UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
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Item ASSESSING THE IMPACTS OF NON-POINT SOURCE FRESHWATER AND NUTRIENT INPUTS ON A SHALLOW COASTAL ESTUARY(2019) Butler, Thomas; Hood, Raleigh R; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Academic research models for Chesapeake Bay have, traditionally, been forced with USGS inputs, flows and nutrient loads from 10 major rivers. These tributaries fail to account for 100% of the inputs entering the Bay. In contrast, models used for determining Total Maximum Daily Load for Chesapeake Bay are forced with output from a watershed model at thousands of locations, presumably, accounting for all these inputs. Our aim is to increase understanding of the impacts different forcing schemes have on water quality model simulation. Simulations were completed using three forcing approaches: 1) using “traditional” USGS-derived input from 10 major rivers; 2) using “concentrated” input from 10 major rivers derived from watershed model output; and 3) using “diffuse” input from 1117 rivers derived from watershed model output. Comparisons of these schemes revealed large impacts on simulations in Chesapeake Bay during periods of high flow and extreme weather events under diffuse forcing.Item ADVANCED MODELING AND REFRIGERANT FLOW PATH OPTIMIZATION FOR AIR-TO-REFRIGERANT HEAT EXCHANGERS WITH GENERALIZED GEOMETRIES(2019) Li, Zhenning; Radermacher, Reinhard K; Aute, Vikrant C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Air-to-refrigerant heat exchangers are key components of the heating, ventilation, air-conditioning and refrigeration systems. The evolving simulation and manufacturing capabilities have given engineers new opportunities in pursuing complex and cost-efficient heat exchanger designs. Advanced heat exchanger modeling tools are desired to adapt to the industrial transition from conventional refrigerants to low Global Warming Potential (low-GWP) refrigerants. This research presents an advanced heat exchanger performance prediction model which distinguishes itself as a cutting-edge simulation tool in the literature to have capabilities, such as to (i) model heat exchangers with variable tube shape and topology, (ii) improved numerical stability, (iv) multiple dehumidification models to improve evaporator prediction, and (v) CFD-based predictions for airflow maldistribution. Meanwhile, HX performance is significantly influenced by the refrigerant flow path arrangements. The refrigerant flow path is optimized for various reasons such as to (i) mitigate the impact of airflow maldistribution, (ii) reduce material/cost, (iii) balance refrigerant state at the outlet of each circuit, and (iv) ensure overall stable performance under a variety of operating conditions. This problem is particularly challenging due to the large design space which increases faster than n factorial with the increase in the number of tubes. This research presents an integer permutation based Genetic Algorithm (GA) to optimize the refrigerant flow path of air-to-refrigerant heat exchangers. The algorithm has novel features such as to (i) integrate with hybrid initialization approaches to maintain the diversity and feasibility of initial individuals, (ii) use effective chromosome representations and GA operators to guarantee the chromosome (genotype) can be mapped to valid heat exchanger designs (phenotype), and (iii) incorporate real-world manufacturability constraints to ensure the optimal designs are manufacturable with the available tooling. Case studies have demonstrated that the optimal designs obtained from this algorithm can improve performance of heat exchangers under airflow maldistribution, reduce defrost energy and assure stable heat exchanger performance under cooling and heating modes in reversible heat pump applications. Comparison with other algorithms in literature shows that the proposed algorithm exhibits higher quality optimal solutions than other algorithms.Item Physical-Biological Interactions Driving the Distribution of the Pelagic Macroalgae Sargassum(2019) Brooks, Maureen Therese; Coles, Victoria J.; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The holopelagic macroalgae of the genus Sargassum are the ecosystem engineers of a unique open-ocean rafting ecosystem in the subtropical North Atlantic and tropical Atlantic. Over the last decade, increases in biomass in the tropics and Caribbean Sea have been observed. The underlying causes of this regime shift have been difficult to discern without a baseline understanding of the drivers of Sargassum distribution. The objective of this dissertation is to fill this knowledge gap using remote and in situ observations, and coupled ocean circulation, biogeochemical, Lagrangian particle, and Sargassum physiology models. A satellite-derived Sargassum abundance climatology shows the center-of-mass of Sargassum shifting between the tropics, Caribbean, Gulf of Mexico, and Sargasso Sea throughout the year. Model experiments demonstrate that advection alone can explain up to 60% of the observed distribution at time scales shorter than two months. At longer time scales, the growth and reproductive strategy of the macroalgae interact with physical processes to drive the overall observed pattern. Sargassum populations in the Western Tropical Atlantic and Gulf of Mexico appear to exert disproportionate influence over the basin-wide distribution. One key physical process influencing both transport and growth is inertia. A novel inverse method, developed from remote sensing to determine the effective radius of Sargassum rafts, facilitates modeling inertial effects. The effective radius is on the order of 0.95 m, much closer