On the physical mechanism of turbulent boundary layer drag reduction under pulsed-dc plasma actuation

Experiments are reported which utilize a novel method of active flow control explicitly designed to intervene in the process of near wall streamwise vortex generation, which is a key component in turbulence production in wall-bounded flows. The flow control method utilizes flush mounted pulsed-DC plasma actuator arrays in a ZPG turbulent boundary layer at Re_τ = 3200. The control flow produced by the actuator consists of a series of near-wall, spanwise opposed wall jets. These have been shown to produce significant reductions in turbulent drag. The focus of the reported experiments is to clarify the mechanism of drag reduction. Hot-wire measurements utilizing the quadrant splitting technique are performed downstream of the actuator. These are used to characterize and contrast both the duration of & time interval between quadrant 2 & 4 events in the actuated and non-actuated flows. The quadrant contributions to the Reynolds stress are compared for natural and actuated cases. The C_f for the actuated flow is computed using the FIK identity. By performing measurements over a range of distances downstream of the actuator, the characteristic streamwise distance over which the near wall organized structures relax back to the natural condition is also determined.