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 HIGH ACCELERATIONS PRODUCED THROUGH SECONDARY IMPACT AND ITS EFFECT ON RELIABILITY OF PRINTED WIRING ASSEMBLIES(2010) Douglas, Stuart Taylor; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The focus of this thesis is the investigation of extremely high accelerations through secondary impact and its effect on reliability of printed wiring assemblies. The test equipment consists of a commercially available drop system and a commercially available attachment termed a Dual Mass Shock Amplifier (DMSA), which extends the impact acceleration range to as much as 30,000 Gs by utilizing secondary impact dynamics. Further secondary impacts between the test vehicle and fixture are intentionally generated in simulation and tested experimentally to imitate board 'slap' phenomena in product assemblies, and to generate even further amplification of the acceleration at various locations on the test specimen. In this thesis a detailed description of the test equipment and modeling techniques are provided. Model complexity ranges from simple analytic closed-form rigid-body mechanics to detailed nonlinear dynamic finite element analysis. The effects of different equipment design parameters (table mass, spring stiffness, table clearance) are investigated through parametric modeling. The effects of contact parameters (constraint enforcement algorithms, stiffness, damping) on model accuracy are explored. Test fixtures for high shock accelerations are discussed and used for board level reliability testing of printed wire assemblies containing WLCSP49s and MEMS microphones.Item Thermal integration of tubular solid oxide fuel cell with catalytic partial oxidation reactor and anode exhaust combustor for small power application(2010) Maxey, Christopher; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the current study, a system configuration of a tubular SOFC with a catalytic partial oxidation (CPOx) reactor and an anode exhaust catalytic combustor is explored to test the feasibility of such a system. A system level model was developed to more fully assess system design and operability issues. For the SOFC, a detailed 1-D SOFC determines local current production and is combined with down-the-channel flow models for the SOFC as well as the catalytic combustor/heat exchanger, and CPOx reactor. System model results showed that variations in fuel flow and air to fuel ratio have large impacts on temperature distribution and power out, with lower fuel flows and air-to-fuel ratios providing higher SOFC power densities (~0.64 W/cm2) at high efficiencies (~45%). The system model also shows that external heat loss greatly reduces system power and efficiency but lower air-to-fuel ratios can offset associated temperature and associate performance losses.Item Direct Numerical Simulation of Non-Premixed Flame Extinction Phenomena(2010) Narayanan, Praveen; Trouve, Arnaud C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Non-premixed flame extinction phenomena are relevant in a variety of com- busting environments, including but hardly limited to diesel engines, pool fires, and fire suppression scenarios. These disparate phenomena are controlled by various parameters that contain information on flame stretch, heat losses, composition of the fuel and oxidizer supply streams, etc. Direct Numerical Simulation (DNS) is used in the present study to provide fundamental insight on diffusion flame extinction under non-adiabatic combustion conditions. The list of DNS configurations include: (C1) counterflow laminar flames with soot formation and thermal radiation transport; (C2) coflow turbulent flames with soot formation and thermal radiation transport; (C3) counterflow laminar and turbulent flames interacting with a mist-like water spray. Configurations C1 and C2 use single-step chemistry while configuration C3 uses detailed chemistry (all cases correspond to ethylene-air combustion). Configuration C1 is also treated using large Activation Energy Asymptotics (AEA). The AEA analysis is based on a classical formulation that is extended to include thermal radiation transport with both emission and absorption effects; the analysis also includes soot dynamics. The AEA analysis provides a flame extinction criterion in the form of a critical Damköhler number criterion. The DNS results are used to test the validity of this flame extinction criterion. In configuration C1, the flame extinction occurs as a result of flame stretch or radiative cooling; a variation of configuration C1 is considered in which the oxidizer stream contains a variable amount of soot mass. In configuration C1, flame weakening occurs as a result of radiative cooling; this effect is magnified by artificially increasing the mean Planck soot absorption coefficient. In configuration C3, flame extinction occurs as a result of flame stretch and evaporative cooling. In all studied cases, the critical Damkohler number criterion successfully predicts transition to extinction; this result supports the unifying concept of a flame Damköhler number Da and the idea that different extinction phenomena may be described by a single critical value of Da.Item A GENERIC RELIABILITY ANALYSIS AND DESIGN FRAMEWORK WITH RANDOM PARAMETER, FIELD, AND PROCESS VARIABLES(2010) Xi, Zhimin; Youn, Byeng D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation aims at developing a generic reliability analysis and design framework that enables reliability prediction and design improvement with random parameter, field, and process variables. The capability of this framework is further improved by predicting and managing reliability even with a dearth of data that can be used to characterize random variables. To accomplish the research goal, three research thrusts are set forth. First, advanced techniques are