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 Increasing Helicity towards Dynamo Action with Rough Boundary Spherical Couette Flows(2022) Rojas, Ruben; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The dynamo action is the process through which a magnetic field is amplified and sustained by electrically conductive flows. Galaxies, stars and planets, all exhibit magnetic field amplification by their conductive constituents. For the Earth in particular, the magnetic field is generated due to flows of conductive material in its outer core. At the University of Maryland, our Three-meter diameter spherical Couette experiment uses liquid sodium between concentric spheres to mimic some of these dynamics, giving insight into these natural phenomena. Numerical studies of Finke and Tilgner (Phys. Rev. E, 86:016310, 2012) suggest a reduction in the threshold for dynamo action when a rough inner sphere was modeled by increasing the poloidal flows with respect to the zonal flows and hence increasing helicity. The baffles change the nature of the boundary layer from a shear dominated to a pressure dominated one, having effects on the angular momentum injection. We present results on a hydrodynamics model of 40-cm diameter spherical Couette flow filled with water, where torque and velocimetry measurements were performed to test the effects of different baffle configurations. The selected design was then installed in the 3-m experiment. In order to do that, the biggest liquid sodium draining operation in the history of the lab was executed. Twelve tons of liquid sodium were safely drained in a 2 hours operation. With the experiment assembled back and fully operational, we performed magnetic field amplification measurements as a function of the different experimental parameters including Reynolds and Rossby numbers. Thanks to recent studies in the hydrodynamic scale model, we can bring a better insight into these results. Torque limitations in the inner motor allowed us to inject only 4 times the available power; however, amplifications of more than 2 times the internal and external magnetic fields with respect to the no-baffle case was registered. These results, together with time-dependent analysis, suggest that a dynamo action is closer than before; showing the effect of the new baffles design in generating more efficient flows for magnetic field amplification. We are optimistic about new short-term measurement in new locations of the parameter space, and about the rich variety of unexplored dynamics that this novel experiment has the potential to reach. These setups constitute the first experimental explorations, in both hydrodynamics and magnetohydrodynamics, of rough boundary spherical Couette flows as laboratory candidates for successful Earth-like dynamo action.Item Simulations of Accretion Mechanisms and Observational Signatures of Black Hole Accretion Disks(2019) Smith, Megan; McKinney, Jonathan C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Black holes have been a subject of fascination since they were first theorized about over a century ago. There are many questions about them left unanswered. One of these questions is how matter is accreted onto these objects when the plasma around them is rotating in an accretion disk. An answer to this question is likely to be found in the magnetohydrodynamic processes that occur in the plasma, which require highly sophisticated numerical simulations to explore. In this thesis, I describe an analysis of one magnetohydrodynamic instability found in these simulations as well as the observational signatures it produces, which might be recognized in observations of these systems. For the remainder of this thesis, I will discuss the formation and evolution of a formal near-peer mentoring program for women in the University of Maryland physics department. Mentoring programs have been shown to have a number of benefits for both mentors and mentees. Primary among them is an increased sense of belonging and science identity, which is linked to increased retention. Given the so-called "leaky pipeline" problem of women leaving physics, a field where they are already underrepresented, efforts to improve retention are vital and peer mentoring is one way to do this.Item Accretion Physics Through the Lens of the Observer: Connecting Fundamental Theory with Variability from Black Holes(2018) Hogg, James Andrew; Reynolds, Christopher S; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Variability is a generic feature of accretion onto black holes. In both X-ray binaries and active galactic nuclei, variability is observed on nearly all accessible timescales and across the entire electromagnetic spectrum. On different timescales and at different wavelengths it has unique signatures that can be used to characterize the accretion processes generating the emission and probe the accretion disks, which would otherwise be impossible. Despite having been observed for over fifty years, interpreting this variability is difficult. Simple phenomenological models have been used to explain the behaviors and geometries of the observed accretion disk, but they have yet to be rigorously tested in a full magnetohydrodynamic framework. In this dissertation we use high-resolution numerical models to investigate: (1) ``propagating fluctuations" in mass accretion rate that give rise to the nonlinear signatures of accretion on viscous timescales, (2) the dynamics of truncated accretion disks which are invoked to explain