DIRECT NUMERICAL SIMULATION OF VORTEX DYNAMICS ASSOCIATED WITH A SEPARATION BUBBLE ON A ROUNDED EDGE

Abstract

The formation of a separation bubble over a generic half-body with a rounded edge is studied by DNS. Front edge curvature and body aspect ratio effects are considered by focusing on the vortex dynamics associated with the breakdown of the bubble through 3D processes. Qualitative and quantitative comparisons with previous experiments are presented. The main influences of curvature and aspect ratio are consistently recovered in present simulations. The structure of the separation bubble is in agreement with experiments, especially the combination of singular points associated with the surface flow. Behind the separated region, the examination of the mean flow reveals the presence of a pair of longitudinal counter-rotating vortices pumping fluid from the side of the body to the top of the flow. The analysis of instantaneous visualisations shows the formation of strong lambda vortices for small aspect ratios that causes ejection of the fluid through a periodic bursting process that seems to be linked to the flapping of the separation bubble. The increase of the curvature of the rounded front edge is found to increase the separation angle, in qualitative agreement with experiments, with a global growing of the size of the separation bubble in longitudinal and vertical directions. Strong curvature also tends to reduce drastically the sensitivity of the flow to upstream conditions, suggesting the occurrence of phenomena that could be interpreted in terms of convective/absolute stability.