Heat transfer characteristics in separated flows controlled by acoustic excitation

An experimental study has been conducted to investigate the heat/mass trans fer and flow characteristics for flow over a backward-facing step and cavit ies with various aspect ratios. A naphthalene sublimation method is employe d to measure the heat/mass transfer coefficients on the bottom wall. Acoust ic excitations are forced on the separated flow using a woofer speaker plac ed above the step edge for Strouhal numbers between 0.2 and 0.4. In the spe ctra 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 ste p, the reattachment length of about 6.7H is reduced by 0.7 similar to1.5H w hen 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 reattac hment of separated flow, which is unclear for a natural case, is produced c learly by the acoustic excitation. For the 5H cavity, the turbulence intens ity and heat/mass transfer coefficient in the recirculation region are slig htly enhanced.