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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Item The Natural Response of Uniform and Nonuniform Plates in Air and Partially Submerged in a Quiescent Water Body(2024) Fishman, Edwin Barry; Duncan, James; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The free vibration of three aluminum plates (.4 m wide, 1.08 m long) oriented horizontally is studied experimentally under two fluid conditions, one with the plate surrounded by air, called the Air case, and the other with the bottom plate surface in contact with a large undisturbed pool of water, called the Half-Wet case. Measurements of the out-of-plane deflection of the upper surfaces of the plates are made using cinematic Digital Image Correlation (DIC) over the center portion of the surface and optical tracking of the center point. Three plate geometries and boundary conditions are studied: A uniform plate with 6.35 mm thickness pinned at the two opposite narrow ends (designated UP), a uniform plate with 4.83 mm thickness simply supported at one narrow end and clamped at the opposite end (UC), and a stepped plate with thickness varying from 12.7 mm to 6.35 mm along its 1.08 m length pinned at two opposite narrow ends (SP). The plate's free response is induced using an impact hammer at three locations along the center-line of the plate. Video frames of the motion of the upper surface of the plate are collected from stereoscopic cameras and processed using DaVis-Strainmaster and MATLAB to extract full-field displacements as a function of time. Two-degree-of-freedom displacements of the plate center are also collected by tracking a target attached to the center of the plate's lower surface. Time and frequency response plots are presented for comparison between the Half-Wet and Air cases and analysis of their dynamics. It is found that the added mass of the water results in lower measured natural frequencies and modified mode shapes. In the Air case, these results are compared to mode shapes/frequencies produced in Creo Simulate and found to agree. Further experiments are discussed.Item Air Entrainment in the Turbulent Ship Hull Boundary Layer(2016) Washuta, Nathan John; Duncan, James H; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Turbulent fluctuations in the vicinity of the water free surface along a flat, vertically oriented surface-piercing plate are studied experimentally using a laboratory-scale experiment. In this experiment, a meter-wide stainless steel belt travels horizontally in a loop around two rollers with vertically oriented axes, which are separated by 7.5 meters. This belt device is mounted inside a large water tank with the water level set just below the top edge of the belt. The belt, rollers, and supporting frame are contained within a sheet metal box to keep the device dry except for one 6-meter-long straight test section between rollers. The belt is launched from rest with an acceleration of up to 3-g in order to quickly reach steady state velocity. This creates a temporally evolving boundary layer analogous to the spatially evolving boundary layer created along a flat-sided ship moving at the same velocity, with a length equivalent to the length of belt that has passed the measurement region since the belt motion began. Surface profile measurements in planes normal to the belt surface are conducted using cinematic Laser Induced Fluorescence and quantitative surface profiles are extracted at each instant in time. Using these measurements, free surface fluctuations are examined and the propagation behavior of these free surface ripples is studied. It is found that free surface fluctuations are generated in a region close to the belt surface, where sub-surface velocity fluctuations influence the behavior of these free surface features. These rapidly-changing surface features close to the belt appear to lead to the generation of freely-propagating waves far from the belt, outside the influence of the boundary layer. Sub-surface PIV measurements are performed in order to study the modification of the boundary layer flow field due to the effects of the water free surface. Cinematic planar PIV measurements are performed in horizontal planes parallel to the free surface by imaging the flow from underneath the tank, providing streamwise and wall-normal velocity fields. Additional planar PIV experiments are performed in vertical planes parallel to the belt surface in order to study the bahvior of streamwise and vertical velocity fields. It is found that the boundary layer grows rapidly near the free surface, leading to an overall thicker boundary layer close to the surface. This rapid boundary layer growth appears to be linked to a process of free surface bursting, the sudden onset of free surface fluctuations. Cinematic white light movies are recorded from beneath the water surface in order to determine the onset location of air entrainment. In addition, qualitative observations of these processes are made in order to determine the mechanisms leading to air entrainment present in this flow.Item The Effect Of Surfactants On The Breakup Of An Axisymmetric Laminar Liquid Jet(2012) Walker, Justin Robert; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The contacting of multiple liquid phases is a complex process, and one that is difficult to study experimentally. Liquid dispersion studies in stirred tanks and high shear mixers frequently involve the use of surfactants without a strong physical understanding of how the surfactants affect the mechanics of droplet production and breakup. In this study, experiments are performed using the axisymmetric laminar jet system. The breakup of a laminar axisymmetric jet is a well-studied fluid dynamics phenomenon. Despite the extensive literature on jet breakup, the impact of surface active agents on jet breakup has received limited attention. An extensive series of experiments with water-air and oil-water jet systems with and without surfactants has been performed, varying fluid flow rate, jet diameter, jet bulk viscosity, surfactant type, and surfactant concentration. Surfactants were found to significantly affect the breakup of laminar liquid jets. Significant effects on both the length of jets and the size of resulting droplets are reported. In general, the effect of surfactants is to reduce the interfacial tension of the system in question, which results in longer jet breakup lengths and larger diameter droplets. However, the interfacial tension alone is insufficient to explain the physics of the jet breakup phenomena. Several breakup mechanisms were identified, and the regimes in which each operates vary not only due to jet geometry and velocity, but on the interfacial properties as well. The effect of surfactants on the breakup phenomena differs in each of these distinct breakup regimes. A mechanistic model for the prediction of breakup length for surfactant laden jets is presented. This model results in good agreement between predicted and experimentally observed values over a wide variety of surfactant concentrations and jet conditions and was shown to be useful for both the oil-water and water-air systems, within the axisymmetric jetting regime.Item Critical Evaluation and Development of One-Equation Near-Wall Turbulence Models(2004-12-20) Diaz, Ricardo H.; Barlow, Jewel B; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A systematic evaluation of one-equation near-wall turbulence models is completed and a new model is developed. The study includes five one-equation near- wall models and one two-equation model such that the performance of the one-equation models can be viewed in context of the performance of this more widely used class of models. It is found that the majority of one-equation near-wall models do not reproduce the variation of the Reynolds shear stress near the wall, do not reproduce the dissipation at the wall, and do not predict the dissipation well in the region near the wall for a boundary layer flow. The new model is found to provide improved performance for the boundary layer and a wavy-wall channel. Specifically, it is found that the new model predicts the turbulent kinetic energy and dissipation in closer agreement with direct numerical simulation data than existing one-equation models for the boundary layer and provides improved predictions of the shear stress distribution for the wavy-wall channel. It is found that the one-equation near-wall models generally predict the shear stress distribution for the wavy-wall channel with greater accuracy than the two-equation model. In addition, it is shown that computations using the one-equation models are less sensitive to wall spacing than those using the two-equation model. This suggests that one-equation near-wall models, and in particular the new model, are ideal for engineering computations of practical flows where computational expense may be a significant factor entering into the choice of turbulence model.