Effect of inflow turbulence on separational flow over smooth-wall axisymmetric hill

Abstract

The turbulent flow around an axisymmetric hill at a Reynolds number ReH∼1.3×105 with respect to the hill height is studied using two resolved large-eddy simulations (LES) aimed at examining the effect of turbulence in the inflow boundary condition on the flow around the hill. LESs were performed for the following inflow boundary conditions: (a) mean turbulent boundary-layer profile obtained experimentally; (b) turbulent velocity profiles from a precursor LES, with the mean profile of the precursor LES forced to agree with experimental results, all other flow parameters being identical in both cases. Comparisons with experimental results showed that the mean flow and turbulence statistics for the upstream side of the hill are correctly reproduced by both simulations. However, the complex separation dynamics in the lee-side of the hill is correctly predicted only in case (b). In case (a) the simulation yields a thicker and longer separation region, a recovery of the flow that occurs farther downstream, and a different vortex structure than was observed experimentally. In both cases the pressure gradient set by the geometry of the hill dictates the separation initiation through the formation of an internal layer. However, in case (b) the turbulence suppresses the separated layer, thereby causing early reattachment as in the experiment. The absence of this effect is responsible for the differences between the results obtained in case (a) and their experimental counterparts.