Turbulence–chemistry interactions in high-enthalpy hypersonic turbulent boundary layer

Direct numerical simulations of spatially developing high-enthalpy hypersonic turbulent boundary layers are performed under non-catalytic wall-cooling conditions. Two distinct levels of chemical reactivity are considered, together with a chemically frozen case for reference. The main objective is to elucidate the underlying mechanisms of high-enthalpy turbulence–chemistry interactions and to develop reliable closure models for high-enthalpy hypersonic turbulent boundary layers. It is well established that high-enthalpy reactions have a negligible influence on the turbulent velocity statistics, and it is further shown here that this behaviour arises from the compensating effects. Specifically, the mean temperature is reduced by endothermic reactions, accompanied by a decrease in mixture molar mass, leading to an approximately unchanged mean density. Furthermore, the chemical time scale is found to be much larger than the large-eddy turnover time, resulting in strong linear correlations between the temperature and species mass fraction fluctuations. Consequently, the refined strong Reynolds analogy (RSRA) can be effectively extended from enthalpy to temperature and species mass fraction fluctuations in high-enthalpy hypersonic turbulent boundary layers. A correlation correction for RSRA is proposed, which corrects the breakdown of the original RSRA in the region near the mean temperature peak, while retaining good performance in the outer layer. An isobaric presumed probability density function model incorporating the correlation-corrected RSRA is further developed, which extends the applicability of the temperature-based presumed probability density function approach to stronger high-enthalpy conditions. This work contributes to the development of engineering models for turbulence–chemistry interactions in high-enthalpy hypersonic turbulent boundary layers.