Characterizing Boundary Layer Mechanisms and Recovery Following Viscous Drag Reduction

Experiments were performed to document changes in and recovery of the turbulence characteristics of a boundary layer whose viscous drag has been reduced by 62%. The drag reduction involves generating a steady spanwise velocity component on the order of uτ, within the sublayer using an array of pulsed-DC plasma actuators. Emphasis is placed on changes in coherent 3-D vortical motions associated with near-wall turbulence production that result from the drag reduction and the temporal response of these structures during and following drag reduction. This involves measurements of all three velocity components in a 3-D region within the boundary layer using a pair of dual [u, v] and [u, w] hot-wire sensors during and following actuation shutoff as the boundary layer recovers to its baseline state. Under reduced drag, these documented a decrease in the turbulence intensity levels of all three velocity components through most of the boundary layer. Conditional sampling is used to construct the coherent 3-D vortical structures associated with “burst-sweep” events. These revealed a reduction of the wall normal vorticity component, ωy, associated with the mean flow distortion caused by quasi-steady streamwise vorticity associated with the wall streak structure[Kline et al., 1967]. The significance of the ωy component comes from Schoppa and Hussain [2002] who proposed it to be a critical parameter in an autonomous mechanism for self-sustained wall turbulence generation. The results support this mechanism in both the observed suppression of the near-wall turbulence producing events, and the resulting decrease in the viscous drag. After quantifying the drag-reduced boundary layer in the drag-reduced space, the temporal response of terminating the actuation was studied to determine how physical properties such as the wall-layer streamwise velocity fluctuations, frequency of “burst-sweep” events, spatially-averaged ωy levels, and the y+ center return from the drag-reduced state to the baseline levels.