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 magnetic field of the three-meter spherical Couette experiment(2021) Burnett, Sarah; Lathrop, Daniel P; Ide, Kayo; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The magnetohydrodynamics of Earth have been explored at the University of Maryland and the Institute of Geosciences in Grenoble, France through experiments, numerical models, and machine learning. The interaction between Earth's magnetic fields and its outer core is emulated in a laboratory using the three-meter spherical Couette device filled with liquid sodium driven by two independently rotating concentric shells and an external dipole magnetic field. Recently, the experiment has undergone modifications to increase the helical flows in the poloidal direction to bring it closer to the convection-driven geodynamo flows of Earth. The experiment has 31 surface Hall probes measuring sparsely the external magnetic field. The numerical model, XSHELLS, solves the coupled Navier-Stokes and induction equations numerically to give a full picture of the internal velocity and magnetic field, however, it cannot resolve all the turbulence. In this thesis we aim to improve the prediction of magnetic fields in the experiment by performing studies both on experimental data and simulation data. First, we analyze the simulation data to assess the viability of using the measured external magnetic field to represent the internal dynamics of the velocity and magnetic field. These simulations also elucidate the internal behavior of the experiment for the first time. Next, we compare the experimental magnetic field measurements with the extrapolated surface magnetic field measurements in simulations using principal component analysis by matching all parameters but the level of turbulence. Our goal is to see if (i) the eigenvectors corresponding to the largest eigenvalues are comparable and (ii) how then the surface measurements of the simulation couple with the internal measurements, which are not accessible in the experiment. Next, we perform several machine learning techniques to see the feasibility of using the current probe setup to predict the magnetic fields in time. In the second to last chapter, we assess the potential locations for magnetic field measurements. These studies provide insight on the measurements required to predict Earth's magnetic field.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.