Conditional Reynolds Stress in the Turbulent Near-wall Region onSmooth and Micro-grooved Surfaces

摘要

Measurements in a zero pressure gradient turbulent boundary layer over a V-shaped micro-grooved surface and a smooth one have been performed using a two-component LDV. The experimental results for the smooth surface are in good agreement with the existing data. Data obtained in the near-wall region above the valley of the grooved surface are compared with those above the smooth surface. The mean velocity profiles show upward shift in the log-law region for the micro-grooved case. Turbulent intensities and turbulent kinetic energy as well as the Reynolds shear stress above the grooves are mitigated. Reynolds shear stress decomposition using quadrant analysis method indicates that grooved surface in drag reduction configuration weakens ejection and sweep events of the turbulent coherent motions, and decrease the burst frequency of low speed streaks. As a result, the momentum and kinetic exchange are impeded, which may serve as a clue for unveiling the drag reduction mechanism by grooves.