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 TOPOLOGICAL ANALYSIS OF DISTANCE WEIGHTED NORTH AMERICAN RAILROAD NETWORK: EFFICIENCY, ECCENTRICITY, AND RELATED ATTRIBUTES(2023) Elsibaie, Sherief; Ayyub, Bilal M.; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The North American railroad system can be well represented by a network with 302,943 links (track segments) and 250,388 nodes (stations, junctions, and waypoints), and other points of interest based on publicly accessible geographical information obtained from the Bureau of Transportation Statistics (BTS) and the Federal Railroad Administration (FRA). From this large network a slightly more consolidated subnetwork representing the major freight railroads and Amtrak was selected for analysis. Recent improvements in network and graph theory and improvements in all-pairs shortest path algorithms make it more feasible to process certain characteristics on large networks with reduced computation time and resources. The characteristics of networks at issue to support network-level risk and resilience studies include node efficiency, node eccentricity, and other attributes derived from those measures, such as network arithmetic efficiency, network geometric central node, radius, and diameter, and some distribution measures of the node characteristics. Rail distance weighting factors, representing the length of each rail line derived from BTS data, are mapped to corresponding links, and are used as link weights for the purpose of computing all pair shortest paths and subsequent characteristics. This study also compares the characteristics of North American railroad infrastructure subnetworks divided by Class I carriers, which are the largest railroad carriers classified by the Surface Transportation Board (STB) by annual operating revenue, and which together comprise most of the North American railroad network. These network characteristics can be used to inform placement of resources and plan for natural hazard and disaster scenarios. They relate to many practical applications such as network efficiency to distribute traffic and a network’s ability to recover from disruptions. The primary contribution of this thesis is the novel characterization of a detailed network representation of the North American railroad network and Class I carrier subnetworks, with established as well novel network characteristics.Item ENHANCING RESILIENCE OF COMPLEX NETWORKS: WASHINGTON D.C. URBAN RAIL TRANSIT AS A CASE STUDY(2020) Saadat, Yalda; Ayyub, Bilal BA; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)According to the United Nation’s Department of Economic and Social Affairs Population Division, 66% of the world’s population will reside in urban areas by 2050; a boost from 30 % in 1950. Urbanization has indeed triumphed and its speed has brought innovation and economic growth. Its synergies within infrastructure systems are undeniable and have increased the demand for such systems. However, urbanization is one reason infrastructure systems are knocked out of equilibrium and show complex dynamical behavior. Most infrastructure systems have been designed without planning for this magnitude of potential demographic changes; thus redesigns are long overdue. Also, climate change looms. Resource scarcity and host of other factors leave their impacts; all pose some incidence of perturbation in the state of the infrastructure system. These perturbations can affect the system’s resilience, which is a defining property of each system for remaining functional in the midst of disruption from an adverse event. Therefore, it is essential to develop appropriate metrics and methods to enhance the resilience of infrastructures at the network level. Such enhancements are critical for sustainable infrastructure development that is capable of performing satisfactorily through intentional and/or stochastic disruptions. A resilience evaluation of a network typically entails assessing vulnerability and robustness as well as identifying strategies to increasing network efficiency and performance and offering recovery strategies ideally taken in a cost-effective manner. This dissertation uses complex network theory (CNT) as the theoretic basis to enhance the resilience of large-scale infrastructure networks, such as urban rail transit systems. Urban rail transit infrastructures are heterogeneous, complex systems consisting of a large number of interacting nodes and links, which can imitate a network paradigm. Any adverse event leading to a disruption in the interaction and connectivity of network components would dramatically affect the safety and wellbeing of commuters, as well as the direct and indirect costs associated with performance loss. Therefore, enhancing their resilience is necessary. Using the Washington D.C. Urban rail transit as a case study, this dissertation develops a methodology to analyze network topology, compute its efficiency, vulnerability, and robustness in addition to provide a unified metric for assessing the network resilience. The steps of methodology are applied to two models of weighted and unweighted networks. For the weighted model two novel algorithms are proposed to capture the general pattern of ridership in the network, and to reflect the weights on assessing network efficiency, respectively. This dissertation then proposes an effective strategy to increase the network resilience prior to a disruptive event, e.g., a natural disaster, by adding several loop lines in the network for topological enhancement. As such, adding a loop line can create redundancy to the vulnerable components and improve network resilience. Expanding on this, the dissertation offers comparative recovery strategies and cost model in the case of disruption. An effective recovery strategy must demonstrate rapid optimal restoration of a disrupted system performance while minimizing recovery costs. In summary, the systematic methodology described above, assesses and enhances the network resilience. The initial results rank the most vulnerable and robust components of the network. The algorithms