An experimental study of large-scale vortices over a blunt-faced flat plate in pulsating flow

Abstract

 Laboratory measurements are made of flow over a blunt flat plate of finite thickness, which is placed in a pulsating free stream, U=U _o (1+A _o cos 2πf _p t). Low turbulence-intensity wind tunnel experiments are conducted in the ranges of St _p≤1.23 and A _o ≤0.118 at Re _H=560. Pulsation is generated by means of a woofer speaker. Variations of the time-mean reattachment length x _R as functions of St _p and A _o are scrutinized by using the forward-time fraction and surface pressure distributions (C _p). The shedding frequency of large-scale vortices due to pulsation is measured. Flow visualizations depict the behavior of large-scale vortices. The results for non-pulsating flows (A _o =0) are consistent with the published data. In the lower range of A _o , as St _p increases, x _R attains a minimum value at a particular pulsation frequency. For large A _o , the results show complicated behaviors of x _R. For St _p≥0.80, changes in x _R are insignificant as A _o increases. The shedding frequency of large-scale vortices is locked-in to the pulsation frequency. A vortex-pairing process takes place between two neighboring large-scale vortices in the separated shear layer.