Cooling-gas injection is crucial to the thermal protection of hypersonic vehicles. As a commonly used coolant, air contains N₂ and O₂, which exhibit different chemical reactivities under temperature conditions where N₂ dissociation is not yet significant. Understanding their behavior in high-enthalpy turbulent boundary layers is of practical importance for vehicle thermal management under realistic flight conditions. Using OpenCFD-Comb, an open-source high-order solver for chemically reacting turbulence developed by our group, direct numerical simulations are performed to investigate species evolution and turbulence–chemistry interactions in a Mach 10 high-enthalpy turbulent boundary layer over a flat plate, with O₂ injected through a slit in the fully turbulent region. Pure O₂ is injected at the free-stream temperature with a blowing ratio of 0.001. Results show that the injection creates a localized oxygen-rich zone downstream, leading to a non-monotonic O₂ distribution and significantly enhanced production of atomic oxygen. Mass-fraction fluctuations of O₂ and N₂ are amplified, while those of N and NO are weakened. The injection strengthens chemical reactions involving O₂, O and NO, but has limited effect on the relatively inactive N₂ and N. Compared with species fluctuations, turbulence–chemistry coupling is more sensitive to temperature fluctuations.

The evolution characteristics of the mean skin friction beneath the supersonic turbulent boundary layer that interacts with incident shock waves at Mach 2.25 are analyzed using Direct Numerical Simulation (DNS). The separated and attached boundary layers in the interaction region that respectively correspond to 33.2° and 28° incident shock angles are considered. The mean skin friction recovery rate for the separated boundary layer is much gentler and distinctly less than that for the attached case where the skin friction completes its recovery within one boundary layer thickness. The novel mean skin friction decomposition method for compressible flows proposed by the recent research is applied in the interaction region to investigate the internal evolution characteristics quantitatively. The results reveal that the three decomposition components are distinctly unequal between the two cases. The contributions of the turbulent motions at different scales to the associated term are focused on using empirical mode decomposition technology. It indicates that the outer large-scale structures dominate separation and reattachment regions, while contributions from inner small-scale structures are limited. In contrast, contributions from the outer large-scale structures are dramatically reduced in the attached case, which results in the outer large-scale and inner small-scale motions being of equal importance.