developed throughout the study advance the weighted network analysis state of art. The topological enhancement strategy offered basis to justify capital improvement. Post failure recovery analysis and the cost model serves to inform decision makers in identifying best recover strategies with special attention not only to restoring performance of a system but also on reducing associated failure and recovery costs. The use of the methodology proposed in this dissertation may lead to significant societal benefits by reducing the risk of catastrophic failures, providing references for mitigation of disruption due to adverse events, and offering resilience- based strategies, and related pursuits.Item GALLIUM NITRIDE BASED ONBOARD CHARGER FOR ELECTRIC VEHICLES(2019) Zhang, Zeyu; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Next generation of electric vehicles will be equipped with high power density and high efficiency onboard charging systems with bi-directional power flow. These benefits can be achieved by utilizing the emerging Wide Bandgap devices, planar magnetic solutions, innovative circuit topologies, and advanced control methods to enable MHz switching frequencies without sacrificing efficiency. However, the advantage of higher switching speed is gained at the expense of higher switching losses in both the semiconductors and the magnetics. Conventional circuit topologies, operation modes and control algorithms would no longer be effective in such conditions. Furthermore, the practical implementation of the system has shown more stringent requirements on the controller speed, layout parasitic and the thermal management. In this Ph.D. dissertation work, aforementioned challenges have been addressed, and the proposed innovations have been validated through design and development of a new bi-directional onboard charger using Gallium Nitride devices. The first part of this work has been focused on a thorough characterization of the front-end AC-DC power factor correction and rectification stage of an onboard charger, utilizing advanced planar magnetics and newly proposed soft-switching control methods. The second part of this work is focused on developing a CLLC DC-DC converter, to interface the AC-DC stage and the high-voltage traction battery. Extended Harmonic Approximation method has been developed and a novel “f-φ” control scheme is proposed to enhance the efficiency at high switching speed. This system allows insights into the practical implementation and evaluation of utilizing Wide Bandgap semiconductors to achieve high power density and efficiency for the transportation industry.Item BIO-INSPIRED PUMPING MECHANISMS IN AN INTERMEDIATE REYNOLDS NUMBER(2018) Saffaraval, Farhad; Kiger, Kenneth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pumps are important to applications across a wide range of scales. Most of traditional applications occur within a range where inertia is the dominating factor influencing the pump performance, and hence many practical designs are based on mechanisms that rely on this assumption. As one moves towards smaller devices, however, the increasing effect of viscosity renders these traditional mechanisms ineffective. The current work looks towards a bio-inspired system consisting of an array of oscillating plates to contend with this challenge. The plates are placed within a channel, and the pumping performance generated is examined for a small range of Reynolds numbers intermediate between inertial and viscous regimes (0.1 < Re < 10). The goal of this work is to observe the effect of how different plate kinematics can be utilized to break the symmetry the system to produce a net pumped flow. Rigid and flexible plates are studied, using both sinusoidal and triangle wave actuation kinematics. The tests are first conducted with a single appendage, and then repeated with an array of 5 closely spaced plates to observe the effect of their interaction on the overall performance. The results of the single plate tests indicate that increased asymmetry introduced in the triangle wave actuation results in increased pumping performance as well as energy consumption. Tests were conducted at two Reynolds number conditions, Re = 0.6 and 6. The pumping performance was found to be an order of magnitude higher for the Re = 6 case. In the case of flexible plates, the results show that a mass specific pumping efficiency was higher for the flexible case with a higher frequency at the same Reynolds numbers. For the plate array, the results indicate five flexible plates with 〖∆θ〗_i=-90 will generate more than 4 times the flow rate in comparison to the single flexible plate. Asymmetric triangle actuation in conjunction with symplectic metachronal motion (〖∆θ〗_i=30) exhibits pumping performance more than 10 times of using a single rigid plate. Total work is noticeably higher for multiple plate system and will result in a reduced overall pumping efficiency in comparison to the single appendage.Item Broadcast and Verifiable Secret Sharing: New Security Models and Round Optimal Constructions(2012) Kumaresan, Ranjit; Katz, Jonathan; Computer Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Broadcast and verifiable secret sharing (VSS) are central building blocks for secure multi-party computation. These protocols are required to be resilient against a Byzantine adversary who controls at most t out of the n parties running the protocol. In this dissertation, we consider the design of fault-tolerant protocols for broadcast and verifiable secret sharing with stronger security guarantees and improved round complexity. Broadcast allows a party to send the same message to all parties, and all parties are assured they have received identical messages. Given a public-key infrastructure (PKI) and digital signatures, it is possible to construct broadcast protocols tolerating any number of corrupted parties. We address two important issues related to broadcast: (1) Almost all existing protocols do not distinguish between corrupted parties (who do not follow the protocol) and honest parties whose secret (signing) keys