Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item The Effects of Gravity on Flow Boiling Heat Transfer(2021) Hammer, Caleb Franklin; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flow boiling is a method of phase change heat transfer used widely in electronics cooling, refrigeration, air conditioning, and other areas where stable temperatures are needed. An area of interest is spaceflight systems, where efficient heat transfer is desired to minimize mass, power requirements, and cost. When compared to terrestrial gravity conditions, the heat transfer of flow boiling in microgravity typically depreciates. This depreciation has been documented across multiple experimental studies performed by teams using different fluids, tube geometries, and flow regimes over the past three decades. Though select experimental microgravity flow boiling heat transfer data are available in the literature, holistic results are sparse due to the cost and limited availability of microgravity research. The two-phase heat transfer mechanisms responsible for the depreciation are therefore not well known, and so heat transfer models for variable gravity flow boiling do not exist. The goal of the proposed study is to develop models for flow boiling heat transfer through a tube as a function of gravity by identifying the effect of gravity on different heat transfer mechanisms. The scope of this proposal involves modeling three microgravity flow regimes (bubbly, slug, and annular flow) to serve as baseline predictions for flow boiling heat transfer without the influence of gravity. Additional gravity effects can be identified using partial and hyper-gravity data. Experiments have been performed aboard parabolic flights and on the ground at various flow rates, heating rates, and inlet subcoolings in microgravity, hyper-gravity, Lunar gravity, Martian gravity, and terrestrial gravity. Results from the experiments showed that negligible slip velocity plays an important role in modeling flow boiling heat transfer. Simulations using modified single-phase models of an accelerating flow were performed which predicted microgravity flow boiling heat transfer well in the nucleate boiling regime.Additional experiments concerning terrestrial gravity quenching heat transfer have been performed to address research gaps in microgravity cryogen chilldown studies. Quenching heat transfer coefficients were recorded in the nucleate boiling regime and compared with correlations. The correlations were able to predict heat transfer for room temperature fluids much more accurately than for cryogenic fluids. Scaling parameters must be tuned to match cryogen data to examine the large disparity between cryogenic quenching heat transfer data and correlations observed in the literature.Item Inference of Mass Anomalies in Planetary Interiors Using a Bayesian Global Gravity Field Inversion(2020) Izquierdo Gonzalez, Kristel Del Carmen; Montesi, Laurent G. J.; Lekic, Vedran; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Knowledge about the interior density distribution of a planetary body can constraingeophysical processes and reveal information about the origin and evolution of the body. Properties of this interior distribution can be inferred by analyzing gravity acceleration data sampled by orbiting satellites. Usually, the gravity data is complemented with additional laser ranging or seismic data in order to reduce the range of possible density models of the interior. However, additional data might not be available and tight prior constraints on model parameters might not be justified. In this case, the flexibility of using non-informative priors and the ability to quantify the non-uniqueness of the gravity inversions are of even greater importance. In this work, we present a gravity inversion algorithm, THeBOOGIe, thatsamples the posterior distribution of density in the interior of a planet or moon according to Bayes theorem, following a Metropolis-Hastings iterative algorithm. It uses non-informative priors on the number, location, shape and magnitude of density anomalies. Different samples of the posterior show different density models of the interior consistent with the observed gravity data. Inversions of synthetic gravity data are ran using point masses, spherical caps and Voronoi regions (VRs) to parametrize density anomalies. THeBOOGIe is able to retrieve the lateral location of shallow density anomalies and the shape, depth and magnitude of a mid-mantle anomaly. The uncertainty of the model parameters increases with depth, as expected. Bouguer gravity data of the Moon obtained by the GRAIL mission was invertedusing a VR parametrization. Shallow anomalies related to the SPA basin, crustal dichotomy and near side basins were found in the correct latitude and longitude and a trade-off in their thickness and magnitude. Positive and negative density anomalies were found in the depth range 500-1141 km. The location of deep moonquakes do not have a clear