the spectral variation of outbursting X-ray binaries and the bifurcation of AGN accretion states, and (3) the large-scale magnetic dynamo behavior in thick and thin accretion disks. We find that the structured variability readily seen in the light curves from accreting black holes (i.e. log-normal flux distributions, linear relations between the RMS and the flux, and radial coherence) quickly and naturally grows from the MRI-driven turbulence and that these properties translate into photometric variability. For the first time, we identify the large-scale magnetic dynamo as the source of the low-frequency modulations of the disk stress that cause this structure. We introduce a bistable cooling law into hydrodynamic and magnetohydrodynamic simulations to study the manifestation of a truncated accretion disk in each regime. We find that rather than a truncation edge, the transition is better described by a ``truncation zone" when the angular momentum transport and heating is governed by MRI-driven turbulence instead of a true viscosity. Additionally, we find that the hot gas in the simulation buoyantly rises in a gentle outflow and eventually fills the entire volume, instead of simply being confined to the innermost region. The outflow interacts with the disk body and enhances the magnetic stresses, which could produce stronger quasiperiodic variability. Finally, we conduct an investigation of the large-scale magnetic dynamo using a suite of four global magnetohydrodynamic disk simulations with scaleheight ratios of $h/r=\{0.05, 0.1, 0.2, 0.4\}$. Most notably, the organization that is prevalent in accretion disk simulations and described as a ``butterfly pattern" does not occur when $h/r \ge 0.2$, despite the dynamo action still operating efficiently.Item Fast Solvers and Uncertainty Quantification for Models of Magnetohydrodynamics(2014) Phillips, Edward Geoffrey; Elman, Howard C; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The magnetohydrodynamics (MHD) model describes the flow of electrically conducting fluids in the presence of magnetic fields. A principal application of MHD is the modeling of plasma physics, ranging from plasma confinement for thermonuclear fusion to astrophysical plasma dynamics. MHD is also used to model the flow of liquid metals, for instance in magnetic pumps, liquid metal blankets in fusion reactor concepts, and aluminum electrolysis. The model consists of a non-self-adjoint, nonlinear system of partial differential equations (PDEs) that couple the Navier-Stokes equations for fluid flow to a reduced set of Maxwell's equations for electromagnetics. In this dissertation, we consider computational issues arising for the MHD equations. We focus on developing fast computational algorithms for solving the algebraic systems that arise from finite element discretizations of the fully coupled MHD equations. Emphasis is on solvers for the linear systems arising from algorithms such as Newton's method or Picard iteration, with a main goal of developing preconditioners for use with iterative methods for the linearized systems. In particular, we first consider the linear systems arising from an exact penalty finite element formulation of the MHD equations. We then draw on this research to develop solvers for a formulation that includes a Lagrange multiplier within Maxwell's equations. We also consider a simplification of the MHD model: in the MHD kinematics model, the equations are reduced by assuming that the flow behavior of the system is known. In this simpler setting, we allow for epistemic uncertainty to be present. By mathematically modeling this uncertainty with random variables, we investigate its implications on the physical model.Item Numerical Simulations of Magnetorotational Turbulence in the Laboratory(2007-04-26) Tillotson, Wilson Andrew; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)When threaded by a weak magnetic field, a differentially rotating electrically conducting fluid may exhibit the Magnetorotational Instability (MRI), a likely mechanism for enhanced angular momentum transport in accretion disks. In this thesis, we investigate the MRI and its role in the transition to magnetohydrodynamic turbulence in laboratory liquid metal flows. In addition to presenting a basic WKB local linear analysis, we use two independently developed, global, nonlinear codes to study the problem of MRI in cylindrical geometry. We verify both codes by demonstrating their ability to simulate well-known nonlinear fluid phenomena, such as the development of Taylor vortices in unstable viscous Taylor-Couette flow. In the presence of magnetic fields, we demonstrate that both codes reproduce the correct MRI stability threshold. Our numerical simulations predict the nonlinear saturation amplitude of excited MRI modes for a range of Prandtl numbers, and results indicate that in laboratory liquid metal investigations, these magnetic excitations saturate at a low level when compared to the background field strength. We address the characteristics of saturated MRI excitations, and investigate their susceptibility to secondary instabilities, such as tearing modes. Finally, we predict the phenomenology of MRI near threshold in realistic cylindrical liquid metal experiments, including the effects of adding a toroidal field in the presence of endcaps. We comment on how the tools created during this research can be used to aid in the design of future experiments to investigate this transition region to magnetic turbulence.