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 PREDICTING THE SEISMIC SIGNATURE OF LAVA TUBES FOR THE EARTH, THE MOON, AND MARS(2024) Wike, Linden; Schmerr, Nicholas; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lava tubes are a type of volcanically-generated subsurface void structure found on Earth, the Moon, and Mars and hold the potential to serve as shelters for crew members, preservation sites of pristine geological samples, and locations of in situ resources. A key question for lava tube science is how to locate them through geophysical methods. Here, we create a workflow that locates and characterizes the geometry of subsurface voids. We build a suite of subsurface synthetic seismic wavefield models that contain lava tube structures and investigate which seismic method best images them. Our models show that the more readily detectable lava tube simulations have a geophone spacing of 0.5 m, variable diameter, and shallow ceiling depth; and reverse-time migration and phase-shift-plus-interpolation migration techniques produce more accurate lava tube reconstructions than the Kirchhoff method. The synthetic modeling serves as a benchmark for understanding seismic wave propagation around lava tubes and helps answer how voids on the Moon and Mars would be imaged through seismology.Item ELECTRICAL AND STRUCTURAL FORMATION OF TRANSIENT LIQUID PHASE SINTER (TLPS) MATERIALS DURING EARLY PROCESSING STAGE(2023) Nave, Gilad; McCluskey, Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The growing demands of electrification are driving research into new electronic materials. These electronic materials must have high electrical conductivity, withstand harsh environments and high temperatures and demonstrate reliable solutions as part of complete electronic packaging solutions. This dissertation focuses on characterizing the initial stage of the manufacturing process of Transient Liquid Phase Sinter (TLPS) alloys in a paste form as candidates for Pb-free high-temperature and high-power electronic materials.The main objective of this dissertation work is to investigate the factors and decouple the multiple cross effects occurring during the first stage of TLPS processing in order to improve the understanding of material evolution. The work proposes, develops, and conducts in-situ electrical resistivity tests to directly measure material properties and analyze the dynamics at different stages of the material's evolution. The research explores various factors, including alloying elements, organic binders, and heating rates, to understand their effects on the development of electrical performance in electronic materials. More specifically, the work examines the performance of Ag-In, Ag-Sn and Cu-Sn TLPS paste systems. Additionally, packing density and changes in cross-section are investigated using imaging techniques and image processing to gain insights into the early formation of the material's structural backbone. An Arrhenius relationship together with Linear Mixed Models (LMM) techniques are used to extract the activation energies involved with each of the processing stages. The study then develops procedures to model different states of the TLPS microstructures at different heating stages based on experimentally observed data. Using these models, the study uses Finite Element Method (FEM) analysis to verify the experimental results and gain a better understanding and visualization into the involved mechanisms. This investigation not only sheds light on the material's behavior but also has implications for robust additive manufacturing (AM) applications.Item Simulating membrane-bound cytoskeletal dynamics(2023) Ni, Haoran; Papoian, Garegin A.; Biophysics (BIPH); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The cell membrane defines the shape of the cell and plays an indispensable role in bridging intra- and extra-cellular environments. The membrane, consisting of a lipid bilayer and various attaching proteins, mechanochemically interacts with the active cytoskeletal network that dynamically self-organizes, playing a vital role in cellular biomechanics and mechanosensing. Comprehensive simulations of membrane-cytoskeleton dynamics can bring insight in understanding how the cell mechanochemically responds to external signals, but a computational model that captures the complex cytoskeleton-membrane with both refined details and computational efficiency is lacking. To address this, we introduce in this thesis a triangulated membrane model and incorporate it with the active biological matter simulation platform MEDYAN ("Mechanochemical Dynamics of Active Networks"). This model accurately captures the membrane physical properties, showing how the membrane rigidity, the structure of actin networks and local chemical environments regulate the membrane deformations. Then, we present a new method for simulating membrane proteins, using stochastic reaction-diffusion sampling on unstructured membrane meshes. By incorporating a surface potential energy field into the reaction-diffusion sampling, we demonstrate rich membrane protein collective behaviors such as confined diffusion, liquid-liquid phase separation and membrane curvature sensing. Finally, in order to capture stretching, bending, shearing and twisting of actin filaments which are not all available with traditional actomyosin simulations, we introduce new finite-radius filament models based off Cosserat theory of elastic rods, with efficient implementation using finite-dimensional configurational spaces. Using the new filament models, we show that the filaments' torsional compliance can induce chiral symmetry breaking in a cross-linked actin bundle. All the new models are implemented in the MEDYAN platform, shedding light on whole cell simulations, paving way for a better understanding of the membrane-cytoskeleton system and its role in cellular dynamics.Item Generating Feasible Spawn Locations for Autonomous Robot Simulations