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 The Effects of Model Scaling on Sediment Transport in Brownout(2013) Glucksman-Glaser, Mark Samuel; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The phenomenon of “brownout” is characterized by a large cloud of sediment or dust that is formed around a rotorcraft when it takes off or lands in arid or dusty environments. To further understand the physics of brownout, a laboratory-scale rotor hovering in water was tested over a ground plane covered with a mobile sediment bed. The sensitivity of the dual-phase flow environment to changes in the values of the similarity parameters that potentially govern the fluid dynamics of the rotor flow and the transport of sediment was explored. First, dye flow visualization was performed to study the general evolution of the rotor flow and its interaction with the ground plane. Then, dual-phase flow visualization was used to expose the details of the processes that mobilize and uplift loose particles from the sediment bed. It was shown using the flow visualization that the trailed vortices from the rotor blades were a primary contributor to the mobilization and suspension of sediment. Particle image velocimetry (PIV) was also used to obtain quantitative measurements of the flow velocities found in the rotor wake and near the ground plane. It is then discussed as to why the steady flow assumptions used in the usual definitions of the classical similarity parameters governing sediment transport are not as applicable to the dual-phase flows produced by a rotor operating over a mobile sediment bed. A Buckingham-π analysis was performed to determine a set of new similarity parameters that potentially better reflect the dual-phase flow characteristics relevant to sediment mobilization and suspension by a rotor wake, including the characteristics of the tip vortices. Sixteen new similarity parameters were initially determined, five of which selected as having particular relevance. Specifically, these new similarity parameters were: 1. The mobile inertia ratio; 2. The stationary inertia ratio, 3. The terminal-swirl velocity ratio; 4. The threshold-swirl velocity ratio; 5. The terminal/threshold-swirl velocity ratio. The values of these similarity parameters were determined using the PIV measurements, and were all found to correlate to the quantity of sediment mobilized and uplifted by the rotor. The terminal/threshold-swirl velocity ratio is proposed as the potentially most important similarity parameter for further characterizing the brownout phenomenon.Item Sediment connectivity between the lower Susquehanna River and upper Chesapeake Bay(2019) Russ, Emily; Palinkas, Cindy; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Excess fine sediment is one of the main pollutants contributing to water quality degradation in the upper Chesapeake Bay. Recent management efforts have focused on reducing sediment inputs within the Bay watershed to achieve water quality standards set in the Chesapeake Bay Total Maximum Daily Load (TMDL). However, the models used to develop the TMDL did not account for the evolving sediment loads to and storage in the Bay, which include reduced sediment capacity in the Conowingo Reservoir, the last reservoir on the Susquehanna River, increased shoreline protection measures, and resurgence of SAV in the upper Bay in a region known as the Susquehanna Flats. The overall goal of this dissertation is to assess the current sediment dynamics of the upper Bay and specifically evaluate the connectivity of sediment transport from the Susquehanna River through the Flats into the upper Bay. First, I evaluated sedimentation on the Susquehanna Flats over seasonal to decadal time scales using radioisotopes within the context of submersed aquatic vegetation (SAV) biomass and geomorphology. Seasonal-scale sedimentation variability was related to river discharge, sediment supply, and geometry over the SAV bed, while decadal-scale sedimentation was influenced by flood events and changes in SAV biomass abundance. Next, I analyzed sediment geochemical patterns in the upper Bay using statistical analyses. Elements associated with aluminosilicate minerals, rare earth elements, and heavy metals explained the most variability in the dataset due to changes in grain size, salinity, and anthropogenic input, respectively. A sediment-provenance analysis was performed using the sediment-geochemistry data and indicated that the Susquehanna is the dominant source of fine-grained material throughout the upper Bay. Finally, I developed an updated sediment budget through quantitative analysis of sediment sources (Susquehanna River and shoreline erosion) and sinks (Susquehanna Flats and mainstem sediment-accumulation rates). Conservation-management practices have reduced Susquehanna