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 Experimental Measurements of Vortex Breakdown with non-Isothermal Inflow(2023) Krupnik, Assaf; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Vortex breakdown occurs in many flow applications such as weather, aerodynamics, swirl combustors, and more, introducing unsteadiness that is often undesired. While vortex breakdown has been previously investigated extensively, researchers have struggled to efficiently characterize these flows due to the flow sensitivity to intrusive probes, and there is no real consensus regarding the process and reason of its formation. Recently, researchers have discovered the blue whirl, a silent and efficient flame which they believe is a mode of vortex breakdown, and suggested mechanisms of stabilization and prevention of it through temperature control. This thesis uses modern time-resolved measurement techniques to investigate breakdown at smaller temporal and spatial scales than previously researched, and shows the effect of heat addition at the inflow of a vortex generator on the onset and behaviour of breakdown modes. Smoke flow visualization experiments were preformed to identify heating and swirl rate effects on vortex flow, and the resulting swirl angle of the flow was measured. Decreasing the incoming swirl was found to delay or fully suppress the formation of a breakdown bubble. Increasing the inlet temperature had a similar effect due to the buoyancy effects on the flow increasing the axial velocity thus reducing the effective swirl. 3D Particle Tracking Velocimetry was used to obtain time-resolved flow fields of the vortex flow and breakdown with and without heating. Individual velocity profiles and velocity fields are presented, showing that flow behavior is dependent on much smaller scales than previously researched. Proper Orthogonal Decomposition is used to isolate energetically dominant modes and determine whether higher order modes are significant to the flow.Item Directed Flow In Heavy Ion Collisions at sqrt{s_NN}=2.76 TeV(2015) Gomez, Jaime Arturo; Mignerey, Alice L; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV relative to the collision Event Plane, defined by the participant plane using various subdetectors. The rapidity-even directed flow component is measured for the first time using an Event Plane weighted by the transverse momentum of the emitted charged hadrons and found to be largely independent of pseudorapidity with a sign change at transverse momenta $p_{T}$ between 1.2 and $1.7$ GeV/c. These results are compared to measurements made by two other experiments at the LHC. Combined with the observation from ALICE of a vanishing rapidity-even $p_{T}$ shift along the spectator deflection plane is strong evidence for dipole like initial density fluctuations in the overlap zone of the nuclei. These observations open new possibilities for investigation of the initial conditions in heavy-ion collisions with spectator nucleons.Item High Reynolds Number Vertical Up-Flow Parameters For Cryogenic Two-Phase Helium I(2014) Mustafi, Shuvo; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The two phase flow characteristics of helium I are of interest since under most operational scenarios this cryogenic fluid exists in both liquid and vapor form because of its extremely low boiling point and latent heat of vaporization. There is a significant knowledge gap in the flow boiling parameters of helium (heat transfer coefficient, pressure drop and dryout heat flux) for high Reynolds number vertical up-flows (Re =10^5-10^6). This dissertation fills this gap and helps to expand the use of helium as an inert simulant for hydrogen. Since no prior correlations for the flow boiling parameters existed for vertical up-flows of helium at these Reynolds numbers, any predictions of these parameters were dependent on correlations that were tested at lower Reynolds numbers, or correlations based on other fluids. The thermophysical properties of helium I are significantly different from most other fluids; therefore the capability of prior correlations in predicting experimental observations was limited. As part of this research new correlations are proposed for the flow boiling parameters. This research begins the investigation of a new regime for two-phase helium I flows at Reynolds numbers above 3e5. The techniques described will enable future work to address other gaps in knowledge for helium I flows that still remain. The prior heat transfer coefficient correlation over-predicted the data that was collected for this research. The new correlation improves the agreement with data by a factor of 98. Two prior models for pressure drop, the separated flow model and the homogeneous flow model, under-predict the observed pressure drop. The newer versions of the separated flow and the homogeneous flow correlations improve agreement with the data by about a factor of 3 and by more than a factor of 2 respectively. The previous dryout heat flux correlation considerably over predicts the observed dryout heat flux. The new correlation improves agreement with the