Flat plate drag reduction using plasma-generated streamwise vortices

We present an experimental study of a turbulent boundary layer (TBL) control on a flat plate using plasma actuators. Three different configurations of the actuators produce spanwise arrays of large-scale streamwise vortices (LSSVs). An ultra-high-resolution floating element (FE) force balance, developed in house and calibrated using mu-particle tracking velocimetry, is employed to measure wall friction. The FE captures a drag reduction (DR) of up to 26% on the FE area (667x1333 wall units), downstream of the actuators. The local DR persists downstream, well after the LSSVs disappear. Both plasma-generated flow and the TBL under control are compared with an uncontrolled TBL. The maximum DR takes place when the LSSVs producing wall jets reach a spanwise velocity of 3.9 in wall units. The flow is altered by up to 29% of the TBL thickness, with a drop in the new vortices due to the control-induced stabilization of the wall streaks. The local friction is characterized by three distinct spatial regions of drag increase, pronounced DR and drag recovery - all connected to the LSSVs. The LSSVs push the streaks to the middle between two adjacent actuators, suppressing transient growth and near-wall turbulent production. A DR mechanism is proposed.