Measurement of wall-shear stress via micro-particle tracking velocimetry

Micro-Particle Tracking Velocimetry （µ-PTV） was used to measure the flow field near the wall with high spatial resolution. By analyzing the velocity distribution of the viscous sublayer, the wall-shear stress can be analyzed by one-time linear regression. The wall-shear stress of the turbulent boundary layer at different Reynolds numbers based on momentum loss thickness was measured and the flow structure of reversal flow events was obtained at Re_θ=1200. The results show that the micro-particle tracking velocimetry technology can accurately measure the wall-shear stress and the measurement error of wall-shear stress is below 2% for Re_θ=1634–4070. Furthermore, the probability of reversal flow is extreme low, i.e., about 0.05% at Re_θ=1200. The measured spatial scale of reversal flow structures is around 8×30 wall units, and therefore the measurement of reversal flow events requires high spatial resolution of the measurement technology. The results show that the reversal flow events occur with the appearance of strong spanwise vortices near the wall.es near the wall.