Conditional mode-elimination and the subgrid-modeling problem for isotropic turbulence

It is shown analytically that different subgrid eddy viscosities are requir ed to characterize the evolution of the velocity field, the energy spectrum and the dissipation rate, respectively. Also, a subgrid viscosity calculat ed recursively from the conditionally averaged "subgrid-subgrid" stress can only be used to renormalize the dissipation relation. It is also shown, by direct numerical simulation, that the spectral correlations of subgrid and resolved stresses are dominated by "resolved-subgrid" terms, in agreement with existing conclusions from energy transfer or other more global methods . We also note the low level of correlation between the exact subgrid stres s and the eddy-viscosity model for all explicit-scales wave numbers. Lastly , we report results from a large-eddy simulation (LES) based on a recursive ly calculated effective viscosity and note that the results are about as go od as for other subgrid models but underline the need to take account of ph ase effects as well as the dissipation rate. (C) 2001 American Institute of Physics.