An Aero-Thermo-Chemo-Mechanical Coupling Framework for the Analysis of Hypersonic Ablative Thermal Protection Systems

Abstract

There are countless challenges associated with the accurate modeling of the hypersonic flight of ablative thermal protection systems (TPS): resolving the relevant coupled physical phenomena through multi-physics simulations, the management of the disparate spatiotemporal scales associated with the fluid and solid responses, and establishing a reliable numerical model able to predict the response of ablative materials exposed to extreme gradients—to name a few. The two-way, loosely coupled framework presented in this thesis consists of ΣMIT, a multi-physics computational solid mechanics (CSM) code, coupled with US3D, a hypersonic computational fluid dynamics (CFD) solver, to form a complete aero-thermochemo-mechanical simulation framework. The ΣMIT-US3D coupling framework provides a step towards high-fidelity simulation capabilities for hypersonic vehicles with ablative TPS, establishing a strong foundation for the simulation of fluid-structure interaction (FSI) phenomena and computation of the mechanical response of porous ablators. The requirement of a robust numerical formulation for the solution of hypersonic pyrolysis problems was made apparent when encountering numerical convergence issues with legacy methods, which sparked the development of a robust semi-implicit pyrolysis material model. The so-called Linearized Pyrolysis model employs simplifying assumptions for the energy and mass balance equations and relies upon the time-lagging of chosen terms to achieve linear convergence and robust performance. The performance of the model has been validated against the Ablation Workshop Test Cases and has increased the range of allowable representative hypersonic boundary conditions significantly compared to the legacy approach. Together, the model and the coupling framework are applied to two aero-thermochemo-mechanical analyses contained within the thesis: a spherical-tipped nose cone and the Orion heat shield. Preliminary results identify the decomposition region as a zone in which high von Mises stress tends to occur—care must be taken to ensure that internal and external flight loads do not exceed allowable limits to prevent catastrophic TPS material failure in this region. However, perhaps the most significant insight resulting from the framework relates to the computation of mass fluxes through the porous ablative material, revealing that for an isotropic monolithic heat shield with at a zero angle of attack, pyrolysis gas flow is driven by the pressure gradient applied to the shield such that the flow exits at the edges of the shield rather than from the base.