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 THE EFFECTS OF SURFACE GRAVITY WAVES ON AIR-SEA MOMENTUM TRANSFER AND VERTICAL MIXING IN A FETCH-LIMITED, ESTUARINE ENVIRONMENT(2017) Fisher, Alexander William; Sanford, Lawrence P.; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Surface gravity waves are the principal pathway through which momentum and energy are transferred from the atmosphere to the ocean. Recent studies have contributed to a growing recognition that wind events can be of leading-order importance for mixing and circulation in estuaries, yet the specific nature of air-sea momentum transfer in coastal environments remains relatively understudied. As part of a collaborative investigation of wind-driven estuarine physics, this dissertation addresses the role that surface gravity waves play in the transfer of momentum from the air to the oceanic surface boundary layer in a fetch-limited, estuarine environment. Using a combination of direct field observations and numerical simulations, the role of surface gravity waves in structuring momentum transfer and vertical mixing were examined for a range of wind, wave, and stratification conditions. Results indicate that inclusion of surface gravity waves in bulk parameterizations of wind stress reduced bias to below 5% for nearly all observed wind speeds and that up to 20% of wind stress variability within Chesapeake Bay was directly attributable to surface wave variability. Furthermore, the 10-meter neutral drag coefficient was shown to vary spatially by more than a factor of two over the extent of Chesapeake Bay as a result of combined wind and wave variability. Anisotropic fetch-limitation resulted in dominant wind-waves that were commonly and persistently misaligned with local wind forcing. Direct observations of stress above and below the water surface demonstrated that, within the oceanic surface layer, stress was more aligned with wave forcing than wind forcing. Accounting for the surface wave field was needed to close the local momentum budget between the atmosphere and the mean flow. Directly observed turbulent profiles showed that breaking waves dominated the transfer of momentum and energy and resulted in a three-layer turbulent response consisting of a wave transport layer, surface log layer, and stratified bottom boundary layer. Comparisons to commonly employed second-moment turbulence closures suggest that the presence of breaking waves homogenized the surface layer to a greater extent than predicted by present parameterizations of turbulent kinetic energy transport away from a source at the surface.Item Longitudinal Control of Intense Charged Particle Beams(2011) Beaudoin, Brian Louis; O'Shea, Patrick G; Kishek, Rami A; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As the accelerator frontier shifts from high energy to high intensity, accelerator facilities are demanding beams with higher quality. Applications such as Free Electron Lasers and Inertial Fusion Energy production require the minimization of both transverse emittance and longitudinal energy spread throughout the accelerator. Fluctuations in beam energy or density at the low-energy side of the accelerator, where space-charge forces dominate, may lead to larger modulations downstream and the eventual degradation of the overall beam quality. Thus it is important to understand the phenomenon that causes these modulations in space-charge dominated beams and be able to control them. This dissertation presents an experimental study on the longitudinal control of a space-charge dominated beam in the University of Maryland Electron Ring (UMER). UMER is a scaled model of a high-intensity beam system, which uses low-energy high-current electron beams to study the physics of space-charge. Using this facility, I have successfully applied longitudinal focusing to the beam edges, significantly lengthening the propagation distance of the beam to 1000 turns (>11.52 km). This is a factor of 10 greater than the original design conceived for the accelerator. At this injected current, the space-charge intensity is several times larger than the standard limit for storage rings, an encouraging result that raises the possibility of operating these machines with far more space-charge than previously assumed possible. I have also explored the transverse/longitudinal correlations that result when a beam is left to expand longitudinally under its own space-charge forces. In this situation, the beam ends develop a large correlated energy spread. Through indirect measurements, I have inferred the correlated energy profile along the bunch length. When the bunch is contained using longitudinal focusing, I have shown that errors in the applied focusing fields induce space-charge waves at the bunch edges that propagate into the middle region of the beam. In some cases, these waves sustain multiple reflections before damping away. I conclude that space-charge in an intense beam without longitudinal focusing can cause the bunch to develop a large correlated energy spread, increasing the risk that the beam is lost to the pipe walls as it requires a larger aperture. When longitudinal focusing is applied however, we are able to transport the beam over a much longer path length and reduce the correlated energy spread.