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 ratios. A naphthalene sublimation method is employed to measure the heat/mass transfer coefficients on the bottom wall. Acoustic excitations are forced on the separated 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 6.7 H is reduced by 0.7 to approximately 1.5 H when the flow is forced by acoustic excitation. The peak Sh is located about 1.1 H upstream from the reattachment point. For the 10 H cavity, the reattachment of separated flow, which is unclear for a natural case, is produced clearly by the acoustic excitation. For the 5 H cavity, the turbulence intensity and heat/mass transfer coefficient in the recirculation region are slightly enhanced.