Characteristics of fluctuating wall-shear stress in a turbulent boundary layer at low-to-moderate Reynolds number

The present work deals with the characteristics of the streamwise component of the fluctuating wall-shear stress tau'(w) in a smooth-wall turbulent boundary layer. A near-wall two-dimensional time-resolved particle image velocimetry measurement was performed in a low-to-moderate Reynolds number range (Re-tau = 400 similar to 2900). The algorithm of single-row cross correlation, together with a newly proposed iterative fitting method, was used to estimate tau'(w) from instantaneous near-wall velocity fields at single pixel row resolution. Additionally, existing direct numerical simulation datasets in the same Rc(tau) range were also analyzed for direct comparison. The fluctuating intensity of tau'(w) is found to follow the empirical log-law correlation, i.e., sigma(+)(tau w) proportional to ln Re tau. This indicates the existence of the influence of outer-layer large-scale or very-large-scale motions (LSMs or VLSMs) on the wall. The following scale decomposition analysis suggests an inner-scaling nature of the cutting-off boundary between small- and large-scale components of tau'(w), the magnitude of which is lambda(+)(xc) approximate to 1000, equivalent to the mean streamwise length of near-wall streaky structures. Combining with the weak Re-dependency of both the skewness of tau'(w) and its convection speed, i.e., S tau(w) approximate to 1 and (u) over bar (+)(c) approximate to 12, it can be inferred that in the present studied Re range, LSMs or VSLMs only leave a mild footprint on the wall. Instead, the amplitude modulation effect is inferred to play a more prominent role in affecting both the statistics and the multiscale characteristics of tau'(w).