Roger Cocle

Investigation of multiscale subgrid models for LES of instabilities and turbulence in wake vortex systems

This paper investigates the capabilities of different subgrid scale (SGS)models, including the recent “multiscale” models, for large-eddy simulation (LES),here in a vortex-in-cell (VIC) method, of complex wake vortex dynamics. More specifically, we here consider the multiscale dynamics developing in a counterrotating four-vortex system, that evolves from a simple state to a turbulent state. The various SGS models are tested and compared on this complex and transitional flow. Comparisons are also made with results obtained using a pseudo-spectral method. Energy diagnostics (global and modal) and spectra are provided and used to support the comparisons. A discussion on the applicability of the various models to LES of complex wake vortex flows is made. The multiscale models are seen to be the most appropriate

Spectral behavior of various subgrid-scale models in LES at very high Reynolds number

This study investigates the capabilities of various recent subgrid-scale (SGS) models (the so-called “multiscale” models) for large-eddy simulation (LES), used either in a vortex-in-cell (VIC) method or in a pseudo-spectral (PS) method, and their applicability to the simulation of decaying homogeneous isotropic turbulence (HIT) in the limit of very high Reynolds number (i.e. LES on a large grid and where the molecular viscosity dissipation is negligible compared to the SGS dissipation). The proper coefficient value for each model investigated was obtained by a calibration performed in an earlier study. Various large grid resolutions (1283, 2563 and 5123) are used to compare and to indeed obtain the asymptotic spectral behavior of each model. We are then able to emphasize the behavior of the models, that is not necessarily observable in “small” LES (i.e. in LES at moderate Reynolds number and/or using a smaller mesh). In particular, we show that the multiscale models perform significantly better than the Smagorinsky model: a much wider inertial range is obtained.