How extreme wall shear stress events contribute to skin friction in compressible flows

The contribution of extreme wall shear stress (WSS) events to skin friction is investigated by direct numerical simulations of compressible turbulent channel flows at bulk Mach numbers of 0.7, 2.0, and 3.0. The results demonstrate that extreme positive events contribute appreciably to the skin friction, whereas backflow events exhibit a notably weaker effect. This behavior is explained by locally intensified Reynolds shear stress and the consequent enhancement of turbulent momentum transfers near the wall. Furthermore, a clear Mach-number dependence is observed. As the Mach number increases, both the probability and the intensity of extreme WSS events decline, thereby reducing their overall contribution to skin friction.