Efficient flow stability analysis method for real gas

Transitions within the boundary layer significantly affect the aerodynamic and aerothermodynamic dynamics of hypersonic vehicles. Accurately predicting these transitions poses a significant challenge in vehicle design. At high speeds and altitudes, thermochemical processes within the hypersonic boundary layer lead to real gas effects that alter flow stability and further complicate transition prediction. Direct numerical simulation and linear stability theory are used to investigate the effects of chemical reaction–induced terms on the second and cross-flow modes, and to identify the main sources of species disturbances. Efficient stability analysis method for real gas is developed by applying multilevel assumptions to the linear stability equation. The results indicate that at lower wall temperatures, species disturbances primarily arise from convective terms, and there is a continuous contribution from chemical reaction source terms. The contributions of the diffusion and chemical source terms to species disturbances increase with the intensity of chemical reactions. When the nitrogen within the boundary layer is not dissociated or is only weakly dissociated, the assumption of complete freezing of the species disturbances can be employed to enhance the computational efficiency of the linear stability analysis. Chemical non-equilibrium linear stability theory based on the freezing assumption is suitable for most experimental and flight conditions, significantly reducing the computational time for real gas transition predictions, making it comparable to that for perfect gas.