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 Experimental Investigation of Boundary-Layer Transition on a Slender Cone at Mach 4(2023) Jones, Benjamin Rhys; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Boundary-layer transition over a 5 degree half-angle straight cone model was examined in University of Maryland's Multiphase flow Investigations Tunnel (MIST), a Mach 4 Ludweig tube. In order to prepare for future studies with particle-laden flow, this study was conducted to characterize the boundary-layer of the cone under dry conditions. Furthermore, both first-mode (Tollmien-Schlichting) and second-mode (Mack) boundary-layer instabilities are expected at Mach 4 (Mack), with the former being not widely studied. The boundary-layer on the top surface of the cone was visualized using high-speed Schlieren and analyzed using image and signal processing techniques. Experimentation was conducted over a range of unit Reynolds numbers from 31.3-40.3 10^6 m^-1 in order to vary the transition location. Power Spectral Density (PSD) and Spectral Proper Orthogonal Decomposition (SPOD) revealed the most coherent wave packets propagating within the boundary layer at frequencies ranging from 10 to 40 kHz, frequencies that are consistent with the first mode. Frequencies between 110-140 kHz contained additional content consistent with the first mode but less coherent. The presence of these structures was verified with a bandpass filter allowing 10-40 kHz and a highpass filter allowing >100 kHz each separately applied to the original footage. Future work is planned to compare the results of this paper with multi-phase flow experiments conducted with the same model at the same freestream conditions.Item Effect of Cooling on Hypersonic Boundary-Layer Stability(2022) Paquin, Laura; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The prediction of boundary-layer transition on hypersonic vehicles has long been considered a primary design concern due to extreme levels of heating and dynamic pressure loading this transition induces. While it has been predicted that the temperature gradient between the vehicle and the local freestream can drastically alter boundary-layer stability, experimental research on the topic over the past fifty years has provided conflicting results. This study investigates the relationship between the wall-to-edge temperature ratio and boundary-layer stability on a slender cone. Campaigns in two wind-tunnel facilities were conducted: one set within the HyperTERP reflected-shock tunnel at the University of Maryland, and one set at the high-enthalpy T5 reflected-shock tunnel at the California Institute of Technology. Both sets of campaigns employed non-intrusive, optical diagnostics to analyze the structures and spectral content within the boundary layer. In the first part of the study, performed in HyperTERP, an experimental methodology was developed to vary the wall temperature of the model using active cooling and passive thermal management. This allowed the wall temperature ratio to be varied at the same nominal test condition (and thus freestream disturbance environment), and three thermal conditions were established for analysis. Simultaneous schlieren and temperature-sensitive-paint (TSP) imaging were performed. Calibrated schlieren images quantified the unsteady density gradients associated with second-mode instabilities, and TSP contours provided insight into the thermal footprint of mean boundary-layer structures. It was found that, overall, cooling shrunk the boundary-layer thickness, increased second-mode disturbance frequencies, and increased the amplification rate of these instabilities. At nonzero angles of attack, cooling appeared to increase the azimuthal extent of flow separation on the leeward side of the cone. In the second part of the study, performed in T5, the disturbance structures and spectral content of laminar and transitional boundary layers were characterized under high-enthalpy conditions. Schlieren images indicated that, at these extremely low wall-to-edge temperature ratios, second-mode waves were confined very close to the wall in the laminar case. During the breakdown to turbulence, structures radiating out of the boundary layer and into the freestream were discovered. A texture-based methodology was used to characterize the Mach angles associated with these structures, and a wall-normal spectral analysis indicated a potential mechanism by which energy was transferred from the near-wall region to the freestream. The study presents some of the first simultaneous imaging of the flow structures and associated thermal footprint of boundary-layer transition within an impulse facility. The work also presents the first time-resolved, full-field visualizations of the second-mode dominated breakdown to turbulence at high enthalpy. Thus, the study imparts significant insight into the mechanics of boundary-layer transition at conditions representative of true hypervelocity flight.Item Jiao Tong: A Grounded Theory of Chinese International Students' Transition to American Tertiary Education(2016) Kavaliauskas Crain, Lena; Griffin, Kimberly; Counseling and Personnel Services; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)University students are more globally mobile than ever before, increasingly receiving education outside of their home countries. One significant student exchange pattern is between China and the United States; Chinese