Wall shear stress and wall heat flux in a supersonic turbulent boundary layer subjected to concave surface curvature

Direct numerical simulation is employed to investigate the characteristics of wall shear stress (WSS) and wall heat flux (WHF) beneath a spatially developing supersonic turbulent boundary layer with a cold-wall thermal condition subjected to a concavely curved compression ramp with a deflection angle of 24 degrees. The Mach number is 2.25 and the friction Reynolds number is 769. Numerical data are exploited to characterize the effect of the concave surface curvature on the statistical and structural properties of the fluctuating WSS and WHF, including the probability density function, space-time correlations, and frequency spectra. Across the curved region, the occurrence probability of extreme negative events is strongly affected by the curvature, as manifested by a noticeable dissimilarity in the large negative tails of the probability density functions. The correlation results show that the streamwise extent of the WSS fluctuations becomes smaller, in contrast to that of the WHF fluctuations, and a considerable decrease in the convection velocity is observed downstream from the curvature. It is found that the curvature qualitatively modifies the WHF frequency spectra, leading to a shift to lower frequencies, while the spectral alternation is less pronounced for the WSS. Importantly, the mean WHF and WSS are also decomposed into different physically informed components, with the finding that the mean WSS generation is fundamentally changed by the curvature but the generation mechanism of the mean WHF is constant throughout the concave surface. Finally, an analysis of velocity and temperature structures is performed with the aid of bidimensional empirical mode decomposition to quantitatively demonstrate the contributions of specific spanwise length scales. We highlight that the outer large-scale organized structures, which are significantly energized by the concave surface, make an increasingly important contribution to the mean WSS and WHF generation.