Coupled Simulation of Material Response using Equilibrium and Finite-Rate Ablation Models

Simulating the response of thermal protection system (TPS) materials in hypersonic flow is a challenging task due to the wide range of physical phenomena present and the difficulty in accurately modeling their behavior and interactions. Internal energy excitation, chemical reactions, radiation, and turbulence can be present in the flow field while thermal conduction, chemical reactions, and pyrolysis are common in TPS materials. At the flow-material interface, gas-surface interactions including ablation add further complexity to the multi-physics problem. In this paper, we present a simulation approach that involves coupling the US3D flow solver and the Kentucky Aerothermodynamics and Thermal-response System (KATS) material response solver with an ablation boundary condition. Two types of ablation models are implemented: the equilibrium model, which assumes the gas is in a saturated thermodynamic equilibrium at the surface, and finite-rate surface chemistry models. We have also implemented different methods for computing diffusion coefficients including a constant Lewis or Schmidt number and an approach based on collision integrals. A set of test cases is used to demonstrate verification and validation of the coupled software before applying it to several problems of interest. Our results show that both the ablation model and diffusion model can have a substantial effect on the ablation rate. Equilibrium and finite-rate ablation models are compared across a range of hypersonic flight conditions finding that even when they predict similar total mass flux, the individual species production rates, surface composition, and heat flux components can differ significantly. The coupled simulation framework is also demonstrated on an atmospheric entry problem to highlight the differences between a coupled and decoupled approach as well as the effect of pyrolysis on the thermal response of the heat shield.