Effects of Eddy Viscosity on Time Correlations in Large Eddy Simulation
نویسندگان
چکیده
Subgrid-scale (SGS) models for large eddy simulation (LES) have generally been evaluated by their ability to predict single-time statistics of turbulent ows such as kinetic energy and Reynolds stresses. Recent applications of large eddy simulation to the evaluation of sound sources in turbulent ows, a problem in which time correlations determine the frequency distribution of acoustic radiation, suggest that subgrid models should also be evaluated by their ability to predict time correlations in turbulent ows. This paper compares the two-point, two-time Eulerian velocity correlation evaluated from direct numerical simulation (DNS) with that evaluated from LES, using a spectral eddy viscosity, for isotropic homogeneous turbulence. It is found that the LES elds are too coherent, in the sense that their time correlations decay more slowly than the corresponding time correlations in the DNS elds. This observation is con rmed by theoretical estimates of time correlations using the Taylor expansion technique. The reason for the slower decay is that the eddy viscosity does not include the random backscatter, which decorrelates uid motion at large scales. An e ective eddy viscosity associated with time correlations is formulated, to which the eddy viscosity associated with energy transfer is a leading order approximation.
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