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

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    QUANTIFYING NITROGEN REMOVAL POTENTIAL OF BOTTOM CAGE (C. VIRGINICA) AQUACULTURE
    (2022) Shenoy, Stefenie; Harris, Lora A; Testa, Jeremy M; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    While management strategies for human-caused nutrient pollution have improved over the last decade, eutrophication and its ecological effects remain primary concerns in many coastal marine systems. In-water nutrient removal techniques are being explored for potential use as a management strategy, including oyster aquaculture operations. Where abundant, oysters have been shown to exhibit denitrifying potential beyond that which is assimilated into shell and tissue biomass. While nitrogen cycling dynamics are well studied and modeled on natural and restored reefs, equivalent processes within oyster aquaculture operations are less defined. This study adapts an existing mechanistic model of oyster filtration, biodeposition, and particle transport to capture the influence of an aquaculture farm on local sediment-water chemical fluxes. Modifications included (1) revising the spatial domain to represent an array of bottom cages, and (2) integrating an existing bioenergetics module to mechanistically couple simulated seston removal from the water column via filtration and subsequent biodeposition by simulating oyster growth. Model simulations included a variety of oyster densities, farm sizes, natural reef, and no oyster scenarios. Two seasonal sampling campaigns of a bottom cage aquaculture site provided model forcing and validation data. Model output revealed complex relationships among oyster density and distribution, farm size, oyster growth and biodeposition. The estimated rates of net nitrogen removal suggest increased potential for oyster aquaculture operations to receive credits above what is currently being realized, and the calculations of such removal for management purposes should consider lease-specific configurations and environmental parameters.
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    NUMERICAL MODELING AND EXPERIMENTAL STUDY OF A NOVEL METAL-POLYMER COMPOSITE HEAT EXCHANGER FOR SENSIBLE AND LATENT THERMAL ENERGY STORAGE APPLICATIONS
    (2022) KAILKHURA, GARGI; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Compact, lightweight, and low-cost heat exchangers (HXs) have the potential to improve efficiencies and save power and carbon foot print in a wide array of applications. The present study investigates an entirely additively-manufactured novel metal-polymer composite heat exchanger, enabled by an innovative and patented cross-media thermal exchange approach, which yields an effective thermal conductivity of 130 W/m-K for the heat exchanger. This record-high thermal conductivity is more than an order of magnitude higher that the previously reported thermal conductivity for polymer and polymer composite HXs. Drawing on the concept of external flow over the tube banks, the proposed HX features a staggered arrangement of fins. This class of HXs are often used for gas-to-liquid sensible cooling applications. However, they can also be designed for latent thermal energy storage applications by employing low-cost and high energy-storage-density phase change materials (PCMs) such as salt-hydrates and alike in either the hot or cold side of the HX, depending on the application. An extensive literature survey on tube banks shows that, though numerous correlations exist in the literature for flow over tube banks, these correlations usually fall outside the range for the current HX design for low-Reynolds number applications (Re<100). Furthermore, the PCM models present in the literature are either very challenging to solve analytically or are computationally expensive. Thus, the dissertation emphasizes developing computationally-efficient and robust numerical models for sensible and latent cooling applications.The numerical models compute the overall thermal and pressure-drop performance metrics based on segment-level modeling, and they integrate the performance parameters such as Euler, Nusselt numbers, or latent thermal energy with the entire HX analytically, thus significantly reducing the computational cost. For steady-state sensible thermal energy storage applications, a realistic 3D CFD-based modeling approach is used, based on the actual dimensions of the printed HXs rather than a traditional 2D CFD-based model. It also resolves the issues due to the 3D velocity field which aren’t captured in the 2D CFD models, and are particularly important for HXs utilizing narrow/micro channels. This modeling approach is used to obtain optimized HXs for case examples of 5-40 kW air-conditioning applications and 250-W electronic cooling applications for nominal operating and flow conditions. The 250-W unit is further validated experimentally and is observed to be within 17% for waterside pressure drop, 11% for airside pressure drop, and within 8% for thermal resistance when compared against experimental measurements. For transient latent thermal storage applications, an analytical-based 1D reduced order model (ROM) for segment-level modeling is developed based on 1D radial conduction inside the PCM. It is numerically validated with commercial CFD tools to within 10% except for cases where axial conduction in PCM is possible due to the high resistance of wire embedded in the PCM. The 1D ROM is used in optimizing a 1.44-MJ TES unit for peak-load building cooling applications and a 19.2-kJ HX for pulsed-power cooling applications. The 1.44-MJ unit is experimentally tested and observed to be within 17% for the melting time of complete PCM and about 8% for the freezing time of the complete PCM. Lastly, another novel and hybrid thermal energy storage design is formulated, which utilizes two different PCMs: shape memory alloys (SMAs) instead of metal wires and salt-hydrates contained inside polymer channels similar to the reference designs. Besides the thermal energy storage design, a novel methodology on Wilson plot for finned surfaces on both fluid-sides is introduced, which is first of its kind in the literature. Ongoing and future work in both these areas is also recommended in the final chapter of the thesis.
