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 PHYSICS AND APPLICATIONS OF EXTENDED AIR HEATING BY FEMTOSECOND LASER FILAMENTATION(2022) Larkin, Ilia; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Femtosecond laser pulses of sufficient energy can propagate as filaments in air due to a dynamic interplay between nonlinear self-focusing and ionization-induced defocusing. A filament in air is characterized by a narrow, 100 m diameter core propagating at high intensity for many Rayleigh ranges corresponding to the core diameter, surrounded by a lower intensity reservoir that exchanges optical energy with the core. The high intensity core ionizes the air and excites molecular rotational wavepackets in N2 and O2. Thermal relaxation of these excitations leads to air heating over very long and narrow volumes, launching acoustic waves and imprinting density profiles in air. These features enable longitudinal mapping of energy absorption, interaction with aerosols in air, guiding of high voltage discharges, and the generation of long air waveguides for subsequent laser pulses. All of these topics are detailed in this dissertation.In particular, we present: (1) Single shot axially resolved energy deposition measurements, using a synchronized array of microphones, to see on a shot-by-shot basis the effect of air turbulence on nonlinear pulse propagation. (2) Measurements of the pre-breakdown evolution of a laser triggered high voltage spark gap, induced by a density channel imprinted by femtosecond laser pulses. By interferometrically measuring air heating and current leakage through the spark gap we clarify the role of laser plasma vs laser air heating in triggering breakdowns. (3) Air waveguiding experiments extended to ranges up to 50 m from the original ~1 m experiments. (4) Fog droplet clearing experiments showing that in natural filamentation of a collimated beam, direct optical interactions are the dominant clearing mechanism rather than acoustic effects.Item NONLINEAR WAVE CHAOS AND THE RANDOM COUPLING MODEL(2019) Zhou, Min; Anlage, Steven M.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Concepts from the field of wave chaos have been shown to successfully predict the statistical properties of linear electromagnetic fields in electrically large enclosures. The Random Coupling Model (RCM) describes these properties by incorporating both universal features described by Random Matrix Theory and the system-specific features of particular system realizations. This Ph.D. thesis studies various approaches to extend the RCM to the nonlinear domain. Nonlinearity has been introduced to study the statistics of generated harmonics and amplitude dependent responses of complex electromagnetic structures. The sources of nonlinearity that have been studied include circuit elements such as diodes, nonlinear dielectrics, and superconducting materials. Nonlinear systems in different scenarios are studied and the RCM is applied and extended to explain the statistical results. This is an important step in the ongoing effort to create the science of nonlinear wave chaos.Item Nonlinear Interactions in Planar Jet Flow with High Frequency Excitation(2016) Kreutzfeldt, Timothy; Chopra, Inderjit; Glaz, Bryan; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An experimental active flow control study was conducted involving excitation of a tabletop planar turbulent jet with a high frequency piezoelectric actuator. The excitation frequencies considered corresponded to the dissipative subrange of turbulent kinetic energy and were orders of magnitude greater than classical shear layer instability modes. Single-wire and dual-wire hot wire probes were used to determine how excitation induces alterations to bulk flow quantities as well as nonlinear interactions. Differences in flow receptivity to high frequency excitation were investigated by varying the development length of the turbulent jet at a Reynolds number of 8,700 and Strouhal number of 21.3. Excitation of developed turbulent flow yielded larger increases in the energy dissipation rate and higher magnitude velocity power spectrum peaks at the forcing frequency than undeveloped turbulent flow. Further tests with excitation of reduced mean velocity flow at a Reynolds number of 6,600 and a Strouhal number of 27.8 demonstrated that high frequency forcing resulted in transfer of energy from large to small scales in the turbulent kinetic energy spectrum. This phenomenon appeared to support past literature that indicated that the mechanics of high frequency forcing are fundamentally different from conventional instability-based forcing. Theoretical arguments are presented to support these experimental observations where it is shown that coupling between the applied forcing and background turbulent fluctuations is enhanced. An eddy viscosity model first proposed under the assumption of instability-based forcing was shown to be an effective approximation for the experimental measurements presented here in which the flow was forced directly at turbulence scales. Dimensional analysis of the coupling between the induced oscillations and the turbulent fluctuations supported experimental findings that receptivity to excitation was increased for forced flow with higher turbulent kinetic energy, higher excitation amplitude, and lower energy dissipation rate. This study is the first to present such results which validate a model that offers theoretical insight into flow control mechanics when directly forcing small scale turbulent fluctuations.Item Network Algorithms for Complex Systems with Applications to Non-linear Oscillators and Genome Assembly(2013) Schmitt, Karl Robert Bruce; Girvan, Michelle; Zimin, Aleksey; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Network and complex system models are useful for studying a wide range of phenomena, from disease spread to traffic flow. Because of the broad applicability of the framework it is important to develop effective simulations and algorithms for complex networks. This dissertation presents contributions to two applied problems in this area First, we study an electro-optical, nonlinear, and time-delayed feedback loop commonly used in applications that require a broad range of chaotic behavior. For this system we detail a discrete-time simulation model, exploring the model's synchronization behavior under specific coupling conditions. Expanding upon already published results that investigated changes in feedback strength, we explore how both time-delay and nonlinear sensitivity impact synchronization. We also relax the requirement of strictly identical systems components to study how synchronization regions are affected when coupled systems have non-identical components (parameters). Last, we allow wider variance in coupling strengths, including unique strengths to each system, to identify a rich synchronization region not