The impact of secondary mean vortices on turbulent separation in 3D Diffusers

Abstract

In rectangular ducts with fully-developed turbulent flow, mean vortices in the corners form secondary flow patterns whose energy contents is orders of magnitude lower than that of the flow in the streamwise direction. In the present numerical experiments, it is demonstrated using Large Eddy Simulations (LES) that these Mean Secondary Vortices (MSV) exert a profound influence on flow separation in three-dimensional asymmetric diffusers following such a duct. By removing, enhancing or switching the sense of rotation of the MSV in the inlet duct of two diffusers the shape, location and extent of separation zones farther downstream were drastically altered and, hence, the performance of the device. These results provide an explanation why eddy-viscosity based Reynolds-Averaged Navier-Stokes (RANS) models, that inherently cannot account for MSV, fail in predicting even the location of the separated flow in such diffusers.