River sediment loads at low flows, but sediment loads at high flows have increased, consistent with a decreasing scour threshold for bottom sediments in Conowingo Reservoir as it has filled. Increases in shoreline stabilization have reduced shoreline erosion inputs.Item The Effects of Model Scaling on Sediment Transport in Brownout(2013) Glucksman-Glaser, Mark Samuel; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The phenomenon of “brownout” is characterized by a large cloud of sediment or dust that is formed around a rotorcraft when it takes off or lands in arid or dusty environments. To further understand the physics of brownout, a laboratory-scale rotor hovering in water was tested over a ground plane covered with a mobile sediment bed. The sensitivity of the dual-phase flow environment to changes in the values of the similarity parameters that potentially govern the fluid dynamics of the rotor flow and the transport of sediment was explored. First, dye flow visualization was performed to study the general evolution of the rotor flow and its interaction with the ground plane. Then, dual-phase flow visualization was used to expose the details of the processes that mobilize and uplift loose particles from the sediment bed. It was shown using the flow visualization that the trailed vortices from the rotor blades were a primary contributor to the mobilization and suspension of sediment. Particle image velocimetry (PIV) was also used to obtain quantitative measurements of the flow velocities found in the rotor wake and near the ground plane. It is then discussed as to why the steady flow assumptions used in the usual definitions of the classical similarity parameters governing sediment transport are not as applicable to the dual-phase flows produced by a rotor operating over a mobile sediment bed. A Buckingham-π analysis was performed to determine a set of new similarity parameters that potentially better reflect the dual-phase flow characteristics relevant to sediment mobilization and suspension by a rotor wake, including the characteristics of the tip vortices. Sixteen new similarity parameters were initially determined, five of which selected as having particular relevance. Specifically, these new similarity parameters were: 1. The mobile inertia ratio; 2. The stationary inertia ratio, 3. The terminal-swirl velocity ratio; 4. The threshold-swirl velocity ratio; 5. The terminal/threshold-swirl velocity ratio. The values of these similarity parameters were determined using the PIV measurements, and were all found to correlate to the quantity of sediment mobilized and uplifted by the rotor. The terminal/threshold-swirl velocity ratio is proposed as the potentially most important similarity parameter for further characterizing the brownout phenomenon.Item Impact of breakwaters on sediment characteristics and submerged aquatic vegetation(2011) Barth, Nicole; Palinkas, Cindy M; Koch, Evamaria W; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study examined the impact of breakwaters, with varying ages (1-19 y) and in 3 salinity regions of Chesapeake Bay, on sediment characteristics and submerged aquatic vegetation (SAV). Sediment and SAV characterisitcs were determined at an adjacent-exposed and a breakwater-protected site in 24 locations. A mesocosm experiment was also conducted to evaluate SAV response to 4 organic-content treatments for 3 SAV species (Ruppia maritima, Vallisneria americana, and Zannichellia palustris). Breakwater effects on sedimentation were site-specific, some sites, having no apparent affect, while others where sandy shoreline erosion was dominant, an increase in grain size and sedimentation rate was observed. At other sites breakwaters facilitated fine-sediment deposition. SAV responses in the mesocosms, were highly variable with organic content. Therefore, SAV biomass in breakwater-protected area was related to the amount and type of sediments that the breakwater retained. Site evaluations should be conducted before breakwater construction if SAV colonization is desired.Item Mechanisms of Sediment Entrainment and Transport in Rotorcraft Brownout(2009) Johnson, Bradley Stephen Curtis; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To advance the understanding of the phenomenon of rotorcraft brownout, a dual-phase flow environment induced by a small-scale rotor hovering above a sediment bed was studied using high-speed flow visualization and particle image velocimetry (PIV). The high frame rate of the camera, combined with advanced particle recognition and tracking software, permitted an understanding of the temporal evolution of the rotor wake in ground effect simultaneously with the processes of sediment entrainment and transport by the rotor flow. High-resolution near-wall PIV measurements showed that large excursions