New variants to the dynamic Subgrid Scale Model

Abstract

We propose an anisotropic modification to the dynamic Subgrid Scale Model for Large Eddy Simulations. The essential feature is that the principal axes of the SGS stress tensor that has to be modelled are not forced to be parallel to the resolved stress tensor's ones. In this way new degrees of freedom are introduced that reduce the isotropy requirements on the small scale structures. As a consequence coarser grids can be used to obtain the same accuracy of previous models.

Numerical results for the plane channel flow using centered finite differences are presented. The computations are performed whithout averaging the model coefficients in space and time. This makes the results more accurate although the method is some-what less robust. We show that for the same numerical grid the new anisotropic model gives more accurate results compared to standard dynamic SGS models, especially near the walls, where the turbulence is highly anisotropic.

We turn next to the modelling of the outlet boundary conditions. We present a new 3D development of boundary conditions that is an extension of a previous 2D model (Jin and Braza 1993). We show on numerical tests of a flow in an annulus that our boundary conditions are totally absorbing both for the velocity field and for a passive scalar.