previously seen. In our second application, we take a complex networks approach to improving genome assembly algorithms. One key part of sequencing a genome is solving the orientation problem. The orientation problem is finding the relative orientations for each data fragment generated during sequencing. By viewing the genomic data as a network we can apply standard analysis techniques for community finding and utilize the significantly modular structure of the data. This structure informs development and application of two new heuristics based on (A) genetic algorithms and (B) hierarchical clustering for solving the orientation problem. Genetic algorithms allow us to preserve some internal structure while quickly exploring a large solution space. We present studies using a multi-scale genetic algorithm to solve the orientation problem. We show that this approach can be used in conjunction with currently used methods to identify a better solution to the orientation problem. Our hierarchical algorithm further utilizes the modular structure of the data. By progressively solving and merging sub-problems together we pick optimal `local' solutions while allowing more global corrections to occur later. Our results show significant improvements over current techniques for both generated data and real assembly data.Item Dynamics of Free Piston Stirling Engines(2009) Choudhary, Farhan; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Free piston Stirling engines (FPSEs) are examples of closed cycle regenerative engines, which can be used to convert thermal energy into mechanical energy. FPSEs are multi-degree-of-freedom dynamical systems that are designed to operate in a periodic manner. Traditionally, the designed periodic orbits are meta-stable, making the system operation sensitive to disturbances. A preferred operating state would be an attracting limit cycle, since the steady-state dynamics would be unique. In this thesis, it is investigated as to how to engineer a Hopf bifurcation of an equilibrium solution in a FPSE. Through a combination of weakly nonlinear analysis and simulations, it is shown that it is possible to engineer a Hopf bifurcation in a FPSE system. Through the analyses, reduced-order-models are developed on the basis of Schmidt formulations and nodal analysis. This thesis effort could serve as a platform for designing FPSEs which take advantage of nonlinear phenomena in either the beta or double acting alpha configuration.Item A COMPARISON BETWEEN AN ORIGIN BASED METHOD AND A NONLINEAR COMPLEMENTARITY BASED METHOD FOR SOLVING THE TRAFFIC ASSIGNMENT PROBLEM(2009) Olarte, Rafael Ernesto; Haghani, Ali; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis compares Bar-Gera's Method and Aashtiani's Method for solving the static traffic assignment problem with fixed demand. Specifically, it compares the computational time spent by their corresponding algorithms in thirteen networks based on real cities. It also verifies whether the assumptions made by both methods and the data used allowed such a comparison. To implement Aashtiani's algorithm, a computer code was appropriately designed. To implement Bar-Gera's algorithm, a non-open source application was used. Numerical results showed mixed results but still showed the following trends: (1) Aashtiani's algorithm seems to be faster when solving complex networks, (2) Bar-Gera's algorithm is almost always faster for very high levels of accuracy while Aashtiani's algorithm is faster for lower levels of accuracy, and (3) Bar-Gera's algorithm almost always increases its speed consistently as more accuracy is demanded. Numerical results also showed that for small networks (specifically, when the number of arcs times the number of links is less than 1.0E+7), both algorithms spent practically no more than one second, rending these networks not recommendable for carrying out future comparisons. As expected, Bar-Gera's method required less memory. This thesis also presents a unified terminology for both methods and adapted Aashtiani's formulation to this specific problem.Item Measurements of doping-dependent microwave nonlinear response in cuprate superconductors(2007-04-25) Mircea, Dragos Iulian; Anlage, Steven M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Near-field microwave techniques have been successfully implemented in the past for the local investigation of magnetic materials and high-temperature superconductors. This dissertation reports on novel phase-sensitive linear- and nonlinear response microwave measurements of magnetic thin films and cuprate superconductors and their interpretation.Item Energy Localization and Transport in Binary Isotopically Disordered Fermi-Pasta-Ulam Chains(2005-05-26) Snyder, Kenneth Alan; Kirkpatrick, Theodore R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Energy transport in binary isotopically disordered (BID) nonlinear Fermi-Pasta-Ulam (FPU) chains is a competition between localization and mode transitions. Starting from an arbitrary localized pulse, energy will dissipate ballistically until either Anderson localization (a disorder effect) or phonon scattering (a nonlinearity effect) slow the rate of dissipation. To reduce computational effort, we propose starting from a localized energy eigenstate so that in the absence of anharmonicity the energy is stationary and there is no transport. The second moment of the site energies is used to characterize an effective thermal conductivity as a function of impurity concentration and nonlinearity strength. Calculating the properties of harmonic BID chains at arbitrary impurity concentration is complicated by the pure-disordered-pure transition that occurs as the impurity concentration varies from zero to one. The localization length of dilute impurity harmonic BID chains is calculated exactly using scaling laws and the scattering cross section of a single impurity, which is calculated for discrete systems, differs from the continuum result. For arbitrary impurity concentration, the localization length is estimated by assuming independent contributions from the two limiting cases of pure material. Information entropy was used to show that the number of modes excited by phonon scattering decreased with increasing impurity concentration, a fact that consistent with density of states calculations. At all impurity concentrations, the second moment of the site energies increases linearly in time, a fact that is corroborated by the number of masses participating in energy transport, as calculated from the localization parameter. The dilute concentration dependence of the effective thermal conductivity was consistent with kinetic theory. At the highest concentrations the thermal conductivity was proportional to the original localization length because mode suppression and dense impurities meant that the same length scale remained dominant over a long period of time.