Characteristics of drag-reduced turbulent boundary layers with pulsed-direct-current plasma actuation

Abstract

Abstract Experiments were performed using an active flow control approach that has shown the ability to significantly reduce the viscous drag in turbulent boundary layers. The purpose of this work was to document the changes in the turbulence characteristics of the boundary layer with the drag reduction. The flow control involved generating a steady spanwise velocity component of the order of $u_{\\tau }$, within the sublayer using an array of pulsed-DC plasma actuators. The intent was to reduce the wall-normal vorticity component, $\\omega _y$, that is associated with the mean flow distortion caused by quasi-steady streamwise vorticity associated with the wall streak structure first observed by Kline et al. (J. Fluid Mech., vol. 30, 1967, pp. 741–773). The significance of the $\\omega _y$ comes from Schoppa & Hussain (J. Fluid Mech., vol. 453, 2002, pp. 57–108), who proposed an autonomous mechanism for self-sustained wall turbulence generation of which the sublayer wall-normal vorticity component is a critical parameter. The results document the characteristics of a turbulent boundary layer in which the viscous drag was reduced by 68 %. This involved measurements of the $u$ and $v$ velocity components in a three-dimensional region within the boundary layer using a pair of dual (X) hot-wire probes. Under the reduced drag, these documented a decrease in $u$ and $v$ turbulence intensity levels through most of the boundary layer. When scaled by $u_{\\tau }$, the impact on the $v$ fluctuations was larger than that on the $u$ fluctuations. Analysis based on [$uv$] quadrant splitting documented a decrease in duration, and an increase in the time between ‘ejections’ (Q2) and ‘sweep’ (Q4) events that substantially lowered the near-wall turbulence production in the drag-reduced boundary layers. Conditional averages used to reconstruct the two- and three-dimensional coherent motions including $\\lambda _2$ vortical structures, indicate a suppression of coherent features in the wall layer. These results are consistent with an underlying mechanism for drag reduction that comes from a suppression of the turbulence producing events in the wall layer associated with the wall streak structure.