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

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    Climate Change Impacts and Adaptations in Eastern US Crop Production
    (2017) Salazar Lahera, Natalia; Hill, Robert L; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Climate change is affecting crop production in the Eastern US and is expected to continue doing so unless adaptation measures are employed. In the first study, we conducted surveys and interviews to identify crop management practices currently used as adaptations in the Mid-Atlantic US. The results pointed to a variety of water and soil management practices, changes in crop characteristics, and changes in planting dates. In the second study, we used the Agricultural Policy/Environmental eXtender (APEX) model to evaluate future climate change impacts and adaptations in Eastern US corn-soybean rotation systems. The effects of climate change on yields ranged from decreases to increases, generally improving with latitude and worsening with time. Climate change affected corn yields more negatively or less positively than soybean yields. No-tillage and rye cover cropping did not serve as effective adaptations in regards to yields. In fact, planting rye after corn and soybeans reduced corn yields.
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    The Impact of Highly Qualified Teachers on Students' Academic Achievement and Graduation Rates
    (2014) Walker, Donald J.; Parham, Carol S; Education Policy, and Leadership; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    ABSTRACT Title of Document: THE IMPACT OF HIGHLY QUALIFIED TEACHERS ON STUDENTS' ACADEMIC ACHIEVEMENT AND GRADUATION RATES Donald Jay Walker, Doctor of Education, 2014 Directed By: Dr. Carol S. Parham, Department of Counseling, Higher Education and Special Education This study examined the relationship between the percentage of classes taught by highly qualified teachers (HQT) and students' academic achievement, as measured by students' passing rates on Maryland's High School Assessment (HSA), and graduation rates among secondary students for three academic school years (2011, 2012 and 2013) in the State of Maryland. All data relating to highly qualified teachers and students' academic achievement and graduation rates was collected from the Maryland's Annual Report Cards, published by Maryland State Department of Education for all 24 school systems in the State of Maryland during three consecutive academic school years (2011, 2012 and 2013). The individuals in this sample were 10th, 11th and 12th grade students from Maryland School System who participated in the Spring Maryland High School Assessment (HSA) for academic school years 2011, 2012 and 2013. A linear regression method was used to answer the question of possible relationships between the dependent variables, academic achievement test scores and graduation rates, and the independent variable, percent of courses taught by highly qualified teachers. System level performance data were analyzed in a descriptive fashion in order to show the percent of students passing each of the Maryland High School Assessments (HSA) in Math, Biology and English, and the percent of students graduating from high school. In addition to the descriptive statistics, Hierarchical Linear Modeling analyses were conducted in order to determine if (1) students performed better on High School Assessments (HSA) in counties having more highly qualified teachers (noted as Between Counties) and (2) if students performed better in years when there were more highly qualified teachers (HQT) in a county, than in those years when there were fewer highly qualified teachers (noted as Within-Counties or Between Years). Results of the data indicated that between-county or counties with more highly qualified teachers (HQT) than counties with fewer highly qualified teachers had a higher percentage of students who passed the Biology 10, 11 and 12, English 10, 11 and 12, and Math 10, 11, and 12 High School Assessments (HSA), over three consecutive academic school years (2011, 2012 and 2013). Within-counties, between-years or years when there were more highly qualified teachers (HQT) in a county compared to years when there were fewer (HQT) teachers, there was no significant relationships between the percentage of highly qualified teachers and students passing rates on the Biology 11 and 12, the Math 10 and 12, and the English 10 and 11 High School Assessments (HSA). However, for Biology 10, Math 11 and English 12 there was a significant relationship between the percentage of HQT in a county in a year, and the percentage of students passing the test that year. Counties where there were more highly qualified teachers (HQT), a higher proportion of students graduated than in counties where there were fewer highly qualified teachers (HQT), over three academic school years (2011, 2012 and 2013). There was no relationship between the percentage of highly qualified teachers (HQT) within a county in a particular school year and the percentage of students graduating in that year.
