Chemistry similarity in turbulent hypersonic boundary layers

This study examines the similarity properties of hypersonic turbulent boundary layers using direct numerical simulations within a two-species mixture, composed of molecular and atomic oxygen. A dissociation–recombination mechanism is considered, at varying reaction rates. The results show that while the hydrodynamic field remains largely unaffected by changes in reaction rates, temperature profiles are slightly altered, with faster reactions leading to lower temperature peaks. The chemical mechanisms significantly influence the wall heat flux, with frozen chemistry overestimating the flux. The reference simulations are compared with companion calculations, where chemical reactions are activated downstream within the fully turbulent region. These calculations represent set-ups in which the computational domain effectively starts with an inflow in a fully turbulent state, where hydrodynamic and thermal quantities are accurately described at the boundary and the chemical inflow profile is derived from a frozen-chemistry assumption. In this set-up, chemical source terms rapidly relax towards the baseline downstream of the chemistry activation location. This behaviour is due to an approximate global self-similarity shown by the chemical species transport in the fully turbulent region. Unlike laminar boundary layers where streamwise fluxes are relevant, source terms are balanced only by wall-normal transport in the turbulent region. A chemical relaxation length scale is introduced to collapse the results of all mechanisms.