Direct numerical simulation of flow separation behind a rounded leading edge: Study of curvature effects

The separation bubble formed over a 2D half-body is studied by direct numerical simulation. The aim of this work is to consider the physical influence of the shape of the body that can be viewed as a thick half-plate with a front edge more or less rounded. The present generic body geometry is defined with a unique parameter η = R / H corresponding to the ratio of the curvature radius R of the front edge over the body height H. In this paper, 18 calculations are presented depending on: (i) the value of η with η = 0.125 , 0.25 , 0.5 , 1 ; (ii) the 2D/3D nature of the computation; (iii) the inflow perturbations used to mimic residual turbulence in the free stream velocity U ∞ . Only one Reynolds number Re = U ∞ H / ν is used for every simulation, allowing us to focus on the curvature effects over the separation bubble dynamics. The value of the Reynolds number ( Re = 2000 ) combined with the resolution demand of the front edge (close to a sharp corner for the highest curvature case) requires to simulate the flow using up to 876 million mesh nodes. The curvature effects are found to deeply influence the separation bubble dynamics, with a significant expansion of the separated region size predicted by 3D computations. This expansion is driven by the separation angle rise combined with the reinforcement of turbulence levels as the curvature is increased. These trends are associated with a change of bubble sensitivity with respect to upstream/downstream perturbations that can be interpreted in terms of convective/absolute stability.