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    DETERMINATION OF SIDEROPHILE ELEMENT CHARACTERISTICS THROUGHOUT LUNAR HISTORY: IMPLICATIONS FOR THE LUNAR MAGMA OCEAN AND LATE HEAVY BOMBARDMENT
    (2014) Sharp, Miriam; Walker, Richard J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Examining the chemical behavior of highly siderophile elements (HSE) in impact events and during planetary differentiation can illuminate geologic processes that have affected the Moon. This dissertation addresses impactor compositions during the putative late heavy bombardment and the chemical composition of the evolving lunar magma ocean at both the times of core segregation and crust formation. Concentrations of the HSE Re, Os, Ir, Ru, Pt, and Pd and 187Os/188Os isotopic compositions are reported for seven Apollo 17 and four Apollo 16 impact melt rocks. Most Apollo 17 samples examined here as in prior studies are characterized by very similar HSE signatures, consistent with a common impactor that had suprachondritic Ru/Ir, Pd/Ir, and Re/Os. In contrast to the Apollo 17 signature, the Apollo 16 impact melts have a wider range of Ru/Ir, Pd/Ir, and Re/Os. This compositional range might be the result of sampling at least three impactor signatures at this site. Experimentally determined plagioclase-melt partition coefficients are also presented. These partition coefficients are used to estimate the concentrations of Sr, Hf, Ga, W, Mo, Ru, Pd, Au, Ni, and Co in a crystallizing lunar magma ocean at the point of plagioclase flotation. Plagioclase-melt derived concentrations for Sr, Ga, Ru, Pd, Au, Ni, and Co are also consistent with prior estimates. Estimates for Hf, W, and Mo, however, are higher. These elements may have concentrated in the residual liquid during fractional crystallization, due to their incompatibility. Experimentally determined metal-silicate partition coefficients are used to constrain the concentrations of W, Mo, Ru, Pd, Au, Ni, and Co in the lunar magma ocean at the time of core formation. The resulting lunar mantle estimates are generally consistent with previous estimates for the concentration of these elements in the lunar mantle. Together, these new results are used to present a compositional timeline for the Moon between the crystallization of the lunar magma ocean and the late heavy bombardment.
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    Adaptive Magnetorheological Sliding Seat System for Ground Vehicles
    (2011) Mao, Min; Wereley, Norman M.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Magnetorheological (MR) fluids (MRFs) are smart fluids that have reversible field dependent rheological properties that can change rapidly (typically 5 - 10 ms time constant). Such an MRF can be changed from a free flowing fluid into a semi-solid when exposed to a magnetic field. The rapid, reversible, and continuous field dependent variation in rheological properties can be exploited in an MRF-based damper or energy absorber to provide adaptive vibration and shock mitigation capabilities to varying payloads, vibration spectra, and shock pulses, as well as other environmental factors. Electronically controlled electromagnetic coils are typically used to activate the MR effect and tune the damping force so that feedback control implementation is practical and realizable. MR devices have been demonstrated as successful solutions in semi-active systems combining advantages of both passive and active systems for applications where piston velocities are relatively low (typically < 1 m/s), such as seismic mitigation, or vibration isolation. Recently strong interests have focused on employing magnetorheological energy absorbers (MREAs) for high speed impact loads, such as in helicopter cockpit seats for occupant protection in a vertical crash landing. This work presents another novel application of MREAs in this new trend - an adaptive magnetorheological sliding seat (AMSS) system utilizing controllable MREAs to mitigate impact load imparted to the occupant for a ground vehicle in the event of a low speed frontal impact (up to 15 mph). To accomplish this, a non-linear analytical MREA model based on the Bingham-plastic model and including minor loss effects (denoted as the BPM model) is developed. A design strategy is proposed for MREAs under impact conditions. Using the BPM model, an MREA is designed, fabricated and drop tested up to piston velocities of 5 m/s. The measured data is used to validate the BPM model and the design strategy. The MREA design is then modified for use in the AMSS system and a prototype is built. The prototype MREA is drop tested and its performance, as well as the dynamic behavior in the time domain, is described by the BPM model. Next, theoretical analysis of the AMSS system with two proposed control algorithms is carried out using two modeling approaches: (1) a single-degree-of-freedom (SDOF) rigid occupant (RO) model treating the seat and the occupant as a single rigid mass, and (2) a multi-degree-of-freedom (MDOF) compliant occupant (CO) model interpreting the occupant as three lumped parts - head, torso and pelvis. A general MREA is assumed and characterized by the Bingham-plastic model in the system model. The two control algorithms, named the constant Bingham number or Bic control and the constant stroking force or Fc control, are constructed in such a way that the control objective - to bring the payload to rest while fully utilizing the available stroke - is achieved. Numerical simulations for both rigid and compliant occupant models with assumed system parameter values and a 20 g rectangular crash pulse for initial impact speeds of up to 7 m/s (15.7 mph) show that overall decelerations of the payload are significantly reduced using the AMSS compared to the case of a traditional fixed seat. To experimentally verify the theoretical analysis, a prototype AMSS system is built. The prototype seat system is sled tested in the passive mode (i.e. without control) for initial impact speeds of up to 5.6 m/s and for the 5th percentile female and the 95th percentile male. Using the test data, the CO model is shown to be able to adequately describe the dynamic behavior of the prototype seat system. Utilizing the CO model, the control algorithms for the prototype seat system are developed and a prototype controller is formulated using the DSPACE and SIMULINK real time control environments. The prototype seat system with controller integrated is sled tested for initial impact speeds of up to 5.6 m/s for the 5th female and 95th male (only the 95th male is tested for the Bic control). The results show that the controllers of both control algorithms successfully bring the seat to rest while fully utilizing the available stroke and the decelerations measured at the seat are substantially mitigated. The CO model is shown to be effective and a useful tool to predict the control inputs of the control algorithms. Thus, the feasibility and effectiveness of the proposed adaptive sliding seat system is theoretically and experimentally verified.