Influence of Thermal Non-Equilibrium on Compressible Turbulence

This study explores how vibrational and chemical non-equilibrium affect the behaviour of turbulence at high temperatures, particularly focusing on flows relevant to aerospace applications. We examine how these effects influence the evolution of two key turbulence properties: the pressure-Hessian tensor and the velocity gradient tensor. Understanding these effects is crucial for accurately predicting turbulent flows during atmospheric re-entry of spacecraft or hypersonic vehicle flights.

We use direct numerical simulations (DNS) of isotropic (uniform in all directions), compressible turbulence that decays over time. These simulations are performed using the hy2Foam solver within the OpenFOAM software, incorporating detailed chemical reactions and energy exchanges involving five chemical species: nitrogen (N₂), oxygen (O₂), nitric oxide (NO), atomic nitrogen (N), and atomic oxygen (O).

Our results show that both vibrational and chemical non-equilibrium have important effects on turbulence statistics in reacting air mixtures. Chemical reactions, particularly those leading to new species formation, strongly influence the pressure-Hessian tensor's evolution. In contrast, vibrational non-equilibrium, which is important in nitrogen-only flows, becomes less important in reacting air mixtures. Additionally, interactions within the air mixture increase the vortical fluctuations and decrease the dilatational fluctuations. This interaction also reduces the strength of the pressure-Hessian tensor compared to the velocity gradient tensor.

These findings emphasise the necessity of accurately modelling chemical reactions and vibrational energy exchange processes when studying high-temperature compressible turbulent flows.