Speaker
Description
During high-speed atmospheric reentry, radiation can make up to 20% of peak heating, as demonstrated by the Electric Arc Shock Tube (EAST) at the Ames Research Center. This means that thermal protection design requires accurate radiation modeling during reentry. This subject is of particular interest at the moment because both NASA [10, 9] and ESA [14] are considering missions that would enter the atmospheres of Ice Giants, where this effect would be significant [4]. However, the chemical reaction rates and pathways relevant to reentry in these atmospheres remain highly uncertain.
There is an ongoing effort to improve radiation modeling; work by Colonna et al. [5] and Carroll et al. [3] propose models that replicate experimental data reasonably well using two different sets of species and reactions, achieved by tuning reaction rates within their uncertainties. This highlights the need for a more systematic calibration approach. More recently, Ninni et al. [11] and Colonna et al. [6], present a new macroscopic model. In this model, the electronic energy levels are resolved, but not the vibrational ones, which are instead represented by a vibrational temperature. This model shows good agreement with the full State-to-State (StS) approach, suggesting that reduced-order models are a viable path forward.
A further distinction between models lies in how they treat radiation. Colonna et al. compare optically thick and optically thin plasma assumptions, while Carroll et al. perform a detailed a posteriori computation of radiation, assuming a quasi-steady state. By contrast, both Coelho and da Silva [4] and Sahai and Johnston [12] present approaches that fully couple to the radiative transfer to CFD. This has been considered necessary for Entry, Descent and Landing (EDL)
missions since 1968 [8].
Rather than proposing a new kinetic-radiative model, this work adopts an existing model and focuses on the systematic calibration of chemical reaction rates, with the sensitivity analysis presented here as the first step toward that goal. The forward model for Ice Giant reentry was constructed using three established software packages: the VKI Shocking code (1D inviscid post-shock relaxation) [1], the VKI Mutation++ library [13], and NASA Ames NEQAIR [2], which handle the fluid, thermochemical, and radiative aspects, respectively. The model includes 9 species (e−, H, H+, H−, H2, H2+ , H3+ , He, He+), StS modeling of H(n=1-10), and 119 reactions, making it broadly comparable to the model by Colonna et al. [5] but with fewer internal states. The sensitivity analysis of the effect of reaction rates on radiation was performed using a range of Quantities of Interest (QoIs) varying in both emission wavelength (Balmer α, β, γ and θ spectral lines) and position after the shock (1-2 cm, 2-3 cm, and 3-4 cm), revealing that H2 dissociation reactions and electron-impact excitation reactions have the greatest impact on the QoIs.
In the future, the forward model will be compared to experimental results from the EAST facility. EAST has had several campaigns at 89:11% volume H2:He [7] corresponding to the Uranus atmosphere, at speeds higher than 25 km/s. Using an Uncertainty Quantification framework, this will allow for the systematic calibration of the reaction rates.
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Summary
Radiative heating can represent up to 20% of peak heating during high speed atmospheric entry. Manual tuning of reaction rates within their large uncertainties has managed to match experimental data rather well but highlights the need for systematic calibration.
We have assembled a model for simulation of H2/He atmospheric entry and performed a sensitivity analysis based on reaction rates as input and radiation at several positions and wavelengths as output. This has shown the relative importance of dissociation and electron impact excitation reactions over the other types of reactions present.