Browsing by Subject "Ablation Modeling"

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    Investigation of Boundary Layer Stability Using the Parabolic Stability Equations on a Coupled Simulation of the Reentry F Flight Experiment
    (2022-07) Rogers, Robert
    Laminar-to-turbulent transition prediction in hypersonic boundary layers is complicatedby the many physical effects present in high-enthalpy flows. For different flight conditions, certain flow phenomena can either stabilize or destabilize a boundary layer, subsequently moving a transition location upstream or downstream. This can have serious effects on surface heating, skin friction drag, and vehicle dynamics. The uncertainty associated with boundary layer prediction is a major limitation in the design of hypersonic flight systems. Previous work has established how thermochemical non-equilibrium, spherical nose tip blunting, surface heating, and Reynolds number variation, among other things, can impact the disturbance environment over canonical geometries like flat plates, wedges, and cones in supersonic and hypersonic flows. Yet, experiments involving all those physics simultaneously are not possible except in flight. Computational studies are a way to couple many flow physics to investigate the effect on boundary layer stability. The objective of this thesis is to couple many of the flight physics and generate laminar base flows to then examine how boundary layer stability is affected on a blunt cone during atmospheric reentry. The coupled physics include ablation chemistry, a realistic wall temperature, altitude variation, and non-spherical nose tip blunting. A new coupled solid-fluid and gas-surface interaction solver called Ares is used to generate base flows for the geometry and flight conditions of Reentry F. The flow physics under investigation are added sequentially to isolate their impact on boundary layer stability. The final simulation includes the effect of shape change, a flight trajectory, a realistic wall temperature profile, and ablation chemistry. Once base flows are generated, the solution is imported into PSE-Chem, a parabolic stability equation (PSE) solver within STABL-2D. PSE analysis then models the most unstable frequencies in the boundary layer and N factor curves are generated to predict how disturbances vary with the additional flow physics and how the transition location is affected. With Ares, the resulting surface temperature profiles match well with Reentry F flight data where it was collected. However, over the graphite nose tip region upstream of 22 cm, no flight temperature data was measured. A study with conjugate heat transfer and gas-surface interactions are presented for the temperature predictions in this range showing that the inclusion of gas-surface chemistry dramatically increases heating and the predicted wall temperature. The boundary layer chemistry also shows CO as the dominant carbonaceous species throughout the boundary layer. The concentrations of other species through the boundary layer are also examined. Carbon species mass loss rates are then used to compute a surface recession rate at the stagnation point. Then, a sensitivity study investigates the effects that non-spherical nose tip blunting has on boundary layer disturbance growth. The stability results show that a realistic wall temperature profile stabilized boundary layer disturbances relative to a cold isothermal wall condition, while increasing Reynolds number destabilizes the boundary layer. In the case of ablation chemistry, the results are mixed with destabilization upstream in some cases, but the overall impact on the predicted transition location is unaffected. Lastly, the non-spherical nose tip blunting has a strong destabilizing effect upstream of the predicted transition location, while changes to N factors around the predicted transition location are negligible.
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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.