Non-equilibrium ionization phenomena behind shock waves

A one-dimensional flow solver tightly coupled to a detailed electronic collisional-radiative (CR) mechanism is used to illustrate how the ionization process in uences the flow quantities, thermodynamic properties and the radiation field. In the application considered here, where entry speeds exceed 10 km/s, the primary contributor to the radiative processes are atomic species (mainly nitrogen atoms), which account for about 90% of the overall radiation output. A realistic representation of the ionization and radiative processes, occurring in shock heated air, can only be achieved through the explicit calculation of the population of the atomic electronic states using a state-to-state description of the gas kinetics, i.e., by treating the quantum states of atoms as separate pseudo-species. Often in the literature the calculation of the radiation field is decoupled from the solution of the flowfield quantities (species densities, temperatures etc.) and escape factors are used in the flow equations to model the effects of the radiative processes on the population of the excited states. The analysis of ionizing flow adopting this simplified treatment of radiation clearly shows the strong dependence of the result on the assumptions made when selecting the escape factors (e.g. thin or thick assumptions). By substituting escape factors with the source terms resulting from the solution of the radiative transfer equation, a fully consistent treatment of the radiation processes is employed in this work. The influence of the radiation processes on the population of the excited states as well as the cooling effects is thus correctly modeled. Modeling examples using this approach are presented using the conditions of FIRE II flight.