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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Item DEVELOPMENT OF A MIXED-FLOW OPTIMIZATION SYSTEM FOR EMERGENCY EVACUATION IN URBAN NETWORKS(2012) Zhang, Xin; Chang, Gang-len; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In most metropolitan areas, an emergency evacuation may demand a potentially large number of evacuees to use transit systems or to walk over some distance to access their passenger cars. In the process of approaching designated pick-up points for evacuation, the massive number of pedestrians often incurs tremendous burden to vehicles in the roadway network. Hence, one critical issue in a multi-modal evacuation planning is the effective coordination of the vehicle and pedestrian flows by considering their complex interactions. The purpose of this research is to develop an integrated system that is capable of generating the optimal evacuation plan and reflecting the real-world network traffic conditions caused by the conflicts of these two types of flows. The first part of this research is an integer programming model designed to optimize the control plans for massive mixed pedestrian-vehicle flows within the evacuation zone. The proposed model, integrating the pedestrian and vehicle networks, can effectively account for their potential conflicts during the evacuation. The model can generate the optimal routing strategies to guide evacuees moving toward either their pick-up locations or parking areas and can also produce a responsive plan to accommodate the massive pedestrian movements. The second part of this research is a mixed-flow simulation tool that can capture the conflicts between pedestrians, between vehicles, and between pedestrians and vehicles in an evacuation network. The core logic of this simulation model is the Mixed-Cellular Automata (MCA) concept, which, with some embedded components, offers a realistic mechanism to reflect the competing and conflicting interactions between vehicle and pedestrian flows. This study is expected to yield the following contributions * Design of an effective framework for planning a multi-modal evacuation within metropolitan areas; * Development of an integrated mixed-flow optimization model that can overcome various modeling and computing difficulties in capturing the mixed-flow dynamics in urban network evacuation; * Construction and calibration of a new mixed-flow simulation model, based on the Cellular Automaton concept, to reflect various conflicting patterns between vehicle and pedestrian flows in an evacuation network.Item SYNTHESIS AND CHARACTERIZATION OF LOW FLAMMABILITY POLYMER/LAYERED SILICATE NANOCOMPOSITES(2009) Zhang, Xin; Briber, Robert M.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There has been significant interest in the applications of polymer nanocomposites in a variety of areas. Polymer/layered silicate nanocomposites have been of interest because of relatively low raw material cost and improved materials properties such as higher Young's modulus, higher thermal deformation temperature, lower small molecule permeability, lower density (compared to metals and traditional glass fiber reinforced composites) as well as low flammability. The relationships between the flammability and the dispersion of the layered silicate platelets inside the polymer matrix is just being established. The complete set of factors that affect the flammability of polymer/layered nanocomposites are not fully identified. In this thesis polymer/layered silicate nanocomposites with different degrees of platelet dispersion were synthesized. The structure of the nanocomposites was characterized by X-ray diffraction (XRD), small angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). The flammability of these nanocomposites was characterized by TGA, cone calorimetry and gasification. By coupling the structural and flammability data it has been concluded that forming a nanometer scale dispersed structure significantly improves the flammability but the details of the degree of dispersion are not critical. The improvement in the flammability arises from the formation of a residue or char layer at the surface of the nanocomposite. This residue layer acts as a radiation shield and as a physical barrier preventing the polymer degradation products from escaping and acting as fuel. It is observed that the stability of the residue layer formed during combustion has major impact on the flammability. This thesis also describes work to improve the flammability of the polymer/layered silicate nanocomposites by enhancing char/residue formation in order to improve the residue layer stability.