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.

The laminar-to-turbulent transition of boundary layers is crucial for the aerodynamic and aerothermal design of high-speed vehicles as it results in a significant increase in skin friction and wall heat flux. In high-enthalpy boundary layers, the transition process becomes even more intricate due to various thermo-chemical processes induced by the extremely high temperature. This study employs chemical non-equilibrium linear stability analysis with 5-species and 11-species atmospheric models to investigate blunt cone boundary layers under reentry conditions, focusing on the effects of Mach number and ionization on the boundary-layer stability. The results indicate that the transport coefficients of ionized air calculated using the Gupta-Wilke (GW) model exhibit significant errors. In contrast, the results of the GW model without considering electrons are more aligned with those of the Chapman-Enskog (CE) model. Under flight conditions, as the Mach number increases, the growth rate of the second mode decreases gradually, while the instability frequency range broadens. Ionization primarily affects the base flow near the nose. Specifically, ionization at the nose slightly reduces the growth rate of the second mode upstream but does not alter the overall N-factor, thereby having a negligible effect on the aircraft's transition prediction. Additionally, the computational efficiency of real-gas transition predictions can be improved by using a five-species model.

Hypersonic boundary layer transition has become a key factor to be considered in aircraft design, and is also a hot and difficult point in hypersonic flow research. There are three typical flow structures and instability characteristics in hypersonic three-dimensional boundary layer, namely cross flow, flow direction vortex and contact line, which have different mechanisms to induce transition. Three global stability analysis methods (BiGlobal, PSE3D and TriGlobal) suitable for three-dimensional boundary layer are introduced. The structure, applicability and application of four commonly used transition models, such as the Low-Reynolds-Number turbulence model, γ-Re_θt , k-ω-γ , laminar-kinetic energy transition model, are summarized. The advantages and disadvantages of eMalik, LASTRAC, LILO, HyTEN and other transition softwares based on e^N method are analyzed. The future of transition research and software development in hypersonic three-dimensional boundary layer is prospected.