Review of nonequilibrium plasma kinetics in hypersonic flows

Abstract

Ionization in hypersonic flows is a critical phenomenon impacting communications with the ground, wake flow radiation, and vehicle radiative heating. Accurate prediction of the formation and decay of these plasmas relies on a detailed treatment of a wide array of nonequilibrium energy exchanges and collisional-radiative kinetics. These processes may be resolved with varying levels of fidelity depending on the simulation quantity of interest and the computational resources available. In this paper, we review the current state of the art in plasma kinetics modeling for hypersonic flows, focusing particularly on species relevant to flight in Earth’s atmosphere for vehicles employing carbon-based ablative thermal protection systems (N ₂ , O ₂ , NO, N, O, CO ₂ , NCO, C ₃ , C ₂ , CO, CN, C, N ₂ ⁺ , O ₂ ⁺ , NO ⁺ , N ⁺ , O ⁺ , CO ⁺ , CN ⁺ , C ⁺ , e ⁻ ). The available modeling approaches for modeling ionized hypersonic flows are discussed, and the use cases for each are highlighted. Rate data are reviewed for nonequilibrium energy exchanges, dissociation, atom exchange, associative ionization, charge exchange, electron impact ionization, radiative recombination, and dielectronic recombination, as well as their reverse processes where relevant. Based on the scatter in published data, uncertainty bounds on the two-temperature rate coefficients involving the considered species are determined and provided. Finally, ground- and flight-test experimental data are reviewed and summarized. Critical areas for further model improvement are identified throughout, and high-priority validation needs are highlighted.