data by a factor of 21. Significant cryogenic challenges were overcome to collect the research data. The strategies described for surmounting the diverse challenges such as thermal acoustic oscillations and low dryout heat flux could be used by future two-phase cryogenic flow researchers.Item CHANNELING THE CURRENT: THE LIVED EXPERIENCE OF MOVING MEDITATION FOR FINDNG A FLOW IN THINKING AND WRITING(2013) Morris, Sarah Lynn; Hultgren, Francine H.; McCaleb, Joseph; Curriculum and Instruction; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)CHANNELING THE CURRENT: THE LIVED EXPERIENCE OF MOVING MEDITATION FOR FINDNG A FLOW IN THINKING AND WRITING Sarah Lynn Morris, Doctor of Philosophy, 2013 Dissertation Directed by: Professor Francine H. Hultgren Department of Teaching and Learning, Policy and Leadership, University of Maryland, College Park This phenomenological study explores lived experience of moving meditation for finding flow in thinking and writing. Moving meditation is intentional practice of mindfulness that brings us deeply into our selves and the world. Connecting to pedagogical implications for teaching composition, this study suggests embodied practices may open a flow of words and ideas for those practicing movement meditation. Grounded in the philosophy of Merleau-Ponty, Heidegger, and van Manen, this work explores embodiment and lived experience, using human science phenomenology as method. Further grounded in writing process and moving meditation texts, this work connects body movement and writing practices through lived experience. I first turn toward my own experience to examine moving meditation as method of finding flow in my thinking and writing. Next, I explore the phenomenon in a range of traditions to further uncover the lived experience of moving writers. The metaphor of the circuit as descriptive of writing process and body process further illuminates the phenomenon. Initial emergent themes include process, practice, flow, solitude, and nature. Recognizing the intersubjective in the particular, this study focuses on lived experience of four high school English teachers as they make meaning through focused movement. In four sessions of meditative contemplation, these teachers walked in the woods, wrote reflections, and considered personal and pedagogical experiences. Renderings of these teachers' journals and conversations suggest themes including fear, care, wholeness, and transcendence. Drawing from these conversants' insights, I explore ways in which meditative movement opens a flow in thinking and writing for these teachers, writers themselves in the current of life. Orienting toward pedagogical implications, I engage with lived experience in order to suggest ways in which teachers of writing may create wholeness of experience for classroom communities: taking students outside, seeing students in wholeness, positioning themselves as more experienced writers, focusing on process rather that product, and being bodies themselves. In doing so, they may generate a culture of care that fosters growth of writing and writers--body, mind, and spirit wholeness-- with the world as classroom and lived life as text.Item Identification of Air Traffic Flow Segments via Incremental Deterministic Annealing Clustering(2012) Nguyen, Alex T; Barras, John S; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many of the traffic management decisions and initiatives in air traffic are based on "flows" of traffic in the National Airspace System (NAS), but the actual identification of the location and time of the flow segments are often left to interpretation based on observations of traffic data points over time. Having an automated method of identifying major flow segments can help to target traffic management initiatives, evaluate design of airspace, and enable actions to be taken on the collection of flights in a flow segment rather than on the flights individually. A novel approach is developed to identify the major flow segments of air traffic in the NAS that consists of a robust method for partitioning 4-dimensional traffic trajectories into a series of great circle segments, and clustering the segments using an Agglomerate Deterministic Annealing clustering algorithm. In addition, a very efficient algorithm to incrementally cluster the segments is developed that takes into account the spatial and temporal properties of the segments, and makes the method very suitable for real-time applications. Further, an enhancement to the algorithm is provided that requires only a small subset of the segments to be clustered, drastically reducing the run time. Results of the clustering technique are shown, highlighting various major traffic flow patterns in the NAS. In addition, organizing the traffic into the flow segments identified using the Incremental Clustering method is shown to have a potential reduction in the number of conflict points. An application of the flow information is presented in the form of a Decision Support Tool (DST) that aids traffic managers in establishing and managing Airspace Flow Programs. In addition, the flow