Extremely high wall-shear stress events in a turbulent boundary layer

The present work studies the fluctuating characteristics of the streamwise wall-shear stress in a DNS of a turbulent boundary layer at Re_τ=1500 from a structural view. The two-dimensional field of the fluctuating friction velocity u′_τ(x,z) is decomposed into the large-A nd small-scale components via a recently proposed scale separation algorithm, Quasi-bivariate Variational Mode Decomposition (QB-VMD). Both components are found to be dominated by streak-like structures, which can be regarded as the wall signature of the inner-layer streaks and the outer-layer LSMs, respectively. Extreme positive/negative wall-shear stress fluctuation events are detected in the large-scale component. The former's occurrence frequency is nearly one order of magnitude higher than the latter; therefore, they contribute a significant portion of the long tail of the wall-shear stress distribution. Both two-point correlations and conditional averages show that these extreme positive wall-shear stress events are embedded in the large-scale positive u′_τ streaks. They seem to be formed by near-wall 'splatting' process, which are related to strong finger-like sweeping (Q4) events originated from the outer-layer positive LSMs.