in the surface boundary layer were produced by the convecting rotor wake vortices. These excursions acted to suppress an equilibrium state in the boundary layer within the zone of vortex impingement on the ground. The highest sediment entrainment levels were observed to occur within this impingement zone, which can be attributed to the increase in groundwash and wall shear produced beneath the vortices. Once entrained, significant quantities of sediment were then trapped and locally suspended by the vortex-induced upwash field. This effect resulted in a noticeable level of intermittency in the initial vertical transport of sediment from the ground. The ground and upwash flow velocities were shown to strengthen significantly during the viscous merging of adjacent wake vortices. This mechanism proved fundamental in defining the concentration of suspended sediment, as well as the maximum height to which sediment could be transported. Sediment particles reaching sufficient heights were observed to recirculate into the rotor wake, and convect back towards the ground at a high speed. This process caused sediment ejection by means of bombardment or "splash." The classical process of saltation bombardment was also visualized for larger particles whose inertia prevented them from being suspended in the vortical flow. While providing new insight into the time- and length-scales associated with sediment transport by a rotor wake, the observations made here also bring into question the validity of equilibrium particle flux models currently being used for brownout simulations.Item NITROGEN CYCLING AND CONTROLS ON DENITRIFICATION IN MESOHALINE SEDIMENTS OF CHESAPEAKE BAY(2009) Owens, Michael Sean; Cornwell, Jeffrey C; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nitrogen is a key nutrient in the eutrophication of coastal and estuarine systems. In shallow water systems, sediment recycling can be an important source of nutrients for phytoplankton growth. The balance between nitrogen recycling and denitrification regulates the importance of sediments as a nitrogen source. To assess controls on denitrification, we conducted intensive seasonal measurements of sediment water exchange and denitrification using sediment core incubations. Peak rates of denitrification were observed in fall and spring (>100 μmol N-N m-2 h) followed by a decrease to 10 μmol N m-2 h in summer. Although denitrification rates were stimulated by labile organic carbon additions from the water column, the overall efficiency of the process sharply declined as temperature increased and bottom water O2 declined. Macrofauna activity was shown to enhance sediment transport of O2 by >5 fold, increase organic matter decomposition and maintain a high rate of denitrification efficiency.Item SEDIMENT SUSPENSION EVENTS FROM RIPPLE BEDS IN OSCILLATORY FLOW: EXPERIMENTS(2009) Knowles, Philip Leland; Kiger, Kenneth T; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An experimental sediment flume is used to investigate sediment transport mechanics within an oscillatory turbulent boundary layer over a mobile sediment bed in the ripple bed regime. Two-phase PIV is utilized to simultaneously capture data from each phase, allowing examination of suspension mechanisms, carrier phase stresses, and to obtain statistics to describe the momentum exchange between the phases. The technique employs median filtering, as well as size and brightness criteria to separate and accurately identify each phase. Independent well-conditioned tests have been conducted to improve the algorithm to account for the imaging conditions encountered in the vicinity of a mobile bed in order to minimize cross-talk between the phases and allow quantification of the dispersed phase concentration. Results show that large-scale vortical structures are responsible for the ejection of sediment from the bed into the outer flow. These structures are a significant source of turbulent transport, but their overall contribution to the bed stress is small compared to the mean flow. Triple decomposition of the equations of motion show that long time averaged sediment flux is of similar magnitude to cyclic fluctuations and the time averaged flow consists of two counter rotating cells. Turbulent kinetic energy created at flow reversal advects over the sediment bed and keeps particles suspended in the flow. Calculation of the vertical particle drag, body force, and convection terms revels that at flow reversal the body force terms are larger than the drag causing the particles on average to settle. The particle convection terms are small compared to particle drag and body force terms. Some of the terms most significant in the particle drag are the fluctuating components indicating that the turbulence is keeping the particles suspended in the outer flow.