relation to the location of these density anomalies.Item QUASIPARTICLES IN SUPERFLUIDS AND SUPERCONDUCTORS(2020) Curtis, Jonathan; Galitski, Victor M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quasiparticle descriptions are a powerful tool in condensed matter physics as they provide an analytical treatment of interacting systems. In this thesis we will apply this tool to theoretically describe two systems: a superconductor interacting with cavity photons and a flowing Bose-Einstein condensate forming a sonic black hole. First we will consider a two-dimensional s-wave BCS superconductor coupled to microwave cavity photons. We show how a nonequilibrium occupation of the photons can induce a nonequilibrium distribution of superconductor Bogoliubov quasiparticles, yielding an enhancement of the superconducting gap. The analytic dependence of this enhancement is provided in terms of the photon spectral and occupation functions, offering a large parameter space over which enhancement exists. Next, we analyze the equilibrium properties of a similar superconductor-cavity structure which has strong sub-dominant d-wave pairing interaction. In this case there is a collective mode known as the Bardasis-Schrieffer mode, which is essentially an uncondensed d-wave Cooper pair. We show that by driving an external supercurrent through the sample the Bardasis-Schrieffer mode can be hybridized with cavity photons, forming exotic Bardasis-Schrieffer-polaritons. We then turn to consider a flowing Bose-Einstein condensate. In the presence of inhomogeneous flow, the long-wavelength motion of quasiparticles can be mapped onto the kinematics of matter fields in a curved spacetime. This mapping allows for the simulation of a black hole and its interactions with quantum fields via analogy. We show that in the case of a step-like jump in the condensate flow the emission of analogue Hawking radiation is accompanied by evanescent modes which are pinned to the event horizon. Finally, we generalize this setup to include pseudo-spin half spinor Bose condensates. In this case, we show that the analogue spacetime the quasiparticles experience can be of the exotic Newton-Cartan type. Newton-Cartan gravity -- the geometric formulation of Newtonian gravity -- is realized when the Goldstone mode disperses quadratically as opposed to linearly. The nature of the analogue spacetime is controlled by the presence or absence of an easy-axis anisotropy in the boson spin-exchange interaction. We conclude by arguing that this Newton-Cartan spacetime can be experimentally realized in current platforms.Item Cryogenic test of gravitational inverse square law below 100-micrometer length scales(2010) Yethadka Venkateswara, Krishna Raj; Paik, Ho Jung; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The inverse-square law is a hallmark of theories of gravity, impressively demonstrated from astronomical scales to sub-millimeter scales, yet we do not have a complete quantized theory of gravity applicable at the shortest distance scale. Problems within modern physics such as the hierarchy problem, the cosmological constant problem, and the strong CP problem in the Standard Model motivate a search for new physics. Theories such as large extra dimensions, ‘fat gravitons,’ and the axion, proposed to solve these problems, can result in a deviation from the gravitational inverse-square law below 100 μm and are thus testable in the laboratory. We have conducted a sub-millimeter test of the inverse-square law at 4.2 K. To minimize Newtonian errors, the experiment employed a near-null source, a disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, were positioned on the two sides of the source mass at a nominal distance of 280 μm. As the source was driven sinusoidally, the response of the test masses was sensed through a superconducting differential accelerometer. Any deviations from the inverse-square law would appear as a violation signal at the second harmonic of the source frequency, due to symmetry. We improved the design of the experiment significantly over an earlier version, by separating the source mass suspension from the detector housing and making the detector a true differential accelerometer. We identified the residual gas pressure as an error source, and developed ways to overcome the problem. During the experiment we further identified the two dominant sources of error - magnetic cross-talk and electrostatic coupling. Using cross-talk cancellation and residual balance, these were reduced to the level of the limiting random noise. No deviations from the inverse-square law were found within the experimental error (2σ) down to a length scale λ = 100 μm at the level of coupling constant |α|≤2. Extra dimensions were searched down to a length scale of 78 μm (|α|≤4). We have also proposed modifications to the current experimental design in the form of new tantalum source mass and installing additional accelerometers, to achieve an amplifier noise limited sensitivity.