have been compromised (but who continue to behave honestly); (2) all existing protocols for broadcast are insecure against an adaptive adversary who can choose which parties to corrupt as the protocol progresses. We propose new security models that capture these issues, and present tight feasibility and impossibility results. In the problem of verifiable secret sharing, there is a designated player who shares a secret during an initial sharing phase such that the secret is hidden from an adversary that corrupts at most t parties. In a subsequent reconstruction phase of the protocol, a unique secret, well-defined by the view of honest players in the sharing phase, is reconstructed. The round complexity of VSS protocols is a very important metric of their efficiency. We show two improvements regarding the round complexity of information-theoretic VSS. First, we construct an efficient perfectly secure VSS protocol tolerating t < n/3 corrupted parties that is simultaneously optimal in both the number of rounds and the number of invocations of broadcast. Second, we construct a statistically secure VSS protocol tolerating t < n/2 corrupted parties that has optimal round complexity, and an efficient statistical VSS protocol tolerating t < n/2 corrupted parties that requires one additional round.Item Open Bid Auctions: A Theoretical and an Experimental Study(2008-02-21) ghosh, dipan; Cramton, peter; Economics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For centuries, auctions have been used as an efficient market mechanism for selling or procuring goods. Over time, auctions have evolved from its very basic price call-out form to the much more sophisticated simultaneous multi goods design, the bulk of this dramatic evolution taking place in the later part of the twentieth century. Even though the earliest use of auction dates back to around 500 B.C. in history, proper scientific research aiming at improving the effectiveness or expanding the scope of this versatile market mechanism started around the 1960's. The pioneering work of William Vickrey in 1961 opened the floodgates for mathematicians and economists alike, to study this fascinating market mechanism, and within a very short period of time, both the understanding of the mechanism and the scope of its application improved vastly. Over these years, a huge mass of theoretical and empirical research has produced results that introduce newer auction designs and characterize the existing ones. This has also allowed scope for continued research in the field of auctions, thriving for improvements and solutions to yet-to-be answered questions. The main goal of this study is to accomplish just that; present improvements that try to address issues that have not been addressed yet. The dissertation is structured as follows. The first chapter highlights the progress that has been made in the field of auctions and introduces the advancements made in the more recent field of auction experiments. This serves as an introduction to the other chapters and briefly outlines the important findings both in the traditional theoretical literature, the more recent operational research literature and the alternative experimental literature. The second chapter introduces a new auction model designed to tackle a specific problem encountered in multiple homogeneous goods auctions, which has not been dealt with satisfactorily thus far. The last chapter presents an extension of the existing auction experiment methodologies in an attempt to reveal possible weaknesses in earlier auction experiments and to improve our understanding of important differences between open and sealed-bid auction formats.Item Essays in Public Finance(2006-05-16) Pang, Gaobo; Rust, John P; Economics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chapter 1 analyzes effects of tax-favored savings plans on savings and retirement decisions in a realistically specified life-cycle model. Individuals face mortality risk and stochastic earnings, allocate assets between conventional savings accounts (CSAs) and tax-deferred accounts (TDAs), make endogenous choice of labor supply and retirement, and make a separate decision on claiming Social Security. The simulations reveal that there is a functional division to some degree between CSAs and TDAs, with the former serving mainly for liquidity and the latter for retirement and bequests. There is tremendous heterogeneity. The tax incentives are generally effective in stimulating new savings for the middle and upper income groups. The higher rate of return on TDAs facilitates wealth accumulation, which consequently and perhaps unintentionally encourages early retirement. Impatient and low-income individuals tend to retire and claim Social Security early. They derive less benefit from TDAs since they face lower marginal tax rates and they have limited resources to take advantage of TDAs. For them, the income effect dominates and TDAs fail to induce new savings. Chapter 2 attempts quantitatively to measure the efficiency of public spending in developing countries. The efficiency is defined as the distance between observed input-output combinations and an efficiency frontier. Both input- and outputefficiencies are estimated for several health and education output indicators by means of the Free Disposable Hull (FDH) and Data Envelopment Analysis (DEA) techniques. This chapter further seeks to verify empirical regularities associated with cross-country efficiency variation. The panel Tobit regressions reveal that countries are more likely to register lower efficiency if they are faced with higher government expenditure levels, larger wage shares in government budget composition, higher ratios of public to private financing in service provision (health), more prevalence of HIV/AIDS epidemic (health), stronger external aid dependency, and/or higher income inequality (education). Though no causality may be inferred from these exercises, they help point at different factors to understand why some countries spend more resources than others to achieve similar educational and health outcomes.