students are the largest population of international students in the U.S. (Institute of International Education, 2014). Differences between Chinese and American culture in turn influence higher education praxis in both countries, and students are enculturated into the expectations and practices of their home countries. This implies significant changes for students who must navigate cultural differences, academic expectations, and social norms during the process of transition to a system of higher education outside their home country. Despite the trends in students’ global mobility and implications for international students’ transitions, scholarship about international students does not examine students’ experiences with the transition process to a new country and system of higher education. Related models were developed with American organizations and individuals, making it unlikely that they would be culturally transferable to Chinese international students’ transitions. This study used qualitative methods to deepen the understanding of Chinese international students’ transition processes. Grounded theory methods were used to invite the narratives of 18 Chinese international students at a large public American university, analyze the data, and build a theory that reflects Chinese international students’ experiences transitioning to American university life. Findings of the study show that Chinese international students experience a complex process of transition to study in the United States. Students’ pre-departure experiences, including previous exposure to American culture, family expectations, and language preparation, informed their transition. Upon arrival, students navigate resource seeking to fulfill their practical, emotional, social, intellectual, and ideological needs. As students experienced various positive and discouraging events, they developed responses to the pivotal moments. These behaviors formed patterns in which students sought familiarity or challenge subsequent to certain events. The findings and resulting theory provide a framework through which to better understand the experiences of Chinese international students in the context of American higher education.Item PERSPECTIVES OF VETERANS IN HIGHER EDUCATION: EXPLORING THE TERM "STUDENT VETERAN" AND THE IDENTITY SHIFTS BETWEEN MILITRAY AND COLLEGE(2014) Hernandez Baron, Paola Maria; Griffin, Kimberly; Counseling and Personnel Services; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Given changes in the G.I Bill, warfare, and higher education, post 9/11 veterans are a unique and expanding college student population. The purpose of this narrative inquiry study was to better understand how post 9/11 student veterans perceive and identify with the term "student veteran." The findings suggest that "student veteran" is more than a label and shares some qualities of a social identity. The participants wanted to be treated as "regular students," but also valued what the term "student veteran" signifies including a unique sociohistorical, cultural, and personal context and history that framed their academic experience. Participants described the term as a way to uphold military culture amidst the more ambiguous college culture. Participants felt the term carries imposed meanings and judgments different from that which participants themselves attribute to it. Findings suggest both theoretical and applied implications for expanded cultural competency around interacting with heterogeneous student veteran populations.Item Why Was Mongolia Successful? Political and Economic Transition in 1990-1996(2012) Chuluunbat, Narantuya; Reuter, Peter; Public Policy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mongolia's historical, geographical, political, and economic circumstances seem to be closest to those of the Central Asian states. Yet, unlike these states, Mongolia was able to successfully transition to a democratic political regime and introduce far-reaching economic reforms. This dissertation analyzes this puzzle by focusing on the early transition period in Mongolia. The dissertation provides detailed account of political and economic processes during the ten years from 1987 to 1996. The account of events is based on primary data drawn from government and party documents, cabinet minutes, daily newspaper accounts, memoirs of the participants, and detailed interviews with the participants of the events. In addition to the detailed description, the dissertation provides alternative theoretical frameworks - as opposed to the structural explanations provided thus far - which could be useful in explaining why things happened the way they did in Mongolia. Namely, the dissertation brings in two groups of political economy theories - institutions and constitutional design and special interest and collective action theories -and attempts to explain the events in Mongolia through the lens of these theoretical arguments. The first chapter provides a comprehensive literature review on Mongolia's political and economic transition and places it in a comparative perspective. The second chapter describes and analyzes the nature and extent of the partial economic and political reforms that were implemented in the late communist period. The third chapter describes in detail the political events that led to the collapse of the long-standing communist regime and the subsequent radical political changes that took