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 Response of hypersonic boundary-layer disturbances to compression and expansion corners(2021) Butler, Cameron Scott; Laurence, Stuart; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An experimental campaign was conducted at the University of Maryland - College Park to examine the impact of abrupt changes in surface geometry on hypersonic boundary-layer instability waves. A model consisting of a 5-degree conical forebody was selected to encourage the dominance of second-mode wavepackets upstream of the interaction region. Interchangeable afterbody attachments corresponding to flow deflections of -5-degree to +15-degree in 5-degree increments were considered. The adverse pressure gradient imposed by the +10-degree and +15-degree configurations caused the boundary layer to separate upstream, creating a region of recirculating flow. High-speed schlieren (440-822 kHz) was employed as the primary means of flow interrogation, with supplemental surface measurements provided by PCB132B38 pressure transducers. A lens calibration was applied to the images to provide quantitative fluctuations in density gradient. The high frame rate made possible the use of spectral analysis techniques throughout the entire field of view. This analysis reveals complex growth and decay trends for incoming second-mode disturbances. Additional, low-frequency content is generated by the deflected configurations. This is most pronounced for the separated cases where distinct, shear-generated disturbances are observed. Spectral proper orthogonal decomposition (SPOD) is demonstrated as a powerful tool for resolving the flow structures tied to amplifying frequencies. Nonlinear interactions are probed through bispectral analysis. Resonance of low-frequency structures is found to play a large role in nonlinear energy transfer downstream of the compression corners, particularly for the separated cases. Concave streamline curvature appears to result in concentrated regions of increased nonlinearity. These nonlinear interactions are shown to be spatially correlated with coherent flow structures resolved through SPOD. Finally, a limited computational study is carried out to demonstrate the ability of linear stability theory and the parabolized stability equations to reproduce experimental results obtained for the +10-degree extension. The development of the second-mode and shear-generated disturbances resolved by the computational analysis shows excellent agreement with the experimental results.Item On the interaction of wind energy with climate and weather(2010) Barrie, Daniel; Kirk-Davidoff, Daniel B; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study focuses on the interaction of large-scale wind energy with the atmosphere; namely, the impact that a substantial development of the wind resource may have on climate and weather as well as the impact that anthropogenic global warming (AGW) may have on the amount of available energy in the wind. A large downstream climate response to wind turbines distributed throughout the central United States is shown in model results from the Community Atmosphere Model (CAM). The mean response takes the form of a stationary Rossby wave. Furthermore, a case study is shown where the wind turbines altered a storm system over the North Atlantic. The resulting magnitude of the anomalous 500 hPa geopotential height field is comparable to the range of forecast uncertainty, which indicates that impacts induced in weather systems may be forecastable Building on this work, a thorough examination of wind farm and atmospheric parameters, including wind farm size, position, and parameterization as well as atmospheric static stability and jet strength is carried out using an idealized version of the Weather Research and Forecasting (WRF) model. Downstream impacts were found to grow in magnitude as wind farm size and the value of damping used to parameterize the wind turbines was increased. Altering the position of the wind farm with respect to the westerlies and synoptic disturbances revealed that the interaction between baroclinic instabilities and the wind farm enables downstream propagation and growth of the wind farm impacts. However, far downstream impacts were observed to be somewhat independent of the wind farm position, i.e., similar downstream effects were noted for model runs initialized with wind farms 20° of longitude from each other. By increasing atmospheric static stability, a fast saturation of wind farm-induced anomalies was observed throughout the atmosphere. This observation is surprising in light of the increased phasing between surface and upper atmospheric anomalies when static stability is low. Anomalies were able to propagate farther downstream over a shorter period of time when jet strength was increased. To study projected climate change impacts on the wind resource, data from the third phase of the Coupled Model Intercomparison Project (CMIP3) and the North American Regional Climate Change Assessment Project (NARCCAP) were studied. The results are dominated by substantial intermodel variability; however, many of the models project an increase in wind speeds and energy over the central United States. This increase in wind energy is related to an increase in low-frequency, high-speed transient wind speeds, which have a high power density due to the cubic relationship between wind speed and power.