Flow-radiation coupling in CO2 hypersonic wakes using reduced-order non-Boltzmann models

The current work presents a computationally tractable simulation methodology combining different model-reduction techniques for resolving non-Boltzmann thermodynamics, chemical kinetics, and radiative transfer in complex three-dimensional flows. The empiricism associated with conventional multitemperature nonequilibrium models is abandoned in favor of the multigroup maximum entropy method combined with kinetics-informed adaptive binning. This group-based approach allows for time-varying optimal reduced-order representation of the internal state population distribution while accounting for all collisional processes included in the state-to-state model. Similarly, radiative transfer calculations are performed efficiently by discretization in the spectral, angular, and spatial spaces using the smeared band, discrete ordinate, and finite-volume methods, respectively. This reduction in computational overhead allows truly non-Boltzmann simulations with two-way coupling between the flow and radiation fields to be realized without simplifying approximations based on the tangent-slab method or local escape-factors. The simulation procedure is used in conjunction with the US3D flow solver to investigate the impact of vibrational nonequilibrium on CO2 wake flows and resultant infrared radiation around the Mars 2020 vehicle. This involves comparing predictions for flow-field properties and radiative transfer obtained using the conventional two-temperature model, the multigroup non-Boltzmann model, and for decoupled/coupled flow-radiation calculations. Conventional two-temperature models overestimate the rate of thermal equilibration in the near-wake region resulting in the population of midlying and upper CO2 vibrational levels being underpredicted by multiple orders of magnitude. Additionally, the two-temperature approach (in comparison to bin-based StS) overpredicts the rate of CO2 dissociation thereby leading to erroneous estimates for flow properties in the postshock region (primary source of afterbody radiative emission). This results in inflated values for surface radiative heat flux with two-temperature modeling, although overall differences in radiative behavior are tempered by the fast characteristic relaxation times for ground vibrational levels.