in Complex Environments(2022) Ropelato, Rafael Florian; Herrmann, Jeffrey W; Systems Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Simulations have become one of the main methods in the development of autonomous robots. With the application of physical simulations that closely represent real-world environments, the behavior of a robot in a variety of situations can be tested in a more efficient manner than performing experiments in reality. With the implementation of ROS (Robot Operating System), the software of an autonomous system can be simulated separately without an existing robot. In order to simulate the physical environment surrounding the robot, a physics simulation has to be created through which the robot navigates and performs tasks. A commonly used platform for such simulations is Unity which provides a wide range of simulation capabilities as well as an interface for ROS. In order to perform multi-agent simulations or simulations with varying initial locations for the robot, it is crucial to find unobstructed spawn locations to avoid undesirable situations with collisions upon start of the simulation. For this purpose, multiple methods were implemented with this research, in order to generate feasible spawn locations within complex environments. Each of the three applied methods generates a set of valid spawn positions, which can be used to design simulations with varying initial locations for the agents. To assess the performance and functionality of these approaches, the algorithms were applied to several environments varying in complexity and scale. Overall, the implemented approaches performed very well in the applied environments, and generated mainly correctly classified locations which are suitable to spawn a robot. All approaches were tested for performance and compared in respect to their fitness to be applied to environments of varying complexity and scale. The resulting algorithms can be considered a efficient solutions to prepare simulations with multiple initial locations for robots and other test objects.Item QUANTIFYING THE ADDED VALUE OF AGILE VIEWING RELATIVE TO NON-AGILE VIEWING TO INCREASE THE INFORMATION CONTENT OF SYNTHETIC SATELLITE RETRIEVALS(2022) McLaughlin, Colin; Forman, Barton A; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Satellite sensors typically employ a “non-agile” viewing strategy in which the boresight angle between the sensor and the observed portion of Earth’s surface remains static throughout operation. With a non-agile viewing strategy, it is relatively straightforward to predict where observations will be collected in the future. However, non-agile viewing is limited because the sensor is unable to vary its boresight angle as a function of time. To mitigate this limitation, this project develops an algorithm to model agile viewing strategies to explore how adding agile pointing into a sensor platform can increase desired information content of satellite retrievals. The synthetic retrievals developed in this project are ultimately used in an observing system simulation experiment (OSSE) to determine how agile pointing has the potential to improve the characterization of global freshwater resources.Item SIMULATION OF MAGNETIC GRANULAR MEDIA USING OPEN SOURCE SOFT SPHERE DISCRETE ELEMENT METHOD(2021) Leps, Thomas; Christine, Hartzell; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic granular media were investigated using a mutual dipole magnetic model integrated into the open source Soft Sphere Discrete Element Method (DEM) framework LAMMPS and LIGGGHTS. Using the magnetic model and the contact force models from LIGGGHTS, we simulated shear behavior of MagnetoRheological Fluids (MRF). We found that the size distribution of simulated particles significantly affects the qualitative and quantitative behavior of MRF in a simple shear cell. Additionally, including cohesion, rolling resistance, friction and other contact forces affect the simulated shear behavior. By using a high fidelity contact force model along with an accurate size distribution and the mutual dipole magnetic model we were able to accurately match experimental data for an example MRF.We used the DEM model to aid in the development of a novel MRF valve operating on an alternative MRF behavior. Our jamming, MRF valve holds pres- sure through stable, but reversible jamming in the flow path, and is actuated by electropermanent magnets, which require no quiescent current to maintain their magnetization states. These valves do not require the large power draw of con- ventional MRF valves to maintain their state. We were able to accurately predict the experimental jamming behavior of the MRF valve using Finite Element Analysis and LIGGGHTS with magnetization, further validating the model with a non-linear, non-continuum behavior. Our jamming MRF valve was demonstrated in a multi- segmented, elastomeric robot, actuated using MRF. Using the magnetic DEM model coupled with self-gravity, the effects of mag- netism on rubble pile magnetic asteroids were examined. We simulated formation, and disruption of metallic asteroids with remnant magnetizations using LAMMPS with permanent dipoles. We found that rubble pile asteroids, formed from clouds of magnetized grains, coalesce more quickly, and have higher porosities than aster- oids coalesced from unmagnetized grains. Distortion and disruption was affected by magnetization during simulated YORP spin-up. Large fragments with high aspect ratios and low densities were formed from highly magnetized asteroids after disrup- tion, matching the shapes of suspected metallic small bodies. Simulations of grain avalanching on the surface of magnetized asteroids found additional morphological differences from their unmagnetized counterparts, with reduced densities, increased angles of repose, and cornicing.Item MODEL-BASED SYSTEMS ENGINEERING SIMULATION FRAMEWORK FOR ROBOT GRASPING(2021) Menaka Sekar, Praveen Kumar; Baras, John