FREQUENCY EFFECT OF AN OSCILLATING PLATE IMMERSED IN A TURBULENT BOUNDARY-LAYER

A phase-averaging technique was employed to investigate flow structures downstream of an oscillating bluff plate immersed in a turbulent boundary layer. The Reynolds number based on the maximum plate height was kept at 7.8 x 10(3). The reduced frequency of the oscillating plate was varied in a range from 0.002 to 0.013. Experimental results obtained clearly indicate that a critical reduced frequency near 0.009 can be identified, which distinguishes two types of flows of different characteristics. Under the subcritical condition (i.e., the reduced frequency less than 0.009), the flow shows a quasisteady behavior, whereas under the supercritical condition, the disturbance generated by the oscillating plate is accumulated behind the plate as the plate extends into the flow, then is released downstream as the plate retracts. A physical model concerning the now development immediately behind the oscillating plate is proposed, which estimates that the critical reduced frequency is about 0.013, 50% higher than the experimental finding.