UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
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Item Studying wildfire spread using stationary burners(2014) Gorham, Daniel Jack; Gollner, Michael J; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A method for performed experiments using stationary gas burners and liquid fuel-soaked wicks to study flame geometry and buoyant instabilities important to fundamental wildland fire behaviour has been developed. This thesis focuses on experiments performed with stationary fires to carefully study instabilities observed in spreading fires that suggest they play a critical role in fire spread. Two types of flow conditions were used to perform experiments similar to wildfire spread conditions: sloped fuel surface and forced-flow (wind aided). Small- scale inclined experiments for performed at the University of Maryland with liquid- fuel soaked wicks and large-scale experiments at the USDA Forest Service Missoula Fire Sciences Laboratory with a gas-burner. These experiments were performed with over a range of heat-release-rates and burner sizes for angles from 0 to 60 degrees from the horizontal. Forced-flow experiments were performed in a large-scale wind tunnel at the Missoula Fire Laboratory and at the University of Maryland with a well characterized wind blower with gas-burners. These experiments were performed for a range of heat-release-rates and burner sizes in wind speeds from 0.2 to 3.0 ms−1 The flame geometry was determined using high-speed videography. Important two-dimensional flame geometry parameters such as centerline flame length and flame tilt angle were measured from these images. Flame intermittency and pulsation close to the surface was measured using high-speed videography and micro-thermocouples. A method was developed to track the extension of the flame close to the surface which would come in direct contact with unburnt fuels ahead of the fire. These methods showed that the pul- sation frequency is complicated suggesting large scale structures in the flow. Using these frequency the stationary experiments follow similar Strouhal-Froude scaling for flame pulsations in spreading fires. Stream-wise streaks in the flow were observed and measured using high-speed videography. Streak spacing has been observed to be associated with possible Gotler votice structures in the fire. The spacing for streaks at the base of the flame for these startionary experiments appear to be dependent on the boundary- layer conditions and could possibly scale with the centerline flame length similar to flame towers observed in spreading fires.Item Turbulent Transport in Global Models of Magnetized Accretion Disks(2011) Sorathia, Kareem; Reynolds, Christopher; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The modern theory of accretion disks is dominated by the discovery of the magnetorotational instability (MRI). While hydrodynamic disks satisfy Rayleigh's criterion and there exists no known unambiguous route to turbulence in such disks, a weakly magnetized disk of plasma is subject to the MRI and will become turbulent. This MRI-driven magnetohydrodnamic turbulence generates a strong anisotropic correlation between the radial and azimuthal magnetic fields which drives angular momentum outwards. Accretion disks perform two vital functions in various astrophysical systems: an intermediate step in the gravitational collapse of a rotating gas, where the disk transfers angular momentum outwards and allows material to fall inwards; and as a power source, where the gravitational potential energy of infalling matter can be converted to luminosity. Accretion disks are important in astrophysical processes at all scales in the universe. Studying accretion from first principles is difficult, as analytic treatments of turbulent systems have proven quite limited. As such, computer simulations are at the forefront of studying systems this far into the non-linear regime. While computational work is necessary to study accretion disks, it is no panacea. Fully three-dimensional simulations of turbulent astrophysical systems require an enormous amount of computational power that is inaccessible even to sophisticated modern supercomputers. These limitations have necessitated the use of local models, in which a small spatial region of the full disk is simulated, and constrain numerical resolution to what is feasible. These compromises, while necessary, have the potential to introduce numerical artifacts in the resulting simulations. Understanding how to disentangle these artifacts from genuine physical phenomena and to minimize their effect is vital to constructing simulations that can make reliable astrophysical predictions and is the primary concern of the work presented here. The use of local models is predicated on the assumption that these models accurately capture the dynamics of a small patch of a global astrophysical disk. This assumption is tested in detail through the study of local regions of global simulations. To reach resolutions comparable to those used in local simulations an orbital advection algorithm, a semi-Lagrangian reformulation of the fluid equations, is used which allows an order of magnitude increase in computational efficiency. It is found that the turbulence in global simulations agrees at intermediate- and small-scales with local models and that the presence of magnetic flux stimulates angular momentum transport in global simulations in a similar manner to that observed for local ones. However, the importance of this flux-stress connection is shown to cast doubt on the validity of local models due to their inability to accurately capture the temporal evolution of the magnetic flux seen in global simulations. The use of orbital advection allows the ability to probe previously-inaccessible resolutions in global simulations and is the basis for a rigorous resolution study presented here. Included are the results of a study utilizing a series of global simulations of varying resolutions and initial magnetic field topologies where a collection of proposed metrics of numerical convergence are explored. The resolution constraints necessary to establish numerical convergence of astrophysically-important measurements are presented along with evidence suggesting that the use of proper azimuthal resolution, while computationally-demanding, is vital to achieving convergence. The majority of the proposed metrics are found to be useful diagnostics of MRI-driven turbulence, however they suffer as metrics of convergence due to their dependence on the initial magnetic field topology. In contrast to this, the magnetic tilt angle, a measure of the planar anisotropy of the magnetic field, is found to be a powerful tool for diagnosing convergence independent of initial magnetic field topology.Item Design of an Overmoded Ka-Band Sheet-Beam Coupled-Cavity Traveling-Wave Tube Amplifier(2011) Larsen, Paul Benjamin; Antonsen, Thomas M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis develops a qualified design for a sheet-beam coupled-cavity slow-wave structure for use in a high-power millimeter wave traveling wave tube amplifier. The main advance realized in the design is the roughly ten-fold increase in power gained by utilizing a sheet, rather than cylindrical, beam while at the same time employing mode-suppression techniques to suppress competing modes that are introduced by the sheet geometry. This design addresses considerations relevant to high-power tubes in general, as well as points specific to the design of a sheet-beam structure. The coupled-cavity structure is designed with the following general characteristics: center frequency of 35 GHz with greater than a 10% bandwidth, and capabilities of 5 kW pulsed output power. The device operating parameters are as follows: a moderate gain of 18 dB, and an experimentally demonstrated sheet electron beam with 3.5 A, 19.5 kV, and 0.3 mm x 4.0 mm beam cross-section. The final design goal has been to limit the interaction length as much as possible to reduce magnet weight and complications. A final design structure is proposed, which produces in excess of 5 kW peak power in simulation with safeguards from instabilities. The structure geometry is based on a novel design for a sheet-beam coupled-cavity slow-wave structure that has been characterized through various analyses, simulations, and experiments. This thesis outlines and details the various techniques used to probe the structure and thus form a full characterization of the structure and proposed amplifier device. The concept espoused by much of this work is to adapt the analyses from cylindrical beam devices for the sheet-beam geometry. Then we make comparisons between the new sheet-beam structure and conventional devices. From these comparisons we draw conclusions on the operation of sheet-beam amplifiers and make design choices accordingly. The final design is validated with fully three-dimensional particle simulations and predicts stable amplification across the range of operation.