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 AUTONOMOUS ESTIMATION AND GUIDANCE OF AN AMPHIBIOUS QUADROTOR FOR BISTATIC UNDERWATER LASER IMAGING(2022) Toombs, Nathan; Paley, Derek; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Underwater object classification by unmanned underwater vehicles (UUVs) is a critical task that is made difficult in shallow waters with concentrated particulate matter. Bistatic laser imaging is a current area of research that is more effective than traditional optical methods, but it requires separation of the laser receiver from the UUV-mounted laser emitter. This work explores the prospect of performing bistatic laser imaging with the receiver mounted to a quadrotor unmanned aerial vehicle (UAV). To facilitate the imaging application, estimation and guidance algorithms are developed to autonomously locate and track a UUV-mounted laser with an amphibious UAV. The UAV is equipped to carry a receiver payload in safe above-water flight and water landings. To represent the received laser measurements, laser intensity models are developed based on the distributions of the decollimated lasers used in the imaging application. The UAV autonomy is validated both in a reduced-order simulation environment and with the hardware testbed.Item APPLICATION OF A BAYESIAN NETWORK BASED FAILURE DETECTION AND DIAGNOSIS FRAMEWORK ON MARITIME DIESEL ENGINES(2022) Reynolds, Steven; Groth, Katrina; Systems Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Diesel engine propulsion has been the largest driver of maritime trade and transportation since its development in the early 20th century and the technology surrounding the operation and maintenance of these systems has grown in complexity leading to rapid advancement in amount and variety of data being collected. This increase in reliability data provides a fantastic opportunity to improve upon the existing tools troubleshooting and decision support tool used within the maritime engine community to enable a more robust understanding of engine reliability. This work leverages this opportunity and applies it to the Coast Guard and its acquisition of the Fast Response Cutter (FRC) fleet powered by two MTU20V4000M93 engines integrated with top of line monitoring and control equipment.The purpose of this research is to create procedures for creating a Failure Detection and Diagnosis (FDD) model of a maritime diesel engine that updates existing Probabilistic Risk Analysis (PRA) data with input from the engine monitoring and control system using Bayesian inference. A literature review of existing work within the PRA and Prognostics and Health Management (PHM) fields was conducted with specific focus on the advancement and gaps in the field specific to their use in maritime engine applications. Following this, a hierarchal ruleset was created that outlines procedures for integrating existing PRA data and PHM metrics into a Bayesian Network structure. This methodology was then used to build a Bayesian Network based FDD model of the FRC engine. This model was then validated by Coast Guard Engineers and run through a diagnostic use case scenario demonstrating the model’s suitability in the diagnostic space.Item STRUCTURAL EVOLUTION DURING THERMAL TREATMENTS AND THE RESULTANT MECHANICAL BEHAVIOR OF HIGH STRENGTH LOW ALLOY STEELS(2018) Draper, Matthew Charles; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)HY steels were designed as a solid solution strengthened grade for both high yield strength and high impact toughness in structural applications for Naval vessels. These alloys are susceptible to both hydrogen and temper embrittlement which necessitates high expense manufacturing processes to preclude these conditions. Successful implementation of lower cost and higher reliability treatments requires an improved understanding of the structural evolution and corresponding changes in mechanical behavior for the alloy. This research combines mechanical and microstructural characterization methods along with thermodynamic and kinetic models to build a comprehensive understanding of the effects of thermal treatments on the structure-property relationship of the alloy system. The embrittlement rate was studied between 315°C and 565°C at varied logarithmic time intervals up to 40,000 minutes. The embrittlement recovery rate was studied between 593°C and 704°C at logarithmic time intervals up to 10,000 minutes. Finally, hydrogen aging was studied between 315°C and 565°C at varied thermodynamically equivalent time intervals. A variety of test methods were employed for characterization including: traditional metallographic techniques, mechanical testing, computational modeling, and a novel image analysis technique for carbide analysis. Metallographic along with computational work supports a conclusion that temper embrittlement and subsequent recovery cannot be solely explained by the segregation of phosphorus and other embrittling elements to grain boundaries. Rather it is shown for the first time that alloy carbides play a key role in embrittlement for this system. The evolution of these carbides serves both to create initiation sites for cleavage fracture and deplete the matrix of Mo, which is a P scavenger. Recovery from embrittlement is thus proposed to be