Mixed modeling for large-eddy simulation: The single-layer and two-layer minimum-dissipation-Bardina models

Predicting the behavior of turbulent flows using large-eddy simulation requires modeling of the subgrid-scale stress tensor. This tensor can be approximated using mixed models, which combine the dissipative nature of functional models with the capability of structural models to approximate out-of-equilibrium effects. The currently existing mixed models, however, are based on ad hoc linear combinations of models and lack mathematical motivation. We, therefore, propose a mathematical basis to mix (functional) eddy-viscosity models with the (structural) Bardina model. With this methodology we obtain the mixed anisotropic minimum-dissipation (AMD) -Bardina model. In order to also obtain a physics-conforming model for wall-bounded flows, we further develop this mixed model into a two-layer approach: the near-wall region is parameterized with the AMD-Bardina model, whereas the outer region is computed with the Bardina model. The original and two-layer AMD-Bardina models are tested in turbulent channel flows at various Reynolds numbers, and improved predictions are obtained when the mixed models are applied in comparison to the computations with the AMD and Bardina models alone. The results obtained with the two-layer AMD-Bardina model are particularly remarkable: both first- and second-order statistics are extremely well predicted and even the inflection of the mean velocity in the channel center is captured. Hence, a very promising model is obtained for large-eddy simulations of turbulent channel flows at moderate and high Reynolds numbers.