place following the peaceful "democratic revolution". The fourth chapter deals in detail with the economic shock the country faced with the withdrawal of the Soviet financial assistance, the first policy response, and the overall politics of economic decision making. I pay special attention to privatization, the cornerstone of early reform attempts. The last chapter summarizes, classifies, and prioritizes the variety of factors - historical, external, political, institutional, and cultural - that were identified as having contributed to the successful political and economic transition.Item THE ROLE OF SELF-DETERMINATION ON QUALITY OF LIFE, STUDENT ENGAGEMENT, AND ENVISIONING A CAREER/LIFE GOAL OR FUTURE FOR STUDENTS PARTICIPATING IN A BEST PRACTICES TRANSITION INTERVENTION(2012) Ferguson, Terri Kay; Fabian, Ellen; Counseling and Personnel Services; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Self-determination has been identified as a major predictive factor for positive postsecondary outcomes for students with disabilities. The purpose of this study was to determine the role of self-determination in productive student engagement, perceived quality of life, and the ability to envision a career/life goal or future for students with multiple disabilities participating in a promising practices transition intervention. Results indicated significant results in the areas of student engagement and quality of life indicating the importance of self-determination on these factors. The results indicated a negative correlation between two of the self-determination subscales and envisioning a career/life goal which was an interesting and contradictory finding to the existing literature. Implications and recommendations are discussed.Item CHARGE TRANSPORT IN GRAPHENE WITH ADATOM OVER-LAYERS ; CHARGED IMPURITY SCATTERING, DIELECTRIC SCREENING, AND LOCALIZATION.(2011) Jang, Chaun; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Graphene, a single atom thick plane of graphite, is a novel two-dimensional electron system in which the low-energy electrons behave as massless chiral Dirac fermions. This thesis explores the effects of disorder in graphene through controlled surface modification in ultra-high vacuum (UHV), coupled with in situ electronic transport experiments. Three different roles of adatom overlayers on graphene are investigated. First, the effects of charged impurity scattering are studied by introducing potassium ions on the graphene at low temperature in UHV. The theoretically expected magnitude and linear density-dependence of the conductivity due to long range Coulomb scattering is verified. Second, the effective dielectric constant of graphene is modified by adding ice overlayers at low temperature in UHV. The opposing effects of screening on scattering by long range (charged impurity) and short range impurities are observed as variations in conductivity, and the changes are in agreement with Boltzmann theory for graphene transport within the random phase approximation. The minimum conductivity of graphene is roughly independent of charged impurity density and dielectric constant, in agreement with the self-consistent theory of screened carrier density inhomogeneity (electron and hole puddles). Taken together, the experimental results on charged impurity scattering and dielectric screening strongly support that long range Coulomb scattering is the dominant scattering mechanism in as-fabricated graphene on SiO2. In addition to the semi-classical transport properties, quantum transport is also studied with cobalt decorated graphene. Strong localization is achieved in the disordered graphene through deposition of cobalt nanoclusters. In finite magnetic field a phase transition occurs from the localized state to the quantum Hall state. Scaling analysis confirms that the transition is a quantum phase transition which is similar to the localization - delocalization transitions in other two dimensional electron systems.Item A Narrative Analysis of the Process of Self-Authorship for Student Affairs Graduate Students(2011) Schoper, Sarah E.; Komives, Susan R; Counseling and Personnel Services; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Research on preparation programs for student affairs professionals has focused primarily on identifying competencies. Limited attention has been paid to the process of how meaning is made of preparation program experiences. Of the scholarship conducted, minimal consideration has been paid to the relationship between development and the environment. The purpose of this study was to explore the process of self-authorship for graduate students within a student affairs preparation program, and the environmental conditions that promoted that process. Utilizing narrative inquiry methodology (Clandinin & Connelly, 2000; Lieblich, Tuval-Mashiach, & Zilber, 1998; Marshall & Rossman, 1999; Shank, 2002), data was collected through in-depth interviews of six graduates of a student affairs preparation program meeting the standards set by the Council for the Advancement of Standards (2009), and analyzed using the constant comparative method (Lieblich et al., 1998). The preparation program studied was located at a public research university in the Midwest. The results were considered in relation to constructive-developmental theory (Boes, Baxter Magolda, & Buckley, 2010), self-authorship theory (Baxter Magolda, 2001; Kegan, 1982, 1994), the