developed to characterize the random field or process. The fundamental idea of these techniques is to model the random field or process with a set of important field signatures and random variables. These techniques enable the use of random parameter, field, and process variables for reliability analysis and design even with a dearth of data. Second, a generic reliability analysis framework is proposed to accurately assess system reliability in the presence of random parameter, field, and process variables. An advanced probability analysis technique, the Eigenvector Dimension Reduction (EDR) method, is developed by integrating the Dimension Reduction (DR) method with three proposed improvements: 1) an eigenvector sampling approach to obtain statistically independent samples over a random space; 2) a Stepwise Moving Least Square (SMLS) method to accurately approximate system responses over a random space; and 3) a Probability Density Function (PDF) generation method to accurately approximate the PDF of system responses for reliability analysis. Third, a generic Reliability-Based Design Optimization (RBDO) framework is developed to solve engineering design problems with random parameter, field, and process variables. This design framework incorporates the EDR method into RBDO. To illustrate the effectiveness of the developed framework, many numerical and engineering examples are employed to conduct the reliability analysis and RBDO with random parameter, field, and process variables. This dissertation demonstrates that the developed framework is very accurate and efficient for the reliability analysis and RBDO of engineering products and processes.Item BAYESIAN BELIEF NETWORK AND FUZZY LOGIC ADAPTIVE MODELING OF DYNAMIC SYSTEM: EXTENSION AND COMPARISON(2010) CHENG, PING DANNY; MODARRES, MOHAMMAD; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The purpose of this thesis is to develop, expand, compare and contrast two methodologies, namely BBN and FLM, which are used in the modeling of the dynamics of physical system behavior and are instrumental in a better understanding on the POF. The paper begins with an introduction of the proposed approaches in the modeling of complex physical systems, followed by a quick literature review of FLM and BBN. This thesis uses an existing pump system [3] as a case study, where the resulting NPSHA data obtained from the applications of BBN and FLM are compared with the outputs derived from the implementation of a Mathematical Model. Based on these findings, discussions and analyses are made, including the identification of the respective strengths and weaknesses posed by the two methodologies. Last but not least, further extensions and improvements towards this research are discussed at the end of this paper.Item Novel Integrated System Architecture for an Autonomous Jumping Micro-Robot(2010) Churaman, Wayne Anthony; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As the capability and complexity of robotic platforms continue to evolve from the macro to micro-scale, innovation of such systems is driven by the notion that a robot must be able to sense, think, and act [1]. The traditional architecture of a robotic platform consists of a structural layer upon which, actuators, controls, power, and communication modules are integrated for optimal system performance. The structural layer, for many micro-scale platforms, has commonly been implemented using a silicon die, thus leading to robotic platforms referred to as "walking chips" [2]. In this thesis, the first-ever jumping microrobotic platform is demonstrated using a hybrid integration approach to assemble on-board sensing and power directly onto a polymer chassis. The microrobot detects a change in light intensity and ignites 0.21mg of integrated nanoporous energetic silicon, resulting in 246µJ of kinetic energy and a vertical jump height of 8cm.Item Sensitivity Analysis Based Approaches for Mitigating the Effects of Reducible Interval Input Uncertainty on Single- and Multi-Disciplinary Systems using Multi-Objective Optimization(2010) Hamel, Joshua Matthew; Azarm, Shapour; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Uncertainty is an unavoidable aspect of engineering systems and will often degrade system performance or perhaps even lead to system failure. As a result, uncertainty must be considered as a part of the design process for all real-world engineering systems. The presence of reducible uncertainty further complicates matters as designers must not only account for the degrading effects of uncertainty but must also determine what levels of uncertainty can be considered as acceptable. For these reasons, methods for determining and effectively mitigating the effects of uncertainty are necessary for solving engineering design problems. This dissertation presents several new methods for use in the design of engineering systems under interval input uncertainty. These new approaches were developed over the course of four interrelated research thrusts and focused on the overall goal of extending the current research in the area of sensitivity analysis based design under reducible interval uncertainty. The first research thrust focused on developing an approach for determining optimal uncertainty reductions given multi-disciplinary engineering systems with multiple output functions at both the system and sub-system levels. The second research thrust extended the approach developed during the first thrust to use uncertainty reduction as a means for both reducing output variations and simultaneously ensuring engineering feasibility. The third research thrust looked at systems where uncertainty reduction alone is insufficient for ensuring feasibility and thus developed a sensitivity analysis approach that combined uncertainty reductions with small design adjustments in