COMPUTATIONAL VISUALIZATION OF SEPARATED/REATTACHED TRANSITIONAL FLOWS ON A BLUNT PLATE

摘要

It is well known that large-scale organized motions, usually called coherent structures, exist in many transitional and turbulent flows (if not all). The topology and range of scales of those large-scale structures change widely from flow to flow such as counter-rotating vortices in wake flows, streaks and hairpin vortices in turbulent boundary layers. However, it is not well established what kind of large-scale structures exists in separated/reattached transitional flows. Large-eddy simulation with a dynamic subgrid-scale model is employed in the current study to investigate the physics of transitional separating/reattaching flows over a blunt plate held normal to a uniform stream. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6500. Statistics of the LES are found to be in acceptable agreement with the available experimental data in the laminar, transitional, and turbulent flow regions. The entire transition process leading to breakdown to turbulence has been shown by flow visualization and large-scale structures have been identified at different stages of the transition process of the separating/reattaching flow. The well-known hairpin vortices commonly associated with boundary-layer transition on a flat plate have been clearly shown at about the mean reattachment point and more large-scale structures are presented as well. The large-scale structures are noticed to persist a considerable distance downstream of reattachment before they eventually break into smaller-scale turbulent structures. In addition, it may shed light on the transition mechanisms and the nature of instabilities involved to understand the formation of these large-scale structures, their spatial and temporal evolution and eventual break-up into smaller structures.