The generation of skin friction in a spatially evolving turbulent boundary layer at a Mach number ${\text{Ma}}_{\infty }=2.9$ is analyzed using direct numerical simulation (DNS). Particular attention is paid to clarifying the impact of the turbulent/nonturbulent interface (TNTI) height on the skin friction. In contrast to the RD identity N. Renard et al. [J. Fluid Mech. 790, 339 (2016)] and its extension to compressible flow W. P. Li et al. [J. Fluid Mech. 875, 101 (2019)] we decompose the instantaneous skin friction into four physics-informed contributions. Taking advantage of this novel decomposition method, the conditional statistics contributing to the skin friction near the TNTI can be analyzed. The instantaneous height of the TNTI follows a Gaussian distribution. At a moderate Reynolds number, the height of the TNTI shows no statistical correlation with the skin friction. More specifically, in the vicinity of the TNTI, the contributions of the viscous dissipation and the spanwise inhomogeneity are close to zero. Both the material derivative term and the streamwise inhomogeneity term are mainly determined by the production of the pressure gradient and the streamwise velocity, but these two terms approximately counterbalance each other. This numerical study contributes to understanding the generation of the skin friction in a supersonic turbulent boundary layer, which is crucial in controlling aerodynamics drag in aerospace engineering.

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• Compressible boundary layers
• Friction
• Turbulence