A rans approximate boundary condition for large-eddy simulation of wall-bounded turbulent flows

Practical application of large-eddy simulation (LES) to wall-bounded turbulent flows has been limited by the need to resolve the energy-containing range in the near-wall region. One solution to this problem was used by Balaras, Benocci, and Piomelli (1996), who applied a "two-layer" approach in which the wall shear stress is determined by solving boundary-layer equations between the wall and the first LES grid point. This allows for the reduction of near-wall grid resolution while retaining a more physical justification for the wall shear stress. In complex geometries, however, the boundary-layer equations may not be able to include the necessary physics to model the near-wall region well. The present study replaces the boundary layer equations with a full set of unsteady Reynolds-averaged Navier-Stokes (RANS) equations. RANS equations are tuned to predict accurately the mean boundary-layer structure, and have been used in many different flow regimes. The RANS equations are used to model the near-wall flow while the LES handles the remainder of the flow. We investigate this approach in the framework of fully developed turbulent channel flow. Since we are using RANS equations, this simulation technique can easily be extended to complex geometries.