Investigation of vortex shedding in transitional separating ― reattaching flow on a blunt plate

摘要

It has been reported in transitional and turbulent separating-reattaching flows that different vortex shedding frequencies exist, in particular the so called low-frequency flapping. In addition it has been reported that a higher-frequency shedding also exists by Tafti and Vanka. It is quite debatable if these low and high frequency vortex shedding exist in all transitional and turbulent separating-reattaching flows. It is also not clear what the working mechanisms are behind the so called low-frequency flapping as reported by several researchers. The major purpose of the paper is trying to address these issues. Large-Eddy Simulation of the transitional flow over a blunt plate held normal to a uniform stream has been carried out to study the physics of separated boundary layer transition. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed to compute the subgrid-scale stresses more accurately in the transitional flow case where the subgrid eddy-viscosity should be zero in the laminar region and starts to increase in the transition region and eventually to full turbulent value. Statistics of the LES are found to be in acceptable agreement with the available experimental data. Time histories of three velocity components and pressure have been taken at many selected points in the flow field, concentrated mainly in the separation bubble region. Detailed processing of these data has been done by Fourier analyses and comprehensive results will be presented.