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    LONGITUDINAL DYE DISPERSION AND SALT FLUX IN ESTUARIES
    (2017) Liu, Wei; Li, Ming; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Estuarine dispersion plays an important role in determining the fate of waterborne materials. It is a long-standing question in estuarine dynamics that is still not well understood. This dissertation revisits this problem by utilizing two tracers: dye and salt. Dye-release experiments and numerical modeling are conducted to investigate horizontal dispersion in a partially mixed estuary. Longitudinal dispersion of a dye patch shows strong flood-ebb asymmetry at early times after a dye release, with most of the dispersion occurring during ebb tides. Tidal straining enhances vertical current shear on ebb tides and promotes longitudinal dispersion. There are also large differences in the dispersion rate between spring and neap tides. Due to strong spring mixing, a dye patch quickly extends from the bottom to the surface, exposing to the full vertical shear in the water column and leading to strong longitudinal dispersion. In contrast most of the dye patch is limited to bottom few meters during neap tides. Although weak vertical mixing facilitates longitudinal dispersion, the vertical shear across the thin dye patch is much weaker, leading to weak longitudinal dispersion during neap tides. In first four tidal cycles, the second moment of the dye patch in the along-channel direction increases with time at a power of between 2 and 3. The longitudinal dispersion rate varies as the four-third power of the dye patch size, indicating scale-dependent diffusion. Salt dispersion and transport are examined in a comparative numerical modeling study between the partially-mixed Chesapeake Bay and the well-mixed Delaware Bay. To investigate how different physical mechanisms drive the salt transport into the estuaries, the longitudinal salt fluxes are decomposed using the Eulerian and quasi-Lagrangian methods. Under the Eulerian framework, the salt flux is decomposed into three parts: an advective term associated with the barotropic forcing, a steady shear dispersion term associated with the estuarine exchange flow, and a tidal oscillatory salt flux. In both estuaries, the advective term is dominant over steady shear dispersion and tidal oscillatory salt flux in the temporal variation of total salt flux. In Chesapeake Bay, the steady shear dispersion is the dominant mechanism and the tidal oscillatory salt fluxis small. In Delaware Bay, the steady shear dispersion and tidal dispersion are comparable. The along-channel variation of tidal oscillatory salt flux is mainly due to changes of the phase difference between the tidal current and salinity. Isohaline analysis using the quasi-Lagrangian methodology yields a new interpretation of the estuarine exchange flows and describes the evolution path of salinity classes.
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    DISSOLVED OXYGEN AND NUTRIENT CYCLING IN CHESAPEAKE BAY: AN EXAMINATION OF CONTROLS AND BIOGEOCHEMICAL IMPACTS USING RETROSPECTIVE ANALYSIS AND NUMERICAL MODELS
    (2013) Testa, Jeremy Mark; Kemp, William M; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Hypoxia, or the condition of low dissolved oxygen levels, is a topic of interest throughout aquatic ecology. Hypoxia has both realized and potential impacts on biogeochemical cycles and many invertebrate and vertebrate animal populations; the majority of the impacts being negative. It is apparent that the extent and occurrence of hypoxic conditions has been on the rise globally, despite a handful of reductions due to management success stories. Efforts to curb the development of hypoxia are well underway in many aquatic ecosystems worldwide, where oxygen levels are a key target for water quality management. Long-term increases in the volume of seasonal bottom-water hypoxia have been observed in Chesapeake Bay. Although there is evidence for the occurrence of low oxygen conditions following initial European habitation of the Chesapeake watershed, as well as direct observations of anoxia prior to the mid 20th century large-scale nutrient load increases, it is clear that hypoxic volume has increased over the last 50 years. Surprisingly, the volume of hypoxia observed for a given nutrient load has doubled since the mid-1980s, suggesting the importance of hypoxia controls beyond nutrient loading alone. I conducted a suite of retrospective data analyses and numerical modeling studies to understand the controls on and consequences of hypoxia in Chesapeake Bay over multiple time and space scales. The doubling of hypoxia per unit TN load was associated with an increase in bottom-water inorganic nitrogen and phosphorus concentrations, suggesting the potential for a positive feedback, where hypoxia-induced increases in N and P recycling support higher summer algal production and subsequent O2 consumption. I applied a two-layer sediment flux model at several stations in Chesapeake Bay, which revealed that hypoxic conditions substantially reduce coupled nitrification-denitrification and phosphorus sorption to iron oxyhydroxides, leading to the elevated sediment-water N and P fluxes that drive this feedback. An analysis of O2 dynamics during the winter-spring indicate that the day of hypoxia onset and the rate of March-May water-column O2 depletion are most strongly correlated to chlorophyll-a concentrations in bottom water; this suggests that the spring bloom drives early season O2 depletion. Metrics of winter-spring O2 depletion were un-correlated with summer hypoxic volumes, however, suggesting that other controls (including physical forcing and summer algal production) are important. I used a coupled hydrodynamic-biogeochemical model for Chesapeake Bay to quantify the extent to which summer algal production is necessary to maintain hypoxia throughout the summer, and that nutrient load-induced increases in hypoxia are driven by elevated summer respiration in the water-column of lower-Bay regions.