environment of reference model (Conyne & Clack, 1981), the learning partnerships model (Baxter Magolda, 2004), and transition theory (Schlossberg, Waters, & Goodman, 1995). Results indicated that although movement toward self-authorship was achieved those who graduated had not fully reached self-authorship. The conditions identified that promoted the process of self-authorship included self-reflection and experiencing different perspectives. For example, participation in self-reflection helped participants separate their own meaning from that of others, as well as determine the value of the meaning made. The results also indicated that the participants sought out support within the environment as they experienced transition. Finally, the findings included a description of conditions within the environment that aided the participants in deciding to select the specific preparation program studied. Although the interaction between the environmental conditions and the participants' meaning making systems varied, the findings can be transferred to student affairs preparation program environments, as well as practitioner environments.Item Education Amidst Transition: The Case of Romania(2010) DiGiacomo, Francis Anthony; Lin, Jing; Education Policy, and Leadership; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)After enduring centuries of colonialism, followed recently by the exogenous forces of globalization and isomorphism, Romania and other countries in Eastern Europe have had unique opportunities amidst formidable challenges since they began their transitions toward democracy. In this case study, I explore these forces and resulting challenges that influenced the Romanian education system between 1989 and 2007. With this approach, I try to elucidate the difficulties endured by the Romanian political and education elite when transitioning the country from totalitarianism toward democracy. I conducted an extensive literature review and document analysis, coupled with in-depth interviews with the Romanian bureaucratic education elite who influenced the country's education reforms. My research investigated the complicated manner in which education plays a role in supporting a country in transition. Two central questions drove my research: (1) What factors impacted Romania's transition from a totalitarian regime toward democracy? (2) What role did education play in Romania's transition from a totalitarian regime toward democracy? My findings suggest that too many poorly constructed short-term focused reforms, developed by an elite deeply entrenched in its communist past, slowed significantly the development of a democratic education system in Romania. Exogenous forces such as colonialism, globalization and isomorphism further compounded the challenges of Romania's political transition. The elements of these findings, in addition to domestic factors such as Romania's history and culture, coupled with its nascent political system and colonized mentality, partially explain the reasons for the core of the education system remaining largely status quo.Item The Associations Among Youth Characteristics, Secondary School Experiences, and Enrollment in Two- and Four-Year Colleges Among Youth with Disabilities(2008-09-22) Miceli, Meredith Anne; McLaughlin, Margaret J; Special Education; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The purpose of this study was to explore the associations between school program and services, high school achievement characteristics, and individual and household characteristics and enrollment in two- and four-year colleges among youth with disabilities. Variables related to youth characteristics and secondary school experiences were identified from the first 5 data collection points of the National Longitudinal Transition Study 2 (NLTS2) database. Chi-squares, t-tests, and logistic regression analyses were used in this study. The results in this study showed that individual and household, high school achievement, and school program characteristics significantly predicted college enrollment among youth with disabilities. The model of variables including individual, household, parental, high school achievement, self-determination, and school program characteristics correctly classified 86.4% of the college enrollers in the analytic sample. These findings confirm the appropriateness of categories included in the NLTS2 Conceptual Framework. Additionally, the study found that high school achievement characteristics such as graduating from high school, participating in extra curricular activities, and experiencing financial management/ responsibility had the most consistent, positive impact on the probability of enrolling in two- and four-year colleges. Additionally, youth characteristics such as disability category, household income, and parental level of education significantly impacted the probability of enrolling in colleges. Finally, participating in vocational education, having a postsecondary goal to attend college, and taking a leadership role in one's transition planning process significantly increased the probability of the youth enrolling in college. The findings emphasize the importance of obtaining a high school diploma, participating in organized activities outside of the classroom, experiencing a level of autonomy or independence, and taking a leadership role in the transition planning process while in high school for youth with disabilities.