segment information is applied to a low-level form of aggregated traffic management, showing that aggregating flights into the flow segments and rerouting the whole flow segment can be efficiently performed as compared to rerouting individual aircraft separately, and can reduce the number of conflict points. Considerations for implementing these techniques in real-time systems are also discussed.Item Flow and interfacial dynamics in vascular vessels and microfluidics(2007-01-19) Wang, Yechun; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates the hemodynamic forces on biological cells adherent on vascular vessels as well as the interfacial dynamics of droplet motion in microfluidic channels. In addition, we develop a novel three-dimensional spectral boundary element algorithm for interfacial dynamics in Stokes flow. In physiological systems, the hemodynamic forces exerted on endothelial cells in vascular vessels affect the behavior of the cells via mechano-transduction. The hemodynamic forces also play a pivotal role in the adhesion of leukocytes onto the surface of blood vessels. This study investigates the relative importance and the nature of the two components of the hemodynamic force, i.e., the shear and normal force, on the cell and its vicinity. Based on computational investigation and scaling analysis, the study demonstrates that the normal force contributes significantly to the total hemodynamic force on the cell. This study points out the importance of the normal force exerted on biological cells attached to blood vessels which has been overlooked. This research may motivate experiments to identify the effects of the normal force on the functions of biological cells adhered in blood vessels. The results of the study are also applicable to the fluid forces over protuberances in microfluidic devices and porous media. For the efficient study of droplet dynamics, we have developed a novel three-dimensional high-order/high-accuracy spectral boundary element algorithm for interfacial dynamics in Stokes flow. This methodology has been employed to several interfacial problems and the results are in excellent agreement with experimental findings, analytical predictions and previous numerical computations. We also investigate the droplet motion in confined geometries which is primarily motivated by the recent development of microfluidic devices and has applications in the enhanced oil recovery, lubrication and coating processes. We consider the buoyancy-driven droplet motion along a solid wall and the pressure-driven droplet motion in a micro-channel. The influence of capillary number, Bond number and viscosity ratio on the droplet motion and deformation is investigated.Item On finding paths and flows in multicriteria, stochastic and time-varying networks(2004-11-24) Opasanon, Sathaporn -; Miller-Hooks, Elise; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation addresses two classes of network flow problems in networks with multiple, stochastic and time-varying attributes. The first problem class is concerned with providing routing instructions with the ability to make updated decisions as information about travel conditions is revealed for individual travelers in a transportation network. Three exact algorithms are presented for identifying all or a subset of the adaptive Pareto-optimal solutions with respect to the expected value of each criterion from each node to a desired destination for each departure time in the period of interest. The second problem class is concerned with problems of determining the optimal set of a priori path flows for evacuation in capacitated networks are addressed, where the time-dependent and stochastic nature of arc attributes and capacities inherent in these problems is explicitly considered. The concept of Safest Escape is formulated for developing egress instructions. An exact algorithm is proposed to determine the pattern of flow that maximizes the minimum path probability of successful arrival of supply at the sink. While the Safest Escape problem considers stochastic, time-varying capacities, arc travel times, while time-varying, are deterministic quantities. Explicit consideration of stochastic and time-varying travel times makes the SEscape problem and other related problems significantly more difficult. A meta-heuristic based on the principles of genetic algorithms is developed for determining optimal path flows with respect to several problems in dynamic networks, where arc traversal times and capacities are random variables with probability mass functions that vary with time. The proposed genetic algorithm is extended for use in more difficult, stochastic, time-varying and multicriteria, capacitated networks, for which no exact, efficient algorithms exist. Several objectives may be simultaneously considered in determining the optimal flow pattern: minimize total time, maximize expected flow and maximize the minimum path probability of successful arrival at the sink (the objective of the SEscape problem). Numerical experiments are conducted to assess the performance of all proposed approaches.