Effects of vibration–vibration–translation reactions in vibrational state-specific modeling of high-enthalpy nitrogen flow over a sphere

The large number of transitions involved in vibration–vibration–translation (VVT) transitions during molecule–molecule collisions in the state-to-state (StS) simulation significantly increases computational cost and limits the multidimensional application. Two methods, multiquantum restrictions and simplifying VVT transitions to vibration–translation (VTm) transitions, are proposed to reduce computational consumption. This study is a continuation of a previous work [Guo et al., “Investigation of high enthalpy thermochemical nonequilibrium flow over spheres,” Phys. Fluids 36, 016122 (2024)]. It aims to systematically analyze the effects of VVT and VTm transitions on the flowfield characteristics and heat flux distributions of high-enthalpy nitrogen flow over a sphere. The numerical results show that the StS VVT and VTm transitions predict nearly identical shock standoff distances and stagnation-point heat fluxes, which agree with experimental data. Incorporating VVT transitions yields minimal differences in the predictions of translational temperature, vibrational temperature, and N mass fraction compared to VTm processes. Near the wall, the VVT and VTm transitions yield almost identical state populations, with gradually pronounced non-Boltzmann distributions. Therefore, the VVT transitions can be effectively reduced to VTm transitions while maintaining essential accuracy for predicting aerodynamic heating.