in the project showed an improvement of results.
• Different approaches for receptivity analysis has been tested and validated against experimental and numerical data from project and literature.
As the secondary instability is a crucial stage of laminar-turbulent transition, it has been investigated thoroughly:
• A new approach to secondary instability analysis has been developed combining the classical approach and nonlinear PSE. This approach improves efficiency of the secondary instability analysis (specially the high frequency one) significantly.
• A new approach for nonlinear transition prediction has been introduced and tools have been developed. Here the idea is to correlate the transition location to onset of secondary instability, which in turn is correlated to shear of the meanflow modified by stationary crossflow vortices.
To studies effects of flow three-dimensionality, two experiments with transition measurements of three-dimensional wings have been identified. RANS and boundary-layer simulations have been performed to generate the mean flow for three-dimensional stability analysis. Stability analysis based on the ray-tracing theory has been performed to investigate the impact of inhomogeneity of flow in spanwise direction on prediction of perturbation growth.
Potential Impact:
The RECEPT project has delivered upstream aerodynamics research that will increase the accuracy ofperformance prediction for aircraft with laminar wings, allowing design of advanced and innovative aircraft. The achievements of the project have improved our understanding of transition in three-dimensional flows and delivered new tools for transition prediction. The generated experimental and numerical data enhances our knowledge about the required surface quality for aircraft with laminar flow technology. These improvements will give the aircraft manufacturers within RECEPT possibility to better predict the flight performance of such an aircraft prior to aircraft project launch. This new knowledge and the greatly improved next generation transition prediction methods can be directly transferred to the more applied work to be performed within the Joint Technology Initiative Clean Sky, through the participation of AIRBUS, SAAB and Piaggio Aerospace.
The performed work, due to its basic research character, has also a great impact on the academic research in aeronautics and fluid mechanics. The phenomenon of laminar-turbulent transition is one of the most important issues for a large number of applications, e.g. combustion, papermaking, atmospheric and ocean flows with direct applications on sustainable energy production such as wind and water power.
The improved transition prediction will increase the capability of designing aircraft with laminar wings. Also it will contribute to better modelling of the important physical phenomena associated with effects of laminar-flow control, thus moving a step towards the design of more advanced flow control devices. Consequently, RECEPT will contribute to achieving the objectives for technology readiness to reduce fuel consumption and hence CO2 and NOx emissions. The gained knowledge will improve the design tools and opens for possible innovation in design of laminar wings.
Drag reduction results to need for less powerful engines for the same flight speed, which in turn results to reduced noise level.
By contributing to the design of greener and smarter European transport systems, the outcome of the RECEPT project will be for the benefit of all citizens. Transport is one of Europe's strengths - the air transport sector contributes around 2% of the EU GDP with 3.7 million jobs. The aeronautics sector alone generates a turnover of EUR 70 billion and exports 60% of its production. Thus, through the increased capability for design of greener aircraft and strengthening of European Aircraft industries, the RECEPT project will also have a direct impact on competition, employment and raised technical skill and knowledge base of the academic and research organisation partners.
List of Websites:
Ardeshir Hanifi
KTH Royal Institute of Technology
Dept. of Mechanics
100 44 Stockholm
Sweden
e-mail: hanifi@kth.se
Phone: +46 8 7908482
The RECEPT project has delivered upstream aerodynamics research that will increase the accuracy of performance prediction for aircraft with laminar wings, allowing design of advanced and innovative aircraft. The achievements of the project have improved our understanding of transition in three-dimensional flows and have delivered new tools for laminar-turbulent transition prediction. The performed experimental and numerical investigations have enhanced our knowledge about the process of disturbance generation in three-dimensional boundary-layer flows caused by free-stream turbulence and surface roughness. The latter gives valuable information regarding the required surface quality for aircraft with laminar flow technology.
Dissemination of novel and high quality research work performed in the project has been insured through presentations at the international conferences and publication in peer-reviewed journals. Participation of industry will guarantee transfer of the new knowledge and improved method to real applications, e.g. those performed within the Joint Technology Initiative – Clean Sky.
