Extreme Wall Shear Stress in High-Speed Lifting-Body Boundary Layer

Revealing the dynamics of wall shear stress in high-speed turbulent boundary layers is critical for improving the flight performance of advanced vehicles. This study investigated extreme events of wall shear stress on the windward surface of a lifting body at a freestream Mach number of 6 and a 2 deg angle of attack, using direct numerical simulation data. It was found that the mean and fluctuating skin-friction coefficients were significantly higher in the side region. Based on wavelet analysis, a strong correlation was unraveled between the extreme events and localized bursts of high-frequency energy, highlighting the critical role of near-wall fine-scale structures. Using conditional volumetric averaging and skin-friction decomposition, it was shown that wall-normal momentum transport, driven by counter-rotating quasi-streamwise vortex pairs, was the primary physical origin of extreme events. To identify the underlying high-order structures, conditional spatial proper orthogonal decomposition was performed. Notably, two basic types of coherent structure were identified: the streamwise single-layer vortex pair and the streamwise stacked vortex pair, which induced near-wall sweep/ejection motions that directly contributed to the formation of extreme events. Furthermore, modal reconstruction demonstrated that the leading mode contributed to the majority of extreme events, although this contribution was not correlated with the mode energy.