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Personne# Michele Mossi

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Simulation des grandes structures de la turbulence

La simulation des grandes structures de la turbulence (SGS ou en anglais LES pour Large Eddy Simulation) est une méthode utilisée en modélisation de la turbulence. Elle consiste à filtrer les petite

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Large-eddy simulation (LES) is a very promising technique for the numerical computation of unsteady turbulent flows, and seems to be the perfect tool to simulate the compressible air flow around a high-speed train in a tunnel, providing unsteady results for aerodynamic and aeroacoustic analysis. To look into this possible future application of LES, two major lines of investigation are pursued in this thesis: first, the study of the effective ability of shock-capturing schemes to predict fundamental turbulent phenomena; second, the analysis of the aerodynamic phenomena induced by a high-speed train in a tunnel. The numerical simulation of compressible flows requires the use of shock-capturing schemes. These schemes can be relatively dissipative and mask the subgrid-scale contribution introduced in a large-eddy simulation to account for the unresolved turbulence scales. To estimate their effective dissipation and their ability to resolve turbulence phenomena, shock-capturing schemes widely used for aeronautical applications, from second- to fifth-order space accuracy, are employed here for simulating well-known fundamental flows in subsonic and supersonic regimes. Direct and large-eddy numerical simulations are undertaken for the inviscid and viscous Taylor-Green vortex decay problem, the freely decaying homogeneous and isotropic turbulence, and the fully developed channel flow. For all the turbulent flows investigated, several turbulence statistics are computed and the numerical dissipation of the schemes tested is interpreted in terms of subgrid-scale dissipation in a LES context, yielding an equivalent Smagorinsky or dynamic coefficient. This coefficient is for all schemes of the same order of magnitude as the commonly accepted values in LES for the subgrid-scale term. On the grounds of this analysis and of the comparisons of the turbulence statistics with accurate data obtained in the literature, the addition of explicit subgrid-scale models to the shock-capturing schemes tested is not recommended. It is thus concluded that the use of the LES technique for compressible turbulent flows is not yet suitable for industrial applications. The aerodynamic phenomena generated by a high-speed train travelling in a tunnel are also discussed, their importance on the design of high-speed lies is pointed out, and the analysis tools commonly employed for their study are reviewed. To study numerically the three-dimensional, compressible and turbulent air flow around a high-speed train accelerating in a tunnel, by accounting for the unsteady effects at inlet and outlet boundaries due to the propagation of pressure waves generated at the train departure, new coupling conditions between one-dimensional and three-dimensional domains are developed. These conditions are applied successfully to the numerical simulation of the unsteady wake developing behind two- and three-dimensional vehicles, where the averaged Navier-Stokes equations are solved with the turbulence modelling approach. The influence on the wake of the length of the vehicle tail is also discussed and results of multi-dimensional simulations are compared with one-dimensional data.

Pascal Comte, Michel Deville, Michele Mossi

"Numerical simulations of freely decaying isotropic fluid turbulence were performed at various Mach numbers (from 0.2 to 1.0) using known shock-capturing Euler schemes (Jameson, TVD-MUSCL, ENO) often employed for aeronautical applications. The objective of these calculations was to evaluate the relevance of the use of such schemes in the large-eddy simulation (LES) context, The potential of the monotone integrated large-eddy simulation (MILES) approach was investigated by carrying out computations without viscous diffusion terms. Although some known physical trends were respected, it is found that the small scales of the simulated flow suffer from high numerical damping. In a quasi-incompressible case, this numerical dissipation is tentatively interpreted in terms of turbulent dissipation, yielding the evaluation of equivalent Taylor micro-scales, The Reynolds numbers based on these are found between 30 and 40, depending on the scheme and resolution (up to 128(3)). The numerical dissipation is also interpreted in terms of subgrid-scale dissipation in a LES context, yielding equivalent Smagorinsky ""constants"" which do not level off with time and which remain larger than the commonly accepted values of the classical Smagorinsky constant. On the grounds of tests with either the Smagorinsky or a dynamic model, the addition of explicit subgrid-scale (SGS) models to shock-capturing Euler codes is not recommended. (C) 1999 Academic Press."

1999