The RECEPT consortium consisted of 12 organisations from 4 different member states (Sweden, Italy, France Germany) and one of International Cooperation Partner Countries, Russia. It contains 3 aircraft manufacturers (Airbus, SAAB, Piaggio), 5 research organisations (CIRA, DLR, FOI, ITAM, ONERA) and 4 universities (Kungliga Teknika Högskolan, Università di Genova, Università di Salerno, Universität Stuttgart).
The high quality research work performed in the RECEPT project will contribute to the need to improve the environmental impact of aircraft with regards to emissions, and to the drive to strengthen the competitiveness of European manufacturing industry.
Project Context and Objectives:
The major objective of the RECEPT project is the development of the capability to predict the in-flight performance of a future laminar flow aircraft through development of more accurate transition prediction tools.
The current procedure of development of new aircraft includes a number of wind tunnel tests (which are usually costly) in order to verify the performance predicted by CFD tools. There is at least one major difficulty with wind tunnel tests for an aircraft designed for Natural Laminar Flow (NLF). The transition of the flow from a laminar state to a turbulent one is highly affected by the level of perturbations in the incoming flow and surface quality. Today these parameters are not included in transition prediction tools. Major goal of RECEPT project is to improve the transition-prediction tools to account for these parameters.
To achieve these goals the RECEPT project will address the following issues
• Receptivity to small and large roughness elements as well as free-stream vortical disturbances;
• Three-dimensional stability analyses where the effects of flow three-dimensionality are strong (wing root area).
• The development of an amplitude based transition prediction method.
The RECEPT project aims at developing the next generation transition-prediction methods by adopting an amplitude-based method. The present work aims at removing the need for empirical relations and therefore making accurate prediction of the onset of transition both under wind tunnel and free-flight conditions possible.
The proposed method will consist of the following steps:
1. Estimation of the disturbance level and spectrum in the wind-tunnel and in free-flight,
2. Receptivity calculations giving the initial amplitudes of the most unstable disturbances,
3. Non-linear stability calculations to estimate the transition location based on the finite amplitude of the disturbances.
Incorporating substantially more physics into the transition prediction methods can be expected to generate a leap in performance and reliability of the methods. The last two proposed steps require new research, outlined in the present proposal.
The project is divided into 5 work packages. The first four deal with scientific aspects of the project and the last one with the administrative issues:
• WP1: This work package is devoted to experimental investigations of receptivity of three-dimensional boundary-layer flows to (i) external vortical perturbations, (ii) small and (iii) medium-size roughness elements. Two models will be manufactured and equipped. Experiments will be performed in a quiet wind tunnel (MTL at KTH) with controlled external perturbations and surface roughness quality.
• WP2: In this WP numerical approaches will be used to replicate the physics approached experimentally in WP1. Methods and numerical tools with different range of complexity and approximation level will be used to attack the problem. Direct numerical simulation will also be employed to provide specific detailed information.
• WP3: In this work package the development of an amplitude-based transition prediction method is investigated. The idea is, based on the results of WP1 and WP2, to take into account the initial amplitude of disturbances obtained from receptivity computations, in effect going from an amplification-based transition prediction criterion to an amplitude-based one.
• WP4: The objectives of this work package are twofold: (i) To apply the new receptivity and transition prediction tools, obtained in WP1-WP3, to geometries and flow parameters for realistic cases. (ii) To estimate the effects of flow three-dimensionality compared to infinite swept-wing approximation. In order to evaluate these effects, two different approaches to account for fully three-dimensional flows will be used.
• WP5: This work package covers the issues related to administration of the project, e.g. cost and time controlling, management of meetings and reports, communication with the European Commission and with the WP and Task leaders.
Project Results:
A series of highly detailed and accurate experiments have been performed with aim to create a database for development and validation of receptivity tools. To perform these experimental works:
• Based on the characteristics of the MTL wind tunnel and performed stability analysis, an airfoil has been chosen and modified. Based on this airfoil two different swept-wing models with high surface quality have been manufactured.
• To insure spanwise flow-homogeneity, contours of wind tunnel sidewalls for experiments at positive and negative angles of attack have been designed and manufactured.
• Necessary instrumentations for generation of controlled roughness elements and generation of free-stream vortical structures has been designed and manufactured.
• An accurate traversing system has been designed and manufactured.
• Detailed measurements have been completed and data were evaluated.
