LARGE-EDDY SIMULATION OF THE TURBULENT MIXING LAYER

Six subgrid models for the turbulent stress tensor are tested by condu cting large-eddy simulations (LES) of the weakly compressible temporal mixing layer: the Smagorinsky, similarity, gradient, dynamic eddy-vis cosity, dynamic mixed and dynamic Clark models. The last three models are variations of the first three models using the dynamic approach. T wo sets of simulations are performed in order to assess the quality of the six models. The LES results corresponding to the first set are co mpared with filtered results obtained from a direct numerical simulati on (DNS). It appears that the dynamic models lead to more accurate res ults than the non-dynamic models tested. An adequate mechanism to diss ipate energy from resolved to subgrid scales is essential. The dynamic models have this property, but the Smagorinsky model is too dissipati ve during transition, whereas the similarity and gradient models are n ot sufficiently dissipative for the smallest resolved scales. In this set of simulations, at moderate Reynolds number, the dynamic mixed and Clark models are found to be slightly more accurate than the dynamic eddy-viscosity model. The second set of LES concerns the mixing layer at a considerably higher Reynolds number and in a larger computational domain. An accurate DNS for this mixing layer can currently not be pe rformed, thus in this case the LES are tested by investigating whether they resemble a self-similar turbulent flow. It is found that the dyn amic models generate better results than the non-dynamic models. The c losest approximation to a self-similar state was obtained using the dy namic eddy-viscosity model.