Measurements of the wall-shear stress distribution in turbulent channel flow using the micro-pillar shear stress sensor MPS3

Abstract

This study investigates the wall-shear stress (WSS) distribution in turbulent channel flow at friction Reynolds numbers of Reτ=860 and 1300 using the micro-pillar shear-stress sensor (MPS3). The probability density functions and the joint probability density functions of the wall-shear stress vectors indicate that the intermittency of the flow increases for higher Reynolds numbers. Extreme events occurring at the wall, such as wall-normal velocity spikes and backflow events, are detected and analyzed based on the wall-shear stress patterns. The events representing the wall-normal spikes are conditionally sampled from the wall-normal velocity component, which is determined from the local wall-shear stress gradients. The results show that the negative velocity spikes tend to co-occur with strong streamwise wall-shear stress motions, and that the positive spikes are likely to accompany large spanwise motions. Rare backflow events are detected from the wall-shear stressdistribution for both Reynolds numbers, serving as an experimental evidence of near-wall flow reversal events in turbulent channel flow. The diameter of the detected backflow region is approximately 20 viscous units. The data confirm the results from previous numerical studies. The wall-normal velocity spikes and the backflow events are likely to be three-dimensional, suggesting an energy and momentum transfer between the viscous sublayer and the outer part of the turbulent channel flow.