Backward Scatter of Subgrid-Scale Energy in Wall-Bounded and Free Shear Turbulence

The energy exchange between the grid scale (GS) and the subgrid-scale (SGS) is investigated in large-eddy simulations (LES) of wall-bounded (fully developed channel) and free shear (mixing layer) turbulent flows. It is shown in the direct numerical simulation (DNS) data analysis that the forward and backward scatters of energy which arise are typically of the same order of magnitude in both flows, with the net transfer being slightly from GS to SGS, but the backward scatter is more pronounced in the mixing layer. A mechanism of the SGS energy generation is discussed in relation to the coherent structures. For the subgrid-scale models, assessment is made of the eddy viscosity coefficient models and the scale-similarity models. In correlation with the DNS data, the scale-similarity model was shown to be more accurate and capable of representing the backward scatter effect, while concurrently providing a drain of the GS energy into SGS. The drawback of the previous scale-similarity model, however, was its overestimate of the backward scatter. A new scale-similarity model that rectifies this drawback is proposed. These SGS models were tested further in actual LES of the channel and mixing layer flows. The results were consistent with those of `a priori' tests.