Turbulent intensities in large-eddy simulation of wall-bounded flows

A persistent problem in wall bounded large-eddy simulations (LES) with Dirichlet no-slip boundary conditions is that the near-wall streamwise velocity fluctuations are over-predicted, while those in the wall-normal and spanwise directions are under-predicted. The problem is particularly pronounced when the near-wall region is under-resolved. The prediction of the fluctuations is known to improve for wall-modeled LES, where the no-slip boundary condition at the wall is typically replaced by Neumann and no-transpiration conditions for the wall-parallel and wall-normal velocities, respectively. However, the turbulent intensity peaks are sensitive to the grid resolution and the prediction may degrade when the grid is refined. In the present study, a physical explanation of this phenomena is offered in terms of the behavior of the near-wall streaks. We also show that further improvements are achieved by introducing a slip boundary condition with transpiration. By using a slip condition, the inner energy production peak is damped, and the blocking effect of the wall is relaxed such that the splatting of eddies at the wall is reduced. As a consequence, the slip condition provides an accurate and consistent prediction of the turbulent intensities regardless of the near-wall resolution.