Hybrid techniques for Large-Eddy Simulations of complex turbulent flows

The paper presents developments for a segregated approach to the coupling of Reynolds-Averaged Navier-Stokes (RANS) calculations with a zone computed as Large-Eddy Simulation (LES). The mean velocity fields are matched at predefined interfaces and velocity fluctuations of the LES zone are treated according to the type of interface. If the RANS zone is downstream of the LES, fluctuations are allowed to leave the domain by employing a convective boundary condition. If the RANS zone is placed between the LES and a wall, the fluctuations at the interface are scaled to match the statistics predicted by the RANS computation. The proposed method was applied to turbulent channel flow and the flow over periodic hills. It was found that the method delivered an improvement over alternative techniques in the literature while removing the need for calibration constants. For incompressible flows, it is also necessary to prescribe conditions for the pressure or an equivalent variable. Several alternatives were tested. Decoupling of the pressure combined with explicitly enforced mass conservation at the interface yielded the best and, for the hill flow with aggressively placed interface, the only acceptable results.