On the structure of streamwise wall-shear stress fluctuations in turbulent channel flows

A growing body of studies in wall-bounded turbulence has shown that the generation of wall-shear stress fluctuations is directly connected with outer-layer large-scale motions. In the present study, we investigate the scale-based structures of the streamwise wall-shear stress fluctuations (tau'(x)) in turbulent channel flows at different Reynolds numbers. The wall-shear stress structures are identified using a two-dimensional clustering methodology, and two indispensable factors, scale and sign, are considered for the analysis. The structures are classified into positive and negative families according to the sign of tau'(x). The statistical properties of the structures, including geometrical characteristics, spatial distribution, population density, fluctuating intensity, and correlations with outer motions are comprehensively investigated. Particular attention is paid to the asymmetries between positive and negative structures and their connection with wall-attached energy-containing eddies. In virtue of our results, only the large-scale structures of negative tau'(x) contain the footprints of the inactive part of wall-attached eddies populating the logarithmic region.