LESFOIL: Large Eddy Simulation of Flow Around a High Lift Airfoil

This volume contains results of a European project on Large Eddy Simulation (LES) of the flow around an airfoil. The main objective of the LESFOIL project was to assess the suitability of LES for airfoil flow. In conclusion, preliminary work was carried out such as development of numerical methods, and subgrid modelling in geometrically simple flows such as fully developed channel flow and periodic flow in a channel with a curved hill-shaped surface. Accurate LES of wall-bounded flow requires fine cells in the near-wall region in all coordinate directions. In an attempt to release this constraint, a large part of the LESFOIL project was aimed at developing and validating different approximate near-wall treatments. In the second half of the book, several LESs of the flow around the Aerospatiale-A airfoil are presented, using different numerical methods, grids, SGS models and near-wall treatments.

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Large Eddy Simulation is a relatively new and still evolving computatio­ nal strategy for predicting turbulent flows. It is now widely used in research to elucidate fundamental interactions in physics of turbulence, to predict phe­ nomena which are closely linked to the unsteady features of turbulence and to create data bases against which statistical closure models can be asses­ sed. However, its applicability to complex industrial flows, to which statisti­ cal models are applied routinely, has not been established with any degree of confidence. There is, in particular, a question mark against the prospect of LES becoming an economically tenable alternative to Reynolds-averaged N avier-Stokes methods at practically high Reynolds numbers and in complex geometries. Aerospace flows pose particularly challenging problems to LES, because of the high Reynolds numbers involved, the need to resolve accura­ tely small-scale features in the thin and often transitional boundary layers developing on aerodynamic surfaces. When the flow also contains a separated region - due to high incidence, say - the range and disparity of the influen­ tial scales to be resolved is enormous, and this substantially aggravates the problems of resolution and cost. It is just this combination of circumstances that has been at the heart of the project LESFOIL to which this book is devoted. The project combined the efforts, resources and expertise of 9 partner organisations, 4 universities, 3 industrial companies and 2 research institu­ tes.

Contenu

I. Introduction.- II. Preparatory Work.- 1 Task 1: Subgrid models.- 1.1 Summary of work progress.- 1.2 Task 1.1: Grid generation.- Unstructured grids.- Structured C-grids.- 1.3 Task 1.2: Generation of database with DNS.- 1.4 Task 1.3: Development and evaluation of subgrid models in simple configurations.- Evaluated subgrid-scale models.- 1.5 Conclusion.- 2 Task 2: Near-wall models.- 2.1 Introduction.- 2.2 Overview of Research.- 2.3 Chalmers.- The Hybrid LES-RANS model.- 2.4 CERFACS.- 2.5 University of Karlsruhe.- 2.6 UMIST/QMW.- 2.7 Conclusions and Overall assessment.- 3 Task 4: Numerical methods.- 3.1 Introduction.- 3.2 Contribution by Chalmers.- Performance Assessment.- Speed-up.- Deferred correction.- PISO and SIMPLEC.- Spatial Discretisation.- 3.3 Contribution by Fluent.- Performance Assessment.- Discretization Scheme.- Accuracy assessment.- Velocity Profiles.- 3.4 Contribution by University of Karlsruhe.- Fourier solver for the p' equation.- Implications of 2D/3D Zonal refinement method on Fourier solver.- 3.5 Contribution by ONERA.- 3.6 Contribution by University of Surrey.- 3.7 Contribution by UMIST.- Solution of momentum equations.- Time-step control.- The pressure equation.- Domain decomposition and parallelization.- Partial diagonalisation.- Multigrid algorithm.- Performance Assessment.- 3.8 Achievements and recommendations.- III. The Airfoil Investigations.- 4 Task 5: Airfoil Computations.- 4.1 Introduction.- The Principal Airfoil Geometry.- Common Mesh.- 4.2 Contribution by Chalmers.- Numerical Method.- Boundary Conditions.- Convergence Criteria.- Computations.- Conclusions.- 4.3 Contribution by Alenia.- Objectives.- Numerical method.- Turbulence models.- Steady flow computations.- Unsteady RANS computations.- Conclusions.- Recommendations for future work.- 4.4 Contribution by CERFACS.- Numerical schemes.- Wall functions.- Airfoil Calculations.- Conclusions.- 4.5 Contribution by Dassault-Aviation.- Description of the Navier-Stokes code.- Towards LES.- Application to the A-airfoil.- Comparison of LES results using different SGS models.- Comparison between RANS and LES.- Conclusions.- 4.6 Contribution by FLUENT.- Model Description.- The Mesh.- Numerical Details.- Results.- Conclusions.- 4.7 Contribution by University of Karlsruhe.- LES resolution requirements.- Computational Efficiency.- Transition modelling.- Airfoil calculations.- Conclusions.- 4.8 Contribution by ONERA.- Simulation method.- Subgrid Scale Modelling.- Euler Flux Discretization.- 2D/3D coupling method.- Computational Setup.- Results and Discussion.- Conclusions.- 4.9 Contribution by QMW.- Overview.- The Numerical Method.- Simulations and Results.- Conclusions.- IV. Lessons Learned.- 5 Synthesis of the Airfoil Flow Simulations.- 5.1 Common Mesh Comparisons.- 5.2 Trailing edge geometry.- 5.3 Final Results Comparisons.- 5.4 Subgrid-Scale Modelling.- 5.5 Near-Wall modelling.- 5.6 Transition Treatment.- 5.7 Synthesis conclusions.- V. Conclusions and Outlook.- VI. References.- VII. Addresses of Partners.