An experimental study of large-scale vortices 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-0(1 + A(0) cos 2 pif(p)t). Low turbulence-intensity wind tunnel experiments are conducted in the ranges of St(p) less than or equal to 1.23 and A(0) < 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 Stp and A, 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(0) = 0) are consistent with the published data.. In the lower range of A(x), as St(p) increases, x(R) attains a minimum value at a particular pulsation frequency. Far large A(0), the results show complicated behaviors of x(R) For St(p) <greater than or equal to> 0.80, changes in x(R) are insignificant as A(0) 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.