Turbulent Drag Reduction by Surface Plasma Through Spanwise Flow Oscillation

An experimental study has been undertaken to reduce skin-friction drag by applying a spanwise oscillation in the near-wall region of a turbulent boundary layer using RF glowdischarge surface plasma. Measurements were taken using hot-wire anemometry at Reτ = 380. The oscillatory plasma forcing was created using two opposing sets of asymmetric plasma actuators. By alternately activating the electrodes, skin-friction drag was reduced by up to 45% in the downstream of the actuators. The plasma action creates a tangential force very close to the wall surface, which creates alternating, co-rotating streamwise vortices in the inner region of the boundary layer. A 40% reduction in mean streamwise velocity and a 30% reduction in turbulent intensity were observed for y < 30. This was accompanied by over 50% reduction in sweep duration and a 30% reduction in sweep intensity. It is suggested that in addition to the spanwise oscillation drag reduction mechanism, the plasmainduced streamwise vortices may be interacting with the naturally occurring quasistreamwise vortices, thus disrupting the turbulence production cycle and leading to weakened near wall events.