related to both the removal of P from the boundary and the dissolution of carbides. From these results a series of kinetic models have been developed for the nucleation, dissolution, and coarsening of alloy carbides. Models developed for the mitigation of monatomic hydrogen show a novel treatment for hydrogen aging via performing the aging within the embrittlement range with follow on treatments designed to recover from embrittlement. This new treatment has the potential to reduce hydrogen aging times by up to 90% in industrial manufacture.Item AN INVESTIGATION ON THE AFFECT OF EXTERNAL CONDITIONS ON THE RELIABILITY OF AIRCRAFT INSPECTIONS(2017) Barrett, Adam David; Modarres, Mohammad; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective of this study is to develop an understanding of how external variables commonly encountered during an inspection affect an inspection systems detection capability. A probability of detection study was performed using representative structural samples, attached to a simulated naval flight asset. For the execution of these tests, common nondestructive testing equipment was utilized by multiple inspectors. For each test inspectors, test samples (with imbedded damage) and inspection locations around the test bed were varied to better simulate field inspection conditions. An understanding of how these variables affect inspection performance will give maintainers, designers, and planners a more realistic idea of what damage can be detected and quantified in field inspection conditions.Item Induced Soil Liquefaction for the Freeing of Grounded Ships(2017) Cerquetti, Jeffrey; Aggour, Mohamed S.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective of this study is to determine the feasibility of freeing a grounded ship by liquefying the surrounding soils. Ships either moored or traveling in near-shore waters and subjected to storm events, will experience waves energetic enough to direct the ship toward the shore. The ship can then become embedded in the soils (grounded) close to the shore. The study included two phases. Phase one was an experimental study where models of three ship sections representing standard classes of ships were constructed. These models were embedded in a saturated sand in an especially constructed tank. Pull tests were done initially to establish benchmark freeing forces and then air blasts were used to produce the dynamic force needed to liquefy the surrounding soils. The models subsequently regained buoyancy. The second phase of the study utilized the data obtained from the testing program to extrapolate those data to a response of an actual-size ship. The conclusion showed that ships grounded can be freed by liquefaction of the surrounding soils. This novel technique of restoring a ship’s buoyancy and thus refloating it was demonstrated experimentally on model ships and analytically by determining the air pressure needed to free an actual ship in a grounding event. This new technique will have an economical value for the shipping industry and could provide an environmentally safe approach in dealing with grounded ships.Item Air Entrainment in the Turbulent Ship Hull Boundary Layer(2016) Washuta, Nathan John; Duncan, James H; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Turbulent fluctuations in the vicinity of the water free surface along a flat, vertically oriented surface-piercing plate are studied experimentally using a laboratory-scale experiment. In this experiment, a meter-wide stainless steel belt travels horizontally in a loop around two rollers with vertically oriented axes, which are separated by 7.5 meters. This belt device is mounted inside a large water tank with the water level set just below the top edge of the belt. The belt, rollers, and supporting frame are contained within a sheet metal box to keep the device dry except for one 6-meter-long straight test section between rollers. The belt is launched from rest with an acceleration of up to 3-g in order to quickly reach steady state velocity. This creates a temporally evolving boundary layer analogous to the spatially evolving boundary layer created along a flat-sided ship moving at the same velocity, with a length equivalent to the length of belt that has passed the measurement region since the belt motion began. Surface profile measurements in planes normal to the belt surface are conducted using cinematic Laser Induced Fluorescence and quantitative surface profiles are extracted at each instant in time. Using these measurements, free surface fluctuations are examined and the propagation behavior of these free surface ripples is studied. It is found that free surface fluctuations are generated in a region close to the belt surface, where sub-surface velocity fluctuations influence the behavior of these free surface features. These rapidly-changing surface features close to the belt appear to lead to the generation of freely-propagating waves far from the belt, outside the influence of the boundary layer. Sub-surface PIV measurements are performed in order to study the modification of the boundary layer flow field due to the effects of the water free surface. Cinematic planar PIV measurements are performed in horizontal planes parallel to the free surface by imaging the flow from underneath