Non-equilibrium plasma formation and electromagnetic-wave scattering in hypersonic transitional flow

Hypersonic flows in the transitional regime exhibit strong thermochemical non-equilibrium, where conventional continuum assumptions fail. This paper presents a hybrid kinetic-fluid framework for simulating partially ionised argon plasmas under such conditions. The heavy-particle component is governed by a kinetic equation and solved using the unified gas-kinetic wave–particle (UGKWP) method, which captures flow physics from the rarefied to the continuum regime without being constrained by the mean free path scale. Electrons are modelled as a fluid with a separate temperature equation that accounts for elastic collisions and ionisation energy loss. An electromagnetic-wave solver, coupled with an auxiliary differential equation for the plasma current density, evaluates the wave scattering by the plasma. Validation against shock-tube experiments confirms the accuracy of the present algorithm. Application to a Mach 22 cylinder flow at Knudsen number 0.05 reveals three notable non-equilibrium transport effects – elevated near-wall heavy-particle density enhances ionisation near the stagnation point; non-equilibrium boundary-layer cooling lowers the electron-density peak in the shock layer; and non-equilibrium transport in the rarefied wake yields a higher downstream electron density. The thermal structure predicted by UGKWP differs significantly from that of the continuum gas-kinetic scheme (GKS), underscoring the important role of multiscale methods. These differences in ionisation further lead to distinct electromagnetic scattering patterns. Our results thus establish a link between multiscale non-equilibrium transport, plasma formation and electromagnetic blackout in hypersonic transitional flows.