Effects of free-stream turbulence on large-scale coherent structures of separated boundary layer transition

Abstract

It has been well established that large-scale structures, usually called coherent structures, exist in many transitional and turbulent flows. The topology and range of scales of those large-scale structures vary from flow to flow such as counter-rotating vortices in wake flows, streaks and hairpin vortices in turbulent boundary layer. There has been relatively little study of large-scale structures in separated and reattached transitional flows. Large-eddy simulation (LES) is employed in the current study to investigate a separated boundary layer transition under 2% free-stream turbulence on a flat plate with a blunt leading edge. The Reynolds number based on the inlet free stream velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed to compute the subgrid-scale stresses more accurately in the current transitional flow case. Flow visualization has shown that the Kelvin–Helmholtz rolls, which have been so clearly visible under no free-stream turbulence (NFST) are not as apparent in the present study. The Lambda-shaped vortical structures which can be clearly seen in the NFST case can hardly be identified in the free-stream turbulence (FST) case. Generally speaking, the effects of free-stream turbulence have led to an early breakdown of the boundary layer, and hence increased the randomization in the vortical structures, degraded the spanwise coherence of those large-scale structures. Copyright © 2005 John Wiley & Sons, Ltd.