the tank, providing streamwise and wall-normal velocity fields. Additional planar PIV experiments are performed in vertical planes parallel to the belt surface in order to study the bahvior of streamwise and vertical velocity fields. It is found that the boundary layer grows rapidly near the free surface, leading to an overall thicker boundary layer close to the surface. This rapid boundary layer growth appears to be linked to a process of free surface bursting, the sudden onset of free surface fluctuations. Cinematic white light movies are recorded from beneath the water surface in order to determine the onset location of air entrainment. In addition, qualitative observations of these processes are made in order to determine the mechanisms leading to air entrainment present in this flow.Item ESTIMATION OF EXTREME BENDING MOMENTS ON SHIPS FROM LIFETIME FATIGUE LOADS(2012) Webb, David; Ayyub, Bilal M; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The U. S. Coast Guard uses a U. S. Navy methodology and a computer program called SPECTRA to probabilistically characterize wave-induced bending moments on surface vessels. SPECTRA is primarily used for fatigue design based on defined cells of vessel operation with specified heading, sea condition and speed in order to calculate bending response using the probability a ship is within each cell in a specified time period. In this study, the SPECTRA output for a hypothetical ship was obtained to examine its appropriateness to be used as a basis to characterize lifetime extreme design bending moments on ship hulls. The objective was to develop a method to utilize the SPECTRA fatigue load output to estimate the parameters of an extreme value distribution, such as the Weibull probability distribution, for the largest bending moment of k years. The study examined how to appropriately interpret and use the mean and variance of the bending moments obtained from SPECTRA for this purpose. A four step method is proposed in this thesis involving first getting the statistical moments of the data from the SPECTRA histograms, estimating the parameters of the Weibull using these moments, finding the moments of the largest in k years from the generated distribution, and finally estimating the parameters of the Weibull for the largest in k years from these moments. The study also includes the development of an efficient and robust method of estimating the parameters and moments that is called the adaptive technique, involving exact calculation and numerical integration. The method is illustrated using a hypothetical case and verified using extreme value computations. It is also observed that the SPECTRA output based on specifying two or more years produces only minor enhancements in the estimated moments for one year and does not produce the statistical moments of extreme loading.Item MODELING OF WATER-BREATHING PROPULSION SYSTEMS UTILIZING THE ALUMINUM-SEAWATER REACTION AND SOLID-OXIDE FUEL CELLS(2011) Waters, Daniel Francis; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis investigates the use of solid oxide fuel cells (SOFCs) to consume waste hydrogen and improve the overall performance of a Hybrid Aluminum Combustor (HAC): a novel underwater power system based on the exothermic reaction of aluminum with seawater. The system is modeled using a NASA-developed framework called Numerical Propulsion System Simulation (NPSS) by assembling thermodynamic models developed for each component. Results show that incorporating the SOFC is not beneficial in cases where venting hydrogen overboard is permissible. However, when venting hydrogen is not permissible - which is the situation for most missions - the HAC-SOFC provides a 5 to 7 fold increase in range/endurance compared to equivalent battery powered systems. The utility of NPSS was demonstrated for evaluating and optimizing underwater propulsion system performance. Methodologies for predicting how system volume and mass scale with power were also developed to enable prediction of power and energy density.Item Mitigation of Frame Acceleration Induced by a Buried Charge(2010) Brodrick, Thomas James; Foruney, William; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, methods to mitigate acceleration delivered to the frame of a vehicle with an attached v-shaped hull are investigated. The frame of a vehicle represents an alternative location for crew seating, as opposed to seats being secured to the floorboard. Mitigation techniques were investigated for three test setups: aluminum frame with a downwardly convex aluminum hull, steel frame with a downwardly convex steel hull, and a steel frame with a downwardly concave steel hull. Accelerations of the frame were measured using piezoelectric accelerometers placed at three different locations on the frame. These acceleration measurements were verified against video recorded by high speed cameras. Each test was intended to reduce peak accelerations experienced by the frame, and to reduce the width of the acceleration envelope at large g levels. Mitigation techniques focused on reducing the initial hull-frame interactions, while damping subsequent responses of the system. Mitigation systems and hull orientation were compared for their ability to reduce blast effects experienced by the frame.