Browsing by Subject "DSMC"

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    Investigation of particle effects on a hypersonic Mars entry
    (2023-06) Kroells, Michael
    Large dust storms periodically form in the Martian atmosphere and pose a threat to future NASA missions. This threat arises from the current lack of understanding of how a Martian dust storm will affect the Thermal Protection System (TPS) of a planetary entry vehicle. While these storms occur infrequently, the long travel-times associated with Martian missions make avoiding these events nearly impossible and therefore the impact of a dust storm on the TPS must be estimated conservatively. However, excessive TPS margins increase the overall entry mass and diminish the allowable mass allocation for mission payload. The overall goal of this thesis is to identify, investigate, and improve underlying computational modeling assumptions relevant to dusty hypersonic Martian entries in order to reduce the conservative margins associated with TPS sizing. The first portion of this thesis covers a recently developed generalized drag coefficient for spherical particles, relevant for Martian dust particles interacting with a hypersonic flow. The proposed model incorporates simple physics-based scaling laws and is valid for a large range of Mach and Knudsen numbers. Additionally, the model retains an explicit dependence on gas type, which is useful for understanding the effect of the Martian atmosphere on particle drag. Next, several studies are performed that utilize Lagrangian particle-tracking to characterize atmospheric particles impacting the surface of high-speed flight vehicles. Two of the studies investigate the effect of Mars entry missions flying through a severe dust storm. The first involves determining the sensitivity of particle-induced surface erosion to underlying particle modeling for the Mars 2020 mission and the second targets an inflatable design that could potentially be used to support human missions to Mars. An additional study is performed in order to understand the impact characteristics of stratospheric particles in Earth's atmosphere on a representative hypersonic flight vehicle. Lastly, a comparison of several numerical strategies for colliding hard-sphere particles is performed. While particle-particle collisions are not likely to play an important role for atmospheric particles because of their low concentrations, particle collisions can play a more important role in characterizing ground experiments that typically have higher particle mass loadings. Specifically, a collision procedure based on the direct-simulation Monte Carlo (DSMC) method is compared to event-driven and time-driven methods for two numerical setups, where the DSMC-inspired collision method is found to be preferable to the other approaches considered because of its improved accuracy and efficiency.
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    Microscale Modeling of Porous Thermal Protection System Materials
    (2015-05) Stern, Eric
    Ablative thermal protection system (TPS) materials play a vital role in the design of entry vehicles. Most simulation tools for ablative TPS in use today take a macroscopic approach to modeling, which involves heavy empiricism. Recent work has suggested improving the fidelity of the simulations by taking a multi-scale approach to the physics of ablation. In this work, a new approach for modeling ablative TPS at the microscale is proposed, and its feasibility and utility is assessed. This approach uses the Direct Simulation Monte Carlo (DSMC) method to simulate the gas flow through the microstructure, as well as the gas-surface interaction. Application of the DSMC method to this problem allows the gas phase dynamics -- which are often rarefied -- to be modeled to a high degree of fidelity. Furthermore this method allows for sophisticated gas-surface interaction models to be implemented. In order to test this approach for realistic materials, a method for generating artificial microstructures which emulate those found in spacecraft TPS is developed. Additionally, a novel approach for allowing the surface to move under the influence of chemical reactions at the surface is developed. This approach is shown to be efficient and robust for performing coupled simulation of the oxidation of carbon fibers. The microscale modeling approach is first applied to simulating the steady flow of gas through the porous medium. Predictions of Darcy permeability for an idealized microstructure agree with empirical correlations from the literature, as well as with predictions from computational fluid dynamics (CFD) when the continuum assumption is valid. Expected departures are observed for conditions at which the continuum assumption no longer holds. Comparisons of simulations using a fabricated microstructure to experimental data for a real spacecraft TPS material show good agreement when similar microstructural parameters are used to build the geometry. The approach is then applied to investigating the ablation of porous materials through oxidation. A simple gas surface interaction model is described, and an approach for coupling the surface reconstruction algorithm to the DSMC method is outlined. Simulations of single carbon fibers at representative conditions suggest this approach to be feasible for simulating the ablation of porous TPS materials at scale. Additionally, the effect of various simulation parameters on in-depth morphology is investigated for random fibrous microstructures.