to the size of an individual plant than that of aggregations which can span kilometers. The inclusion of inertia alters modeled distributions of Sargassum by increasing retention in the Gulf of Mexico and the Caribbean, while increasing export from the Sargasso Sea by up to 20%. Inertia acting on buoyant Sargassum rafts also leads to their increased entrainment in cyclonic eddies. These eddies propagate toward the north-west in the northern hemisphere providing transport for Sargassum from the tropics through the Caribbean to the Gulf of Mexico and leading to increased biomass due to transport into regions with better growing conditions. Sargassum biology and its interaction with ocean circulation and mesoscale features is central to improving understanding of the changes in its distribution and for prediction of costly beaching events.Item Accretion Physics Through the Lens of the Observer: Connecting Fundamental Theory with Variability from Black Holes(2018) Hogg, James Andrew; Reynolds, Christopher S; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Variability is a generic feature of accretion onto black holes. In both X-ray binaries and active galactic nuclei, variability is observed on nearly all accessible timescales and across the entire electromagnetic spectrum. On different timescales and at different wavelengths it has unique signatures that can be used to characterize the accretion processes generating the emission and probe the accretion disks, which would otherwise be impossible. Despite having been observed for over fifty years, interpreting this variability is difficult. Simple phenomenological models have been used to explain the behaviors and geometries of the observed accretion disk, but they have yet to be rigorously tested in a full magnetohydrodynamic framework. In this dissertation we use high-resolution numerical models to investigate: (1) ``propagating fluctuations" in mass accretion rate that give rise to the nonlinear signatures of accretion on viscous timescales, (2) the dynamics of truncated accretion disks which are invoked to explain the spectral variation of outbursting X-ray binaries and the bifurcation of AGN accretion states, and (3) the large-scale magnetic dynamo behavior in thick and thin accretion disks. We find that the structured variability readily seen in the light curves from accreting black holes (i.e. log-normal flux distributions, linear relations between the RMS and the flux, and radial coherence) quickly and naturally grows from the MRI-driven turbulence and that these properties translate into photometric variability. For the first time, we identify the large-scale magnetic dynamo as the source of the low-frequency modulations of the disk stress that cause this structure. We introduce a bistable cooling law into hydrodynamic and magnetohydrodynamic simulations to study the manifestation of a truncated accretion disk in each regime. We find that rather than a truncation edge, the transition is better described by a ``truncation zone" when the angular momentum transport and heating is governed by MRI-driven turbulence instead of a true viscosity. Additionally, we find that the hot gas in the simulation buoyantly rises in a gentle outflow and eventually fills the entire volume, instead of simply being confined to the innermost region. The outflow interacts with the disk body and enhances the magnetic stresses, which could produce stronger quasiperiodic variability. Finally, we conduct an investigation of the large-scale magnetic dynamo using a suite of four global magnetohydrodynamic disk simulations with scaleheight ratios of $h/r=\{0.05, 0.1, 0.2, 0.4\}$. Most notably, the organization that is prevalent in accretion disk simulations and described as a ``butterfly pattern" does not occur when $h/r \ge 0.2$, despite the dynamo action still operating efficiently.Item CFD MODELING AND ANALYSIS OF ROTOR WAKE IN HOVER INTERACTING WITH A GROUND PLANE(2014) Kalra, Tarandeep Singh; Baeder, James d; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The action of the rotor wake on loose sediment on the ground is primarily responsible for inducing the rotorcraft brownout phenomenon. Therefore, any simulation of brownout must be capable of accurately predicting the velocity field induced by the rotor when it is operating in ground effect. This work attempts to use a compressible, structured, overset Reynolds-Averaged Navier-Stokes (RANS) based solver to simulate hovering rotors in ground effect (IGE) to demonstrate the capability of the code to provide accurate tip vortex flow field predictions, and provide a good understanding of the ground-wake interactions. The computations are performed for a micro-scale rotor (0.086m radius, aspect ratio of 4.387 operating at a tip Mach number of 0.08 and Reynolds number of 32,500) and a sub-scale rotor (0.408m radius, aspect ratio of 9.132 operating at a tip Mach number of 0.24 and Reynolds number of 250,000) in order to compare to experimental measurements. The micro-scale rotor has a rectangular tip shape and is simulated three rotor heights: 1.5R, 1.0R and 0.5R above ground (R = Rotor radius). The sub-scale rotor is simulated at one particular rotor height (i.e. 1R) but with four different tip shapes: rectangular, swept, BERP-like and slotted tip. Various mesh placement strategies are devised to efficiently capture the path of the tip vortices for both regimes. The micro-scale rotor simulations are performed using the Spalart Allmaras (S-A) turbulence model. The examination of the IGE tip vortex flow field suggests high degree of instabilities close to the ground. In addition, the induced velocities arising from the proximity of the rotor tip vortices causes flow