An implicit coupling framework for numerical simulations between hypersonic nonequilibrium flows and thermal responses of charring materials in the presence of ablation

An implicit coupling framework for hypersonic nonequilibrium flows and material thermal responses is proposed for numerical simulations of ablative thermal protection materials during flight trajectories. When subjected to aerodynamic heating from hypersonic flows, charring ablative materials undergo complex processes such as ablation and pyrolysis, which involve heterogeneous and homogeneous chemical reactions. These multiphysical phenomena are simulated using a multicomponent material thermal response (MTR) solver that accounts for the complexity of the components of pyrolysis gases. The species concentrations are calculated to enhance the accuracy of the transport and thermophysical parameters of pyrolysis gases. The efficiency of the MTR solver is improved by 92% through implicit time integration in the test cases. The numerical solutions of hypersonic flows and material thermal response are coupled through a gas-surface interaction (GSI) interface, which is based on the surface mass and energy balance on the ablating surface. An explicit coupling method and an implicit coupling mechanism are developed depending on when the GSI interface is updated. The explicit coupling method updates the interfacial quantities at physical time steps, which improves computational efficiency by 40%, but introduces time discretization errors and numerical oscillations in wall properties. In contrast, the implicit coupling mechanism updates the interfacial quantities at pseudo time steps, reducing temporal discretization errors by up to 5.8% and suppressing numerical oscillations. Additionally, a simplified ablation boundary based on a steady-state ablation assumption is proposed to approximate the MTR solution, providing quasi-steady flow solutions in the presence of ablation.