Large-Eddy Simulations of a Turbulent Boundary Layer Separation Over a Bump With Strong Pressure Gradients and Free-Stream Turbulence

Abstract

Abstract. This study examines the flow over a wall-bounded bump geometry using wall-modeled large-eddy simulations (WMLES) in the limit of very thin wall resolution. The geometry and flow conditions are based on an experimental investigation of a canonical geometry designed to replicate diffusion rates similar to those in low-pressure turbine blades. The high subsonic Mach and low Reynolds number upstream of the bump generate strong pressure gradients, both favorable and adverse. As the incoming turbulent boundary layer encounters the bump surface, it undergoes relaminarization, shock–wave interaction and subsequent separation. An inadequate grid resolution near the wall is found to suppress the formation of streamwise vortices during relaminarization, resulting in a different mixing process within the separated boundary layer and a longer separation bubble. To prevent this, extending the spatial region of high refinement to at least 3 δ99 of the relaminarization region is recommended. The study also investigates the impact of freestream isotropic turbulence on the pressure distribution and separation, revealing that even a small variation in inflow turbulence intensity is sufficient to induce relevant changes in the mean flow results.