Influence of thermal non-equilibrium on homogeneous turbulence in a chemically reacting air mixture

This study investigates the role of vibrational and chemical non-equilibrium mechanisms in the evolution of pressure-Hessian and velocity gradient tensors in high-temperature compressible turbulence. Specifically, it focuses on reacting air mixtures relevant to aerospace applications. Understanding these mechanisms is essential for accurately predicting turbulent flows encountered during atmospheric re-entry of spacecraft and cruise flights of hypersonic vehicles. We employ direct numerical simulation (DNS) of isotropic compressible decaying turbulence using the hy2Foam solver on the OpenFOAM platform, with detailed finite-rate chemistry and vibrational energy exchanges among five species (N ₂, O ₂, NO, N and O). Our findings reveal that vibrational and chemical non-equilibrium mechanisms do influence the statistics of turbulent flows in a reacting air mixture. Specifically, chemical non-equilibrium processes associated with species production dominate the evolution of the pressure-Hessian tensor in air mixtures. Vibrational non-equilibrium, significant in a nitrogen-only flow, becomes insignificant in a reacting air mixture. Additionally, air mixture interactions result in an increase in the vortical fluctuations and a decrease in the dilatational fluctuations, along with a reduction in the strength of the pressure-Hessian tensor relative to the velocity gradient tensor. These results highlight the importance of accurately modelling chemical and vibrational non-equilibrium mechanisms in high-temperature compressible turbulent flows.