Effects of thermal non-equilibrium during transition to turbulence of a high enthalpy free shear layer

Under the high-enthalpy conditions encountered in hypersonic flight, non-equilibrium effects from vibrational excitation and chemical reactions complicate the prediction of transition to turbulence and the developed turbulent state. This study assesses the impact of thermal non-equilibrium on turbulence properties for a canonical compressible mixing layer and evaluates the influence of vibration modeling approaches, particularly the difference between using a single vibration energy equation vs separate equations for each molecular species. Preliminary two-dimensional simulations reveal two distinct patterns of thermal non-equilibrium dependent on temperature: one characterized by thermally hot vortex cores and the other by a combination of hot and cold regions around developing vortices. Chemical non-equilibrium effects are minimal, as the timescales of chemical reactions are significantly shorter than those of the flow. The importance of intermolecular vibration modeling is highlighted by comparing results from a single fully coupled vibration equation with an artificial limiting case of three separate vibration equations without intermolecular coupling. Three-dimensional direct numerical simulations show trends consistent with the two-dimensional cases, with hot vortex cores forming in the low-temperature case and more complex hot–cold patterns surrounding vortex tubes when thermal and flow timescales are comparable. At later stages of breakdown and turbulence decay, differences between cases are enhanced by amplification of small perturbations, i.e., non-linear flow dynamics that result in symmetry breaking, and cannot be attributed directly to non-equilibrium effects. Nevertheless, turbulence statistics show that increased thermal non-equilibrium is correlated with increased translational temperature fluctuations, related to variation in mean translational temperature across the shear layer.