Author ORCID Identifier

https://orcid.org/0000-0002-7459-4521

Date Available

2-5-2027

Year of Publication

2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Mechanical Engineering

Faculty

Savio J. Poovathingal

Faculty

Jonathan Wenk

Abstract

Atmospheric entry at hypersonic speeds produces extreme thermal environments in which radiative heating from thermochemical non-equilibrium shock gases can dominate the heating load. In porous, semi-transparent thermal protection systems (TPS), incident radiation penetrates into the material and thermal emission originates over a finite depth; consequently, radiation acts as a volumetric heating/cooling mechanism that depends on the medium radiative properties. In addition, fibrous TPS exhibits strongly anisotropic scattering (forward- and backward-peaked) that conventional low-order approximations (e.g., diffusion/Rosseland and P1) cannot reliably capture, motivating higher-fidelity radiative transport modeling in TPS.

This dissertation develops and applies a pathlength-based reverse Monte Carlo ray-tracing (RMCRT) solver for the radiative transfer equation (RTE). The RMCRT method is selected because it is well-suited for parallelization and incorporating complex physics, including anisotropic scattering, heterogeneous radiative properties, and directional dependency. The development of the RMCRT solver is presented and verified against analytical relations and benchmark solutions from the literature. The RMCRT solver is then coupled to a one-dimensional material response (MR) solver to compare against a diffusion-approximation scheme representative of current state-ofthe- art practices. The fully coupled MR–RMCRT scheme explicitly resolves in-depth radiative transport and provides the radiative source term for the energy equation in the MR solver. The comparison study includes the effects of anisotropic scattering and changes in radiative properties during charring/ablation.

Using RMCRT-generated data, this work quantifies the impact of anisotropic scattering, spectral, and directional dependencies on TPS radiative behavior. It establishes practical relationships linking surface radiative properties (emissivity/ absorptivity) to medium radiative properties (extinction, scattering albedo, and scattering phase function), and introduces a new metric, the radiative coupling length, to characterize the radiation penetration depth and guide when diffusion-type models are likely insufficient. To enable reliable yet computationally efficient radiative modeling of TPS, a physics-based anisotropic radiative transfer (ART) framework is proposed. The ART framework combines an exponential-weighted effective temperature (EWET) emission model for a non-isothermal medium with an exponential-decay (ED) absorption model for externally incident radiation in an anisotropic-scattering medium. Verifications are performed against the verified RMCRT-generated data across a wide range of radiative properties. Additional case studies extend to study the radiation trapping in surface defects and their impact on surface recession, estimation of effective radiative properties from computed-tomography microstructure scans of TPS materials, and inverse estimation of radiative properties from experimental reflectance/transmittance data.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2026.07

Funding Information

This study was supported by the National Aeronautics and Space Administration and the Kentucky EPSCoR Program under award number 80NSSC22M0174 in 2022.

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Available for download on Friday, February 05, 2027

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