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 Floquet Heating and Relaxation of Interacting Bose Einstein Condensates(2022) Maslek, James; Porto, James V; Rolston, Steve; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Floquet’s theorem says that any unitary, periodically driven system can be described by an effective time-independent Hamiltonian, where the effective Hamiltonian can have completely different properties than the static, undriven system. Floquet engineering makes use of this idea to simulate new Hamiltonians that would otherwise not be possible in the undriven case. For interacting systems, this approach can be used to realize interesting correlated many-body states, but drive-induced heating must be understood and mitigated. Cold atoms in optical lattices provide a controllable, well-isolated system in which these ideas can and have been realized. I describe research into two areas of Floquet engineering for interacting Bose-Einstein condensates in periodically driven optical lattices. The first half of this thesis focuses on the study of heating mechanisms for condensates in periodically driven lattices. In the weakly interacting limit, one might expect that heating could be described with a Fermi Golden Rule approach. Parametric driving of fluctuations in the condensate, however, can lead to runaway heating that cannot be described perturbatively. We experimentally study heating in shaken 2D square lattices and demonstrate heating consistent with the theoretical predictions of parametric instabilities. The second half of this thesis describes experiments that realize Floquet-induced effective staggered magnetic fields, and the relaxation dynamics of interacting particles subject to these fields. Interestingly, we observe pre-thermal relaxation dynamics, where an initially heated cloud suddenly subject to the effective Hamiltonian condenses into a state governed by the drive-induced effective Hamilton on a timescale faster than heating.Item SEPARATIONS OF WATER-IN-OIL EMULSIONS BY ELECTROSTATIC FIELD AT THE ELEVATED TEMPERATURE(2019) Lee, Hak Seung; Yang, Bao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Separation of water from oil has been a significant subject for a crude-oil purification in the petroleum industry and chemical processing. This dissertation reports theoretical and experimental studies on separation of water from water-in-oil emulsions under combined treatment of a radial electric field and elevated temperature. Compared to macroemulsion, there are fundamental differences when considering microemulsion. It is difficult to separate microemulsion by an electric field due to its tiny droplet size. Microemulsion can be transformed to macroemulsion state over the cloud point. Therefore, heating is applied to the microemulsion to change its phase, then an electric field is applied to the system to expedite the separation speed. The enhanced separation performance using the combined method can be mainly attributed to the reduced viscosity and dissociated surfactants at elevated temperature and the accelerated droplet coalescence under the radial electric field. Theoretical analysis shows that temperature and electric strength can strongly affect the movement of the water droplets, and these effects are also experimentally validated by water/oil separation tests. In our experiments, a cylindrical cone-shaped separation tube equipped with coaxial cylindrical electrodes was built to improve the separation using a non-uniform radial electric field. From the result of the numerical calculations, the precipitation and collision times of water droplets are rapidly reduced as the operating temperature increases. This theoretically expected values accurately predict the experimental results of the water-in-oil emulsion separation tests. It is experimentally observed that increasing the applied voltage and/or temperature can significantly reduce the separation time and the residual water concentration in the emulsion. Lastly, the dynamic water/oil separation system was designed using the combined method of heating and electric field. The effects of operating temperature and flow rate on the quality of the separated oil are investigated by executing several tests at different temperatures and flow rates. From the results of the dynamic separation tests, the residual water concentration in the separated oil reduces as the operating temperature increases under the boiling point. Also, the water concentration of the separated oil increases as the flow rate increases due to the reduced residence time.Item Ionospheric Turbulence Near the Upper Hybrid Layer: Theory and Experiment(2016) Najmi, Amir Christopher; Papadopoulos, Konstantinos; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The thesis presents experimental results, simulations, and theory on turbulence excited in magnetized plasmas near the ionosphere’s upper hybrid layer. The results include: The first experimental observations of super small striations (SSS) excited by the High-Frequency Auroral Research Project (HAARP) The first detection of high-frequency (HF) waves from the HAARP transmitter over a distance of 16x10^3 km The first simulations indicating that upper hybrid (UH) turbulence excites electron Bernstein waves associated with all nearby gyroharmonics Simulation results that indicate that the resulting bulk electron heating near the upper hybrid (UH) resonance is caused primarily by electron Bernstein waves parametrically excited near the first gyroharmonic. On the experimental side we present two sets of experiments performed at the HAARP heating facility in Alaska. In the first set of experiments, we present the first detection of super-small (cm scale) striations (SSS) at the HAARP facility. We detected density structures smaller than 30 cm for the first time through a combination of satellite and ground based measurements. In the second set of experiments, we present the results of a novel diagnostic implemented by the Ukrainian Antarctic Station (UAS) in Verdansky. The technique allowed the detection of the HAARP signal at a distance of nearly 16 Mm, and established that the HAARP signal was injected into the ionospheric waveguide by direct scattering off of dekameter-scale density structures induced by the heater. On the theoretical side, we present results of Vlasov simulations near the upper hybrid layer. These results are consistent with the bulk heating required by previous work on the theory of the formation of descending artificial ionospheric layers (DIALs), and with the new observations of DIALs at HAARP’s upgraded effective radiated power (ERP). The simulations that frequency sweeps, and demonstrate that the heating changes from a bulk heating between gyroharmonics, to a tail acceleration as the pump frequency is swept through the fourth gyroharmonic. These simulations are in good agreement with experiments. We also incorporate test particle simulations that isolate the effects of specific wave modes on heating, and we find important contributions from both electron Bernstein waves and upper hybrid waves, the former of which have not yet been detected by experiments, and have not been previously explored as a driver of heating. In presenting these results, we analyzed data from HAARP diagnostics and assisted in planning the second round of experiments. We integrated the data into a picture of experiments that demonstrated the detection of SSS, hysteresis effects in simulated electromagnetic emission (SEE) features, and the direct scattering of the HF pump into the ionospheric waveguide. We performed simulations and analyzed simulation data to build the understanding of collisionless heating near the upper hybrid layer, and we used these simulations to show that bulk electron heating at the upper hybrid layer is possible, which is required by current theories of DAIL formation. We wrote a test particle simulation to isolate the effects of electron Bernstein waves and upper hybrid layers on collisionless heating, and integrated this code to work with both the output of Vlasov simulations and the input for simulations of DAIL formation.Item Performance of Residential Heating and Cooling Control Strategies using Distributed Wireless Sensor Networks(2010) Siemann, Michael; Kim, Jungho; Chopra, Nikhil; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Previous work has suggested that residential space heating and cooling control strategies that partition the structure into individual zones using wireless sensor networks might result in lower energy consumption compared to systems using a single-sensor thermostat. Questions have been posed as to whether these strategies can achieve the same level of performance in a variety of geographic locations and climates. This study compared four control strategies that utilized a wireless temperature and humidity sensor network to regulate the comfort of a residence in the mid-Atlantic region of the United States during the summer and winter. In particular, the energy consumption and comfort levels of each multi-sensor strategy were compared to a baseline strategy that mimicked a single thermostat. The difference in energy usage measured by each control strategy was found to be statistically insignificant. However, experiments indicated that these strategies may nevertheless result in improvements in thermal comfort.