Near-Wall Flow Statistics in High- Re_τ Drag-Reduced Turbulent Boundary Layers

Manipulating the organized flow in the near-wall region of a turbulent boundary layer is a direct path to achieving skin-friction drag reduction. However, near-wall flow measurements in high Reynolds number ( Re_τ ) wall flows can be challenging, due to this region’s small physical size and measurement resolution issues. The present study demonstrates the capability of hot-wire (HW) and stereoscopic particle image velocimetry (PIV) techniques of accurately estimating the trends of near-wall flow statistics in high- Re_τ drag-reduced turbulent boundary layers. The drag reduction strategy considered involves imposition of streamwise travelling waves of spanwise wall oscillations, well known for attenuating the drag-producing near-wall streaks via unsteady cross-flow straining. A flow phase identification methodology is proposed, based on real-time tracking of the wall-oscillation cycle, to estimate the near-wall phase-based statistics from PIV experiments. This methodology is leveraged to investigate phase-specific orientations of the near-wall flow features, which have been shown in the literature to mimic the characteristics of the shear strain vector, dictating the efficacy of this drag reduction scheme. Reconciliation of the HW and PIV measurements demonstrates that the trends exhibited by higher-order moments of the near-wall streamwise velocity fluctuations, with increasing drag reduction, are representative of the inherent flow physics of the drag-reduced flow. Apart from assisting with the design of high- Re_τ experiments over drag-reducing devices (riblets, plasma actuators, etc.), the present outcomes also inform high- Re_τ studies in more general three-dimensional wall flows.