separation at the ground. The sub-scale rotor simulations show a smeared out flow field even at early wake ages due to excessive turbulence levels. The distance function in the S-A turbulence model is modified using the Delayed Detached Eddy Simulation (DDES) approach and a correction to length scaling is included for anistropic grids. The resulting computational flow field after these modifications compares well with the experiments. The slotted tip is seen to diffuse the tip vortices at early wake ages through injection of momentum and increased turbulence, and generates the least amount of unsteady pressure variation at the ground plane when compared with other three tip shapes.Item The tectonics of intraplate regions: Quantifying stress and surface deformation in the central and eastern U.S. and planetary analogs on Mercury and the Moon(2013) Walsh, Lisa Schleicher; Martin, Aaron J; Montesi, Laurent G.J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Occurring ~ 1 year apart, the magnitude 3.4 Germantown, Maryland, (16 July 2010) and magnitude 5.8 Mineral, Virginia, (23 August 2011) earthquakes rocked the U.S. national capital region, drawing renewed attention to the occurrence of seismicity within continental interiors. While the majority of earthquakes concentrate at tectonic plate boundaries, the processes that promote spatially diffuse zones of seismicity in intraplate regions are not well understood. The Mineral earthquake was one of the largest earthquakes to occur east of the Rocky Mountains in the past century and offers a rare opportunity to examine the role of stress transfer, long-distance triggering, and aftershock decay within an intraplate region. Stress transfer from the Mineral and Germantown earthquakes relieved stress on the majority of Cenozoic faults in the Mid-Atlantic region, moving these faults further away from future failure. The Everona fault and southern portion of the Mountain Run fault zone were the only locations (except in the aftershock region) that were loaded from the Mineral earthquake, although by only ~1 mbar. Accumulation of stress over time is required in order to significantly affect regional seismicity. There is no evidence of remote triggering due to the passage of seismic waves in any of the major seismic zones in the central and eastern U.S. However, the slow decay rate of aftershocks suggests seismicity in the epicentral region might continue for a decade or longer. Aftershocks triggered by stress imparted by the mainshock imply that Coulomb stress transfer plays an important role in earthquake triggering processes within intraplate regions. Processes in the aftershock zone likely have the greatest influence on seismic hazard. New imagery and altimetry data returned from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) and Lunar Reconnaissance Orbiter (LRO) spacecraft provide new insight into processes driving intraplate tectonic deformation. Mercurian wrinkle ridges are ~2.2 larger in mean relief than wrinkle ridges on the Moon, suggesting a larger component of global contraction on Mercury. Patterns of faulting on Mercury and the Moon, as well as in the central and eastern U.S., indicate that intraplate seismicity can concentrate in zones of pre-existing weakness and spatially migrate.Item Advection-Diffusion Controlled Lithium Isotopic Distribution in Contact Aureoles: A Case Study from the Florence County Pegmatites, Wisconsin(2009) Liu, Xiaoming; Hier-Majumder, Saswata; Rudnick, Roberta L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Stable isotopes are useful tracers of fluid-rock interactions in contact aureole settings. To date, only a few case studies have used Li isotopes to study fluid-rock interactions in contact aureole settings. These studies highlight the very large Li isotopic fractionation that can be generated in these settings via diffusion of Li from the pluton into the country rocks, but none of these studies have generated a complete and detailed section of the contact aureole needed to understand the Li distributions. Here, I report the results from a combination of Li isotope analyses and 2-D advection-diffusion modeling for two detailed profiles through country rocks adjacent to Li-rich pegmatite dikes in the Florence County pegmatite field. The results show that the Li concentration and isotopic distribution in the two contact profiles is consistent with fluid infiltration and diffusion of Li through a grain boundary fluid from the pegmatites into their country rocks.Item Thermo-Optic Aspects of Large Screen Plasma Display Panels(2007-05-16) Kahn, Jeffry Joseph; Bar-Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma Display Panels (PDPs) are a popular technology for large size television displays. Screen inefficiencies, which result in significant localized heat generation, necessitate the use of advanced thermal management materials to reduce the peak temperatures and spatial temperature variations across the screen. In the current study, infrared thermography was used to obtain thermal maps of a typical, 42", high-definition PDP screen for different illumination patterns and for several configurations of externally controlled heaters simulating PDP heat generation. The results were used to validate a three-dimensional numerical thermal model of the PDP designed to predict the beneficial effects of anisotropic graphite heat spreaders on the temperature distribution. In addition, a color analyzer was used to determine the spatial and temporal variations in luminosity across the PDP when operated continuously for 1750 hours. The thermal model and experimental luminosity characteristics were used to evaluate the deleterious effects of temperature on PDP performance.