S; Systems Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Constant rise in industrial usage of robots for commercial applications has led to the need for rapid, efficient, and reliable robotic system development processes. Integration of tools from various disciplines to perform design space exploration,taking into consideration the stakeholder and system requirements, is one major step in regards to this. In this thesis, we apply Model-Based Systems Engineering (MBSE) principles to a simple pick and place task. We do this by integrating Cameo Systems Modeling Language (SysML) tool, CoppeliaSim robot simulator, and Gurobi Optimizer to facilitate and accelerate the design process for a robot grasping system. A simulation based Verification & Validation approach supports design space exploration to obtain optimal design solutions, thereby leading to successful and profitable deployment and operation.Item Simulations of Fire Smoke Movement in High-rise Buildings with FDS(2021) Xu, Hongda; Trouve, Arnaud AT; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Fire Dynamics Simulator (FDS) developed by the National Institute of Standards and Technology (NIST) solves a form of the Navier-Stokes equations appropriate for low-speed (Ma < 0.3), thermally-driven flow with an emphasis on smoke and heat transport and has been shown to be capable of simulating the flow and temperature conditions in the vicinity of a fire [1]. In the present study, we evaluate the ability of FDS to simulate pressure dynamics in high-rise buildings, a pre-requisite to the correct simulation of smoke transport far from the fire.The objective of this study is to test the accuracy of FDS for determining the conditions throughout the entire expanse of a 40-story high-rise building featuring an elevator shaft and four stairwells. The output from FDS is first compared to the results generated by a network model called COSMO. The comparison of the two outputs shows that correct results are predicted by FDS. Additionally, more realistic scenarios are simulated with FDS and the results are compared with those of a network model called CONTAM and an in-house MATLAB program. The network model CONTAM and the MATLAB program do not represent the time-dependent thermal mixing process taking place inside the elevator shaft and the stairwells whereas FDS does. The comparison shows the importance of this thermal mixing process that impacts the pressure dynamics and smoke movement inside the building, with implications for the evacuation capability provided by the stairwells.Item Morphing Waveriders for Atmospheric Entry(2019) Maxwell, Jesse R; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The primary challenge for vehicles entering planetary atmospheres is surviving the intense heating and deceleration encountered during the entry process. Entry capsules use sacrificial ablative heat shields and sustain several g deceleration. The high lift produced by the Space Shuttle geometry resulted in lower rates of heating and deceleration. This enabled a fully reusable vehicle that was protected by heat shield tiles. Hypersonic waveriders are vehicles that conform to the shape of the shock wave created by the vehicle. This produces high compression-lift and low drag, but only around a design Mach number. Atmospheric entry can reach speeds from zero to as high as Mach 40. A morphing waverider is a vehicle that deflects its flexible bottom surface as a function of Mach number in order to preserve a desired shock wave shape. It was demonstrated in this work that doing so retains high aerodynamic lift and lift-to-drag ratio across a wide range of Mach number. Numerical simulations were conducted for case-study waveriders designed for Mach 6 and 8 for flight at their design conditions as well as with variations in angle-of-attack and Mach number. A single-species air model was used between Mach 1 and 12 with the RANS k-omega SST and LES-WALE turbulence models. A seven-species air model was used for Mach 15 at 60km altitude and Mach 20 at 75km. Analytical methods were used to construct a reduced-order model (ROM) for estimating waverider aerodynamic forces, moments, and heating. The ROM matched numerical simulation results within 5-10% for morphing waveriders with variations in angle-of-attack, but discrepancies exceeded 20% for large deviations of rigid vehicles from their design Mach numbers. Atmospheric entry trajectory simulations were conducted using reduced-order models for morphing waverider aerodynamics, the Mars Science Laboratory (MSL) capsule, and the Space Shuttle. Three morphing waveriders were compared to the Space Shuttle, which resulted in reduced heating and peak deceleration. One morphing waverider was compared to the MSL capsule, which demonstrated a reduction in the peak stagnation heat flux, a reduction in the peak and average deceleration, and a reduction in the peak area-averaged heating.Item MODELING AND SIMULATION OF A SEMICONDUCTOR MANUFACTURING FAB FOR CYCLE TIME ANALYSIS(2018) Shinde, Aditya Ramaji; Fu, Michael; Systems Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of the thesis is to conduct a study of the effects of scheduling policies and machine failures on the manufacturing cycle time of the Integrated Circuit (IC) manufacturing process for two processor chips, namely Skylake and Kabylake, manufactured by Intel. The fab simulation model was developed as First in First Out (FIFO), Shortest Processing Time (SPT), Priority based (PB), and Failure FIFO (machine failures) model, and the average cycle times and queue waiting times under the four scheduling policy models were compared for both the Skylake and Kabylake wafers. The study revealed that scheduling policies SPT and PB increased the average cycle time for Skylake wafers while decreasing the average cycle time for the Kabylake wafers, when compared to the base FIFO model. Machine failures increased the average cycle time for both types of wafers.