Chemistry similarity in turbulent hypersonic boundary layers*

Vehicles traveling at hypersonic speed experience intense mechanical and thermal stresses due to aerodynamic effects, which become significantly more severe as the flow transitions to turbulence. At high Mach numbers, the elevated temperatures trigger thermochemical phenomena, leading to the coupling of chemical dissociation with turbulent dynamics.

Despite recent advances in hypersonic wall-bounded turbulent flows simulations, the lack of data still prevents drawing broad conclusions about universal behavior in high-enthalpy regime. For instance, self-similar solutions, where flow characteristics remain unchanged under certain scale transformations, offer crucial insights into the fundamental aspects of hypersonic boundary-layer phenomena. Currently, it is understood that the introduction of additional timescales due to chemistry restricts the solution to being only locally self-similar in laminar conditions. With this study, we aim at investigating these similarity properties when chemical reactions compete with turbulent mixing.

To this purpose, we employ direct numerical simulations of several flat-plate turbulent boundary layers, integrating a simplified dissociation-recombination mechanism within a two-species gas mixture. By manipulating the reaction rates, we explore various degrees of chemical activity. Additionally, we compare the reference results with companion simulations, where chemical reactions are activated downstream in the fully turbulent region, aiming to observe whether the mean profiles collapse to the reference and assess chemistry similarity properties.