A Tensor-diffusivity Subgrid Model for Large-Eddy Simulation

Subgrid-scale models for large-eddy simulation that are based on exact series expansions for ltered products are considered. In particular, if the rst two terms are retained, the result is a di usive subgrid term with a tensor di usivity. This tensor is proportional to the rate-of-strain tensor of the large-scale velocity eld. This leads to negative di usion in the stretching directions. Implications of this result are considered for the ltered scalar advection-di usion equation and for the momentum equation for incompressible uid ow. When coupled with a dynamic Smagorinsky term to form a mixed model, very encouraging results are shown for turbulent, isotropic decay and for turbulent channel ow. In addition, it is shown that the model, mixed or not, transforms appropriately when di ering frames of reference are considered. Modi cations to the model are suggested for the case in which the un ltered eld(s) has discontinuities.