Implicit Coupling Approach for Simulation of Charring Carbon Ablators

This study demonstrates that coupling of a material thermal response code and a flow solver with nonequilibrium gas–surface interaction for simulation of charring carbon ablators can be performed using an implicit approach. The material thermal response code used in this study is the three-dimensional version of fully implicit ablation and thermal response program, which predicts charring material thermal response and shape change on hypersonic space vehicles. The flow code solves the reacting Navier–Stokes equations using data-parallel line relaxation method. Coupling between the material response and flow codes is performed by solving the surface mass balance in the flow solver and the surface energy balance in the material response code. Thus, the material surface recession is predicted in the flow code, and the surface temperature and pyrolysis gas injection rate are computed in the material response code. It is demonstrated that the time-lagged explicit approach is sufficient for simulations at low surface heating conditions, in which the surface ablation rate is not a strong function of the surface temperature. At elevated surface heating conditions, the implicit approach has to be taken because the carbon ablation rate becomes a stiff function of the surface temperature, and thus the explicit approach appears to be inappropriate, resulting in severe numerical oscillations of predicted surface temperature. Implicit coupling for simulation of arc-jet models is performed, and the predictions are compared with measured data. Implicit coupling for trajectory-based simulation of Stardust forebody heat shield is also conducted. The predicted stagnation point total recession is compared with that predicted using the chemical equilibrium surface assumption.