Optical Measurement and Statistical Analysis of Instantaneous Wall-Shear Stress in a Turbulent Boundary Layer

The near-wall PIV technique was utilized to measure the instantaneous wall-shear stress (WSS) in a smooth-wall turbulent boundary layer at low-to-moderate Reynolds numbers. The algorithm of single-row cross correlation, together with a newly proposed iterative fitting method, was used to estimate the WSS. The spatial resolution of WSS was about 2~4 pixel in the wall-normal direction. Existing direct numerical simulation datasets in the same Re_τ range were also analyzed for direct comparison. The fluctuating intensity of WSS was found to follow the empirical log-law correlation with Re_τ, indicating the existence of the influence of outer-layer large-scale or very-large-scale motions (LSMs/VLSMs) on the wall. A further spectra analysis indicates that the near wall low-speed streaks dominate the dynamic behavior of the WSS. Combined with weak Re-dependency of both the skewness and probability density function of fluctuating WSS, it can be inferred that the LSMs/VLSMs leave a mild footprint on the wall. Instead, the amplitude modulation effect is inferred to play a more prominent role in affecting the statistics and multi-scale characteristics of WSS.