Browsing by Subject "Coupling"

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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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    Multi-Physics Modeling of Ablative Processes
    (2023-12) Schroeder, Olivia
    As reliability requirements for entry systems become increasingly stringent, the need for predictive modeling of complex configurations grows. Such configurations, like micro-meteoroid and orbital debris (MMOD) impacted thermal protection systems (TPS), often involve highly coupled physics that evolve at disparate length and time scales. This work addresses modeling efforts to characterize thermal response of TPS materials with MMOD impact cavities in high enthalpy flow. The importance of cavity geometric parameters, material properties and multi-dimensionality is quantified with regards to design criteria for system failure. The key findings were that length of conduction path and thermal mass play a significantly more important role in TPS survivability than heating augmentation on the surface due to cavities. It was also shown that 1-dimensional analysis for material response of cavity damaged TPS is highly inaccurate particularly after long exposure times. To enable modeling of complex and highly coupled ablation problems, a multi-physics framework is developed. A methodology for modeling shape change in coupled systems is presented and assessed for validity and performance. The approach taken to model gas-surface interactions and translate coupled surface phenomena to physically meaningful boundary conditions in the distinct solvers is discussed. Particularly, the nature of coupled boundary conditions pertaining to surface energy and mass balance as well as surface chemistry modeling. The developed methodology was used to simulate a shear test in arc-jet conditions in order to assess the validity of the fully coupled approach as well as the implementation of the distinct relevant physical processes. It was found that the current work improves upon the agreement of approaches from literature with experimental results.