Direct numerical simulation of a strongly reacting turbulent hypersonic boundary layer in chemical nonequilibrium

The combined presence of leading-edge shock waves together with near-wall viscous dissipation activates several high-enthalpy effects in hypersonic boundary layers, including both vibrational excitation and molecular dissociation. With chemical processes proceeding at finite rates, on time scales comparable to that of turbulent mixing, the thermodynamic fluctuations introduced by eddy motions result in significant interaction between the thermochemical and hydrodynamic fields. In order to further characterize this turbulence-chemistry interaction, we present a direct numerical simulation of a reacting high-Mach turbulent boundary layer undergoing significant chemical activity. Leading-edge effects and the resultant near-wall recombination layer are considered with a five-species chemical mechanism. The direct numerical simulation demonstrates that the breakdown to turbulence is accompanied not only by the expected increase in heat flux, but also by a considerable variation in the chemical composition within the boundary layer. Implications for reduced-order modeling of turbulent reacting hypersonic boundary layers are assessed, and a detailed analysis of the closure problem for the modeling of subgrid chemistry in the context of large-eddy simulation is presented.