On Coherent Structures in a Separated/Reattached Flow

It is well known that large-scale organised motions, usually called coherent structures, exist in many transitional and turbulent flows. The topology and range of scales of coherent structures change widely in different flows. However, it is not well established what kind of large-scale coherent structures exists in separated/reattached transitional flows. Large Eddy Simulation (LES) with a dynamic subgrid-scale model is employed to investigate a separated boundary layer transition under 2% free stream turbulence level and without free stream turbulence on a flat plate with a blunt leading edge. Flow visualization is employed to show the entire transition process leading to breakdown to turbulence and large-scale coherent structures have been identified at various stages of the transition process. However, there are some noticeable difference between the flow case with and without free stream turbulence. The Kelvin-Helmholtz rolls, which are clearly visible under no free stream turbulence (NFST), are not so clearly visible under 2% free steam turbulence (FST) case. The Lambda-shaped vortical structures which can be clearly seen in the NFST case can hardly be identified in the FST case. Generally speaking, the effects of free-stream turbulence have led to an early breakdown of the separated boundary layer and hence increased the randomisation in the vortical structures, degraded the spanwise coherence of those large-scale structures.