Petrov-Galerkin model reduction for thermochemical nonequilibrium gas mixtures

State-specific thermochemical collisional models are crucial to accurately describe the physics of systems involving nonequilibrium plasmas, but they are also computationally expensive and impractical for large-scale, multi-dimensional simulations. Historically, computational cost has been mitigated by using empirical and physics-based arguments to reduce the complexity of the governing equations. However, the resulting models are often inaccurate and they fail to capture the important features of the original physics. Additionally, the construction of these models is often impractical, as it requires extensive user supervision and time-consuming parameter tuning. In this paper, we address these issues through an easily implementable and computationally efficient model reduction pipeline based on the Petrov-Galerkin projection of the nonlinear kinetic equations onto a low-dimensional subspace. Our approach is justified by the observation that kinetic systems in thermal nonequilibrium tend to exhibit low-rank dynamics that rapidly drive the state towards a low-dimensional subspace that can be exploited for reduced-order modeling. Furthermore, despite the nonlinear nature of the governing equations, we observe that the dynamics of these systems evolve on subspaces that can be accurately identified using the linearized equations about thermochemical equilibrium steady states, and we shall see that this allows us to significantly reduce the cost associated with the construction of the model. The approach is demonstrated on two distinct thermochemical systems: a rovibrational collisional model for the O₂-O system, and a vibrational collisional model for the combined O₂-O and O₂-O₂ systems. Our method achieves high accuracy, with relative errors of less than 1% for macroscopic quantities (i.e., moments) and 10% for microscopic quantities (i.e., energy levels population), while also delivering excellent compression rates and speedups, outperforming existing state-of-the-art techniques.