Hybrid LES-RANS using synthesized turbulent fluctuations for forcing in the interface region

The main bottleneck in using Large Eddy Simulations at high Reynolds number is the requirement of very fine meshes near walls. One of the main reasons why hybrid LES-RANS was invented was to eliminate this limitation. In this method unsteady RANS (URANS) is used near walls and LES is used away from walls. The present paper evaluates a method for improving standard LES-RANS. The improvement consists of adding instantaneous turbulent fluctuations (forcing conditions) at the matching plane between the LES and URANS regions in order to trigger the equations to resolve turbulence. The turbulent fluctuations are taken from synthesized homogeneous turbulence assuming a modified von Kármán spectrum. Both isotropic and non-isotropic fluctuations are evaluated. The new approach is applied to fully developed channel flow and it is shown that the imposed fluctuations considerably improve the predictions. It is found that increasing the prescribed turbulent length scale of the synthesized turbulence provides excellent agreement with the classical log-law.