Effect of a localized time-periodic wall motion on a turbulent boundary layer flow

The effect of a localized time-periodic wall motion on a turbulent boundary layer flow is experimentally investigated in this study. A part of the wall is replaced with a circular membrane (560 wall units in diameter) which moves up and down periodically in the wall-normal direction (56 wall units in amplitude) at three different frequencies, corresponding to one, two, and three times the bursting frequency, by a wall-deformation actuator. The phase-averaged mean velocity decreases and increases with some phase delay, respectively, when the wall moves up and down. At all the actuation frequencies investigated, the region of the mean-velocity decrease is more widely distributed than that of the mean-velocity increase, indicating that the upward wall motion affects the downstream flow more greatly than the downward wall motion. The effect of the wall motion persists in the downstream, but the recovery of the flow to an equilibrium boundary layer flow becomes slower at a lower actuation frequency. At relativ...