an effort to again reduce output variations and ensure feasibility. The fourth and final research thrust looked to relax many of the assumptions required by the first three research thrusts and developed a general sensitivity analysis inspired approach for determining optimal upper and lower bounds for reducible sources of input uncertainty. Multi-objective optimization techniques were used throughout this research to evaluate the tradeoffs between the benefits to be gained by mitigating uncertainty with the costs of making the design changes and/or uncertainty reductions required to reduce or eliminate the degrading effects of system uncertainty most effectively. The validity of the approaches developed were demonstrated using numerical and engineering example problems of varying complexity.Item CHARACTERIZING SMOKE DISPERSION ALONG BEAMED CEILINGS USING SALT-WATER MODELING(2010) Chan, Chau Siang; Marshall, Andre W.; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The study successfully validated the use of salt-water analog modeling as an effective diagnostic, predictive and scaling tool for understanding fire dispersion in a beam-ceiling complex compartment using the Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) techniques. Dimensionless dispersion signatures and front arrival times were compared between the fire and salt-water experiments which showed excellent agreement. Prediction of the detector lag times using fire and saltwater data agreed with that of fire experiments.Item Air Bubble Entrainment by Breaking Bow Waves Simulated by a 2D+T Technique(2010) Tavakolinejad, Mohammadreza; Duncan, James H; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Experimental studies of air entrainment by breaking bow waves are essential for advancing the understanding of these flows and reating valid models. This experimental research is part of a larger project whose broad goal is to improve our understanding of the dynamics of breaking bow waves including the entrainment of air bubbles into the flow and the generation of turbulence and vorticity. A mechanical wave maker creates waves similar to those formed at the bow of a moving ship, utilizing the 2D+T approximation. The rimary mechanisms for air entrainment are the impact of the plunging wave jet and individual droplets in the splash region on the free surface. The air entrainment process is observed in small scale stationary model experiments, and the air bubbles are entrained in spatially periodic bubble clouds. The specific objectives of this project are to develop a shadowgraph technique for measurement of bubble size distributions and motions and to measure and analyze the void fraction as a function of the equivalent forward speed of the ship model. Three key bubble characteristics that will be emphasized and studied in particular detail are bubble distributions and motions, void fractions and relationships between the properties of bubble and the geometrical characteristics of the wave. The air entrainment in ship bow waves simulated by a 2D+T technique and the methods used to analyze these phenomena are all novel aspects of this particular study and should provide a valuable set of data and analyses for the study of breaking bow wave mechanics.Item A FLUID STRUCTURE INTERACTION STRATEGY WITH APPLICATION TO LOW REYNOLDS NUMBER FLAPPING FLIGHT(2010) Vanella, Marcos; Balaras, Elias; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this work a structured adaptive mesh refinement (S-AMR) strategy for fluid-structure interaction (FSI) problems in laminar and turbulent incompressible flows is developed. The Eulerian computational grid consists of nested grid blocks at different refinement levels. The grid topology and data-structure is managed by using the Paramesh© toolkit. The filtered Navier-Stokes equations are evolved in time by means of an explicit second-order projection scheme, where spatial derivatives are approximated with second order central differences on a staggered grid. The level of accuracy of the required variable interpolation operators is studied, and a novel divergence-preserving prolongation scheme for velocities is evolved. A novel direct-forcing embedded-boundary method is developed to enforce boundary conditions on a complex moving body not aligned with the grid lines. In this method, the imposition of no-slip conditions on immersed bodies is done on the Lagrangian markers that represent their wet surfaces, and the resulting force is transferred to the surrounding Eulerian grid points by a moving least squares formulation. Extensive testing and validation of the resulting strategy is done on a numerous set of problems. For transitional and turbulent flow regimes the large-eddy simulation (LES) approach is used. The grid discontinuities introduced in AMR methods lead to numerical errors in LES, especially if non-dissipative, centered schemes are used. A simple strategy is developed to vary the filter size for filtered variables around grid discontinuities. A strategy based on explicit filtering of the advective term is chosen to effectively reduce the numerical errors across refinement jumps. For all the FSI problems reported, the complete set of equations governing the dynamics of the flow and the structure are simultaneously advanced in time by using a predictor-corrector strategy. Dynamic fluid grid adaptation is implemented to reduce the number of grid points and computation costs. Applications to flapping flight comprise the study of flexibility effects on the aerodynamic performance of a hovering airfoil, and simulation of the flow around an insect model under prescribed kinematics and free longitudinal flight. In the airfoil simulations, it is found that peak performance is located in structural flexibility-inertia regions where non-linear resonances are present.