Heat Transfer Characteristics in Separated Flows Controlled by Acoustic Excitation

An experimental study has been conducted to investigate the heat / mass transfer and flow characteristics for flow over a backward-facing step and cavities with various aspect rations. A naphthalene sublimation method is employed to measure the heat / mass transfer coefficients on the bottom wall. Acoustic excitations are forced on the spearated flow using a woofer speaker placed above the step edge for Strouhal numbers between 0.2 and 0.4 In the spectra of streamwise velocity fluctuations for the flow excited at St_H=0.2, a sharp peak appears at the forcing frequency. For the backward-facing step, the reattachment length of about 6.7H is reduced by 0.7〜1.5H when the flow is forced by acoustic excitation. The peak Sh is located about 1.1H upstream from the reattachment point. For the 10H cavity, the reattachment of separated flow, which is unclear for a natural case, is produced clearly by the acoustic excitaition. For the 5H cavity, the turbulence intensity and heat / mass transfer coefficient in the recirculation region are slightly enhanced.