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 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 Finite-Discrete Element Method Simulations of Colliding Red Blood Cells(2014) Warner, Benjamin; Solares, Santiago D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The implantation of artificial heart valves can lead to a large decline in red blood cell life. There has been much research in the last few years dedicated to understanding the cause of this decline. One theory states that collisions at large velocity can lead to spontaneous hemolysis which leads to the premature recycling of cells by the body. Currently, there is no suitable method for modeling the complex intersection interaction of blood cells in a computer code. The Finite-Discrete Element Method (FDEM) is a relatively new computer modeling technique that seeks to combine modeling of continuum-based deformability and discontinuum based motion and element interaction. This thesis utilizes FDEM to model the collision of erythrocytes with other erythrocytes. A method of approximating volume of arbitrary discrete element meshes is proposed and tested for general colliding bodies for accuracy. Red Blood cell simulations are presented with experimentally verifiable data to allow for validation of the model. Future steps are presented for further development of themodel for more specialized applications, such as sedimentation and resting contact. The volume-based FDEM method appears to recreate reasonable results for colliding deformable bodies.Item The Development and Implementation of Numerical Tools for Investigation into the Granular Dynamics of Solid Solar System Bodies(2013) Schwartz, Stephen Ross; Richardson, Derek C; Michel, Patrick; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The work advanced in this thesis joins together the disciplines of planetary science and granular physics. Grain dynamics have played a prominent role in the evolution of our Solar System from planetesimal formation billions of years ago to the surface processes that take place today on terrestrial planets, moons, and small bodies. Recent spacecraft images of small Solar System bodies provide strong evidence that the majority of these bodies are covered in regolith. This regolith ranges in size from the fine powder found on the Moon to large rocks and boulders, like the 27 m Yoshinodai boulder on the small asteroid, Itokawa. Accordingly, the processes that take place on the solid bodies of the Solar System vary widely based upon the material properties of the regolith and the gravitational environments on their surfaces. An understanding of granular dynamics is also critical for the design and operations of landers, sampling devices and rovers to be included in space missions. Part of my research is concerned with the development of numerical tools that have the ability to provide explanations for the types of processes that our spacecraft have observed. Granular processes on Earth are incredibly complex and varied, and constitute an enormous field of study on their own, with input taken from across the broad disciplines of engineering and the physical sciences. In micro-gravity, additional forces, which on Earth are relevant only to micron-size particles or smaller, are expected to become important for material up to the size of large rocks, adding further complexity. The numerical tools developed in this work allow for the simulation of grains using an adaptation of the Soft-Sphere Discrete Element Method (SSDEM) along with implementations of cohesive forces between particles into an existing parallel gravity tree code.Item Estimation of the Time of Concentration with High-Resolution GIS Data: Limitations of Existing Methods and Analysis of New Methods.(2007-05-02) Pavlovic, Sandra; Moglen, Glenn E.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Differences in the calculation of the time of concentration using the velocity method result from different degrees of discretization along the longest flowpath in the watershed. We examined an idealized system for which an analytical solution could be derived. Next, we studied a dataset compiled from watersheds across the State of Maryland, for which the observed time of concentration was known. In both cases we show that the time of concentration estimate increases with the degree of discretization. Two different models were developed that show good predictive agreement with the observed time of concentration. One method uses, gradually varied flow concepts to allow velocity to vary more realistically along the discretized flowpath. The other method uses a regression approach to guide the merging of GIS pixel-based flowpath elements into larger segments. Strengths and limitations of both methods are discussed in the context of future application in Maryland and elsewhere.