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
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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 Reversibility, memory formation and collective rotations in dense granular media(2021) Benson, Zackery; Losert, Wolfgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Granular matter is a broad term that describes materials comprised of macroscopic grains. Granular material has unique properties that can mimic either a solid- or fluid-like system and has macroscopic behaviors such as segregation, shear- jamming, reversibility, and compaction. The finite size of the grains suggests that thermal fluctuations are neglibible compared to the macroscopic interactions such as gravitational potential. This implies that tools developed in thermodynamics cannot be readily applied. Instead, research into granular material uses a combination of bulk measurements (packing density, pressure) with grain-scale tracking of position, orientation, and forces. This thesis presents four main studies utilizing three-dimensional experiments and simulations to probe the dynamics of individual grain subject to cyclic compression.The first study uses numerical simulations to connect granular rotations to translations in sheared granular packings. It is proposed that rotations play an extensive role in the formation of shear zones in granular packing, in which the rotations allow for ball-bearing like motion that could reduce the stress from external pressures. In this study, we quantify the effect of friction on the shear-zone rotations and translations. We find a direct connection between average rotations and the vorticity of translations independent of the friction. The second study explores reversibility of grain translations and rotations in the context of memory formation. In granular matter, memory is formed via a response to an external perturbation, ranging from compression and shear to thermal cycling. In this project, we encode and read out memory of compression amplitudes for a cyclically driven granular system. We find that memory is significantly affected by the interparticle friction of each grain and is most readily extracted by quantifying reversible displacements within the sample. The third study experimentally measures three-dimensional orientations of granular spheres using our refractive index matched scanning setup. We apply a combination of deep learning and image processing to extract the position and orientation of individual grains subject to cyclic compression. Using this, we can quantify the spatial distribution of sliding and rolling motion of contacts. We find that sliding occurs deep within the sample where the grains are mostly fixed in place. This occurs with an increase in internal stress within the material. Finally, we explore collectively rotating states in three-dimensions. We introduce a new measure in which we can identify affine (collective) and non-affine rotations. We find that grain rotations are generated by minimizing sliding mo- tion between all contacts independent of the forces between each contact. Further, we find that the collective rotation component is more directly correlated to irreversible translations than the residual rotations. This result identifies that collective rotations are important to reversible states in sheared granular systems.Item Modeling and Experimental Measurement of Triboelectric Charging in Dielectric Granular Mixtures(2020) Carter, Dylan Patrick; Hartzell, Christine M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Triboelectric charging, the phenomenon by which electrical charge is exchanged during contact between two surfaces, has been known to cause significant charge separation in granular mixtures, even between chemically identical grains. This charging is a stochastic process resulting from random collisions between grains, but creates clear charge segregation according to size in dielectric granular mixtures. Experiments in grain charging are frequently conducted with methods that may introduce additional charging mechanisms that would not be present in airless environments, and often aren't capable of measuring the precise charge of each grain. We resolved these issues through the development of a model that predicts the mean charge on grains of a particular size in an arbitrary mixture, and through experiments that do offer controlled measurement of precise grain charges. These results can be used to develop methods for electrostatic sorting to enable \textit{in situ} resource utilization of silica-based regoliths on airless extraterrestrial bodies. Beginning from a basic collision model for a mixture of hard spheres, we developed a robust semi-analytical model for making predictions about the charge distribution in a dielectric granular mixture. This model takes a set of assumptions about a mixture, including the continuous size distribution, collision frequencies, and charge transfered per collision, and calculates the mean charge acquired by grains of each size after all charges have been exchanged. This model allows us to explore experimental results through many different lenses. To test our predictions and provide a repeatable and flexible method for analyzing charging in a variety of granular mixtures, we designed and built our own experimental test stand. This device is housed entirely in a vacuum chamber, allowing us to induce tribocharging in dielectric grains in a controlled airless environment and measure individual charge and diameter of a grain by dropping samples through a transverse electric field. We observed that mixtures of zirconia-silica grains containing two primary size fractions exhibited size-dependent charge segregation when charged in vacuum. Unlike in other experiments with grains charged by fluidization with a gas, we consistently observed that the small grains charged predominantly positive, while the large grains were primarily negative. We considered a variety of charge transfer mechanisms and generated predicted charge distributions for each using the modeling framework we developed. Comparing these models to the collected data, we are able to assess the viability of each potential transfer mechanism by examining properties of its resulting distribution, including the relative charge magnitudes for each size fraction, the point at which the polarity changes, and the polarity and magnitude of the charge carrier density. The results of this work provide solid supporting evidence for the role of positive charge carriers in dielectric tribocharging. While some prior work has suggested positive ions from the atmosphere and/or adsorbed water are responsible, we have observed that even when these environmental factors are reduced or eliminated, silica-based materials still exhibit positive charge transfer. The modeling framework developed in search of a descriptive model for this effect is a useful, adaptable tool. The experimental apparatus itself, and especially in conjunction with these modeling tools, overcomes some of the more difficult challenges faced by experimentalists investigating granular tribocharging, enabling further investigation into tribocharging in regolith and other dielectric materials.