Direct numerical simulation of a separation bubble on a rounded finite-width leading edge

The formation of a separation bubble over a generic half-body with a rounded edge is studied by direct numerical simulation. A single Reynolds number Re = 1250 (based on the body height H s and the inflow velocity U ∞ ) corresponding to reference experimental results is investigated. Various body geometries are considered through the change of its width L (four aspect ratios L / H s addressed) and its front edge curvature R (two rounded shapes R / H s addressed). The combined effects of aspect ratio and curvature are considered by focusing on the vortex dynamics associated with the breakdown of the bubble through three-dimensional 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 on the top-boundary of the body. 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 visualizations shows the formation of strong lambda vortices for small aspect ratios. These vortices cause ejection of the fluid through a periodic bursting process which 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. The sensitivity of the flow to upstream conditions is discussed by considering different levels of inflow fluctuations (with a root mean square from zero to 1% of U ∞ ) while evaluating the deterministic response of the bubble dynamics with respect to cyclic inlet excitation. Strong curvature is found to reduce drastically the upstream receptivity of the flow, the resulting phenomena being interpreted in terms of convective/absolute stability.