On The Physical Mechanism of Turbulent Boundary Layer Drag Reduction Under AC-DBD Plasma Actuation

The results of a series of experiments are reported which use near-wall active flow control designed to intervene in the process of streamwise vortex (SWV) generation, which is primarily responsible for turbulence production in wall-bounded flows. The flow control method utilizes an array of flush mounted AC-DBD plasma actuators in a ZPG TBL over the range of Re$_{\mathrm{\tau }}=$550-1750. The control flow consists of a series of near-wall, span-wise oriented unidirectional wall jets with velocity comparable to the friction velocity and has been shown to produce significant reductions (around 20{\%}) in drag. The control flow is fully characterized using PIV. The span-wise wall jets inhibit the formation of near-wall SWVs {\&} thus reduce the turbulence production. This manifests itself in the reduction of near wall turbulent Reynolds stress producing events. The focus of the reported experiments is to further clarify the~mechanism of drag reduction. X-wire measurements utilizing the quadrant splitting technique are performed downstream of the actuator. These are used to characterize {\&} contrast both the duration of {\&}~time interval between quadrant 2 {\&} 4 events in the actuated {\&} non-actuated flows. The quadrant contributions to the Reynolds stress are compared for natural {\&} actuated cases. Effort has been made to correlate the observed drag reduction {\&} the change in Reynolds stress profile. The turbulence statistics have also been compared to similar statistics obtained from a ZPG TBL under pulsed-DC plasma actuation where even higher drag reduction was achieved.