A large number of state-of-art high-resolution direct numerical simulations have been performed to investigate a number of different receptivity phenomena:
• Direct numerical simulations of large roughness elements in a three-dimensional boundary layer at high speed (Mach 0.65 and Reynolds number 6.9 millions) have been carried out to identify the critical size of roughness elements. Simulations show clear differences between the structures created behind sub- and super critical roughness elements.
• Several low speed (incompressible) cross-flow dominated flows corresponding to the experimental studies from literature have been simulated. These simulations include both receptivity to small surface roughness elements and transition control by means of micron-sized roughness elements.
• Interaction of freestream perturbations with the three-dimensional boundary-layer flow over wings at low and high speeds has been numerically investigated. These studies included interaction of freestream turbulence with different intensities and surface roughness. The results have been compared to available experimental data.
Development of efficient and robust receptivity tools and methods has been one of focus point of the project:
• A theory based on the random forcing of flow by thermal noise has been developed for three-dimensional flows.
• A new theory for computation of receptivity theory to surface roughness elements have been developed which accounts for the aspect ratio of the roughness. Its application to test cases analyzed in the project showed an improvement of results.
• Different approaches for receptivity analysis has been tested and validated against experimental and numerical data from project and literature.
As the secondary instability is a crucial stage of laminar-turbulent transition, it has been investigated thoroughly:
• A new approach to secondary instability analysis has been developed combining the classical approach and nonlinear PSE. This approach improves efficiency of the secondary instability analysis (specially the high frequency one) significantly.
• A new approach for nonlinear transition prediction has been introduced and tools have been developed. Here the idea is to correlate the transition location to onset of secondary instability, which in turn is correlated to shear of the meanflow modified by stationary crossflow vortices.
To studies effects of flow three-dimensionality, two experiments with transition measurements of three-dimensional wings have been identified. RANS and boundary-layer simulations have been performed to generate the mean flow for three-dimensional stability analysis. Stability analysis based on the ray-tracing theory has been performed to investigate the impact of inhomogeneity of flow in spanwise direction on prediction of perturbation growth.
Potential Impact:
The RECEPT project has delivered upstream aerodynamics research that will increase the accuracy ofperformance prediction for aircraft with laminar wings, allowing design of advanced and innovative aircraft. The achievements of the project have improved our understanding of transition in three-dimensional flows and delivered new tools for transition prediction. The generated experimental and numerical data enhances our knowledge about the required surface quality for aircraft with laminar flow technology. These improvements will give the aircraft manufacturers within RECEPT possibility to better predict the flight performance of such an aircraft prior to aircraft project launch. This new knowledge and the greatly improved next generation transition prediction methods can be directly transferred to the more applied work to be performed within the Joint Technology Initiative Clean Sky, through the participation of AIRBUS, SAAB and Piaggio Aerospace.
The performed work, due to its basic research character, has also a great impact on the academic research in aeronautics and fluid mechanics. The phenomenon of laminar-turbulent transition is one of the most important issues for a large number of applications, e.g. combustion, papermaking, atmospheric and ocean flows with direct applications on sustainable energy production such as wind and water power.
The improved transition prediction will increase the capability of designing aircraft with laminar wings. Also it will contribute to better modelling of the important physical phenomena associated with effects of laminar-flow control, thus moving a step towards the design of more advanced flow control devices. Consequently, RECEPT will contribute to achieving the objectives for technology readiness to reduce fuel consumption and hence CO2 and NOx emissions. The gained knowledge will improve the design tools and opens for possible innovation in design of laminar wings.
Drag reduction results to need for less powerful engines for the same flight speed, which in turn results to reduced noise level.
By contributing to the design of greener and smarter European transport systems, the outcome of the RECEPT project will be for the benefit of all citizens. Transport is one of Europe's strengths - the air transport sector contributes around 2% of the EU GDP with 3.7 million jobs. The aeronautics sector alone generates a turnover of EUR 70 billion and exports 60% of its production. Thus, through the increased capability for design of greener aircraft and strengthening of European Aircraft industries, the RECEPT project will also have a direct impact on competition, employment and raised technical skill and knowledge base of the academic and research organisation partners.
List of Websites:
Ardeshir Hanifi
KTH Royal Institute of Technology
Dept. of Mechanics
100 44 Stockholm
Sweden
e-mail: hanifi@kth.se
Phone: +46 8 7908482
