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 Development of the Maryland Tiltrotor Rig (MTR) and Whirl Flutter Stability Testing(2022) Tsai, Frederick; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tiltrotor aircraft encounter an aeroelastic instability called whirl flutter at high speeds. Whirl flutter is caused by the complex interaction between the aerodynamics and dynamics of the rotating proprotor blades, hub, and the wing. Current tiltrotors are limited to about 280 kt in cruise. While many computational analyses have been performed to assess potential improvements in whirl flutter stability, few have been validated by test data. There is a scarcity of publicly available test data along with documented model properties. A new tiltrotor rig is developed in this work to address this gap. The new rig, henceforth called the Maryland Tiltrotor Rig (MTR), is a semi-span, floor-mounted, optionally-powered rig with a static rotor tilt mechanism, capable of testing 3-bladed proprotors of up to 4.75-ft diameter in the Glenn L. Martin Wind Tunnel (7.75- by 11-ft section with 200 kt maximum speed). The objective is to experimentally characterize the parameters that affect the onset of whirl flutter which is vital to validating computational models and analyses. The MTR supports interchangeable hubs (gimballed and hingeless), interchangeable blades (straight and swept tip), and interchangeable wing spars, to allow a systematic variation of components important for tiltrotor flutter and loads. The vision for this rig is to conduct research towards flutter-free tiltrotors capable of achieving 400 kt and higher speeds in cruise. This dissertation lays the groundwork toward that vision by describing the test and evaluation of a baseline gimballed hub model. The features, controls, instrumentation, data acquisition, and all supporting equipment of the rig are described. A simple whirl flutter analysis model is developed, verified, and used for pre-test stability prediction of the MTR. The damping measurement methods are detailed. The first whirl flutter tests of the MTR were carried out at the Naval Surface Warfare Center-Carderock Division wind tunnel between 26 October - 2 November, 2021. Frequency and damping data were measured for four parametric configurations of wing on versus wing off, gimbal free versus gimbal locked, freewheel versus powered rotor, and straight versus swept-tip blades. The tests were conducted up to 100 kt windspeed, restricted only by the tunnel precautionary measures. Since the baseline model is loosely a 1/5.26-scale XV-15, 100 kt translates to a full-scale speed of 230 kt. It was observed that the baseline rig was stable up to 100 kt with an average wing damping lower than 1% critical in beamwise and 1.5% critical in chordwise motion. The effect of wing aerodynamics was insignificant up to 100 kt. Locking the gimbal affected mostly the chord mode and increased is damping significantly. Powering the rotor also affected mostly the chord mode and increased its damping significantly. The swept-tip blades showed interesting trends near 100 kt but higher speeds are needed for definitive conclusions. Overall, the MTR allowed the controlled variation of parameters that are important for fundamental understanding and analysis validation, but are impossible to carry out on an actual aircraft.Item An Experimental Investigation of Hypersonic Boundary-Layer Transition on Sharp and Blunt Slender Cones(2019) Kennedy, Richard Edward; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the instabilities leading to the laminar-to-turbulent transition of a hypersonic boundary layer is a key challenge remaining for the design of efficient hypersonic vehicles. In the present study, experiments are performed in three different facilities at freestream Mach numbers between 6 and 14 to characterize instability mechanisms leading to transition on a 7-degree half-angle slender cone. Second-mode instability waves are visualized using a high-speed schlieren setup with the camera frame rate and spatial resolution optimized to allow individual disturbances to be tracked. In order to facilitate quantitative time-resolved measurements, a method of calibrating the schlieren system and novel image-processing algorithms have been developed. Good agreement is observed between the schlieren measurements, surface pressure measurements, and parabolized stability equation computations of the second-mode most-amplified frequencies and N factors. The high-frequency-resolution schlieren signals enable a bispectral analysis that reveals phase locking of higher harmonic content leading to nonlinear wave development. Individual disturbances are characterized using the schlieren wall-normal information not available from surface measurements. Experiments are also performed to investigate the effect of nose-tip bluntness. For moderate to large bluntness nose tips, second-mode instability waves are no longer visible, and elongated structures associated with nonmodal growth appear in the visualizations. The nonmodal features exhibit strong content between the boundary-layer and entropy-layer edges and are steeply inclined downstream. Simultaneously acquired surface pressure measurements reveal high-frequency pressure oscillations typical of second-mode instability waves associated with the trailing edge of the nonmodal features.Item INTERPOLATION OF RIGID-BODY MOTION AND GALERKIN METHODS FOR FLEXIBLE MULTIBODY DYNAMICS(2019) Han, Shilei; Bauchau, Olivier A.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Traditionally, flexible multibody dynamics problems are formulated as initial value problems: initial states of the system are given and solving for the equations of motion yields the dynamic response. Many practical problems, however, are boundary rather than initial value problems; two-point and periodic boundary problems, in particular, are quite common. For instance, the trajectory optimization of robotic arms and spacecrafts is formulated as a two-point boundary value problem; determination of the periodic dynamic response of helicopter and wind turbine blades is formulated as a periodic boundary value problem; the analysis of the stability of these periodic solutions is another important of problem. The objective of this thesis is to develop a unified solution procedure for both initial and boundary value problems. Galerkin methods provide a suitable framework for the development of such solvers. Galerkin methods require interpolation schemes that approximate the unknown rigid-body motion fields. Novel interpolation schemes for rigid-body motions are proposed based on minimization of eighted distance measures of rigid-body motions. Based on the proposed interpolation schemes, a unified continuous/discontinuous Galerkin solver is developed for the formulation of geometrically exact beams, for the determination of solutions of initial and periodic boundary value problems, for the stability analysis of periodic solutions, and for the optimal control/optimization problems of flexible multibody systems.Item MORPHOLOGY OF CELLULOSE AND CELLULOSE BLEND THIN FILMS(2017) Lu, Rui; Briber, Robert M.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cellulose is the most abundant, renewable, biocompatible and biodegradable natural polymer. Cellulose exhibits excellent chemical and mechanical stability, which makes it useful for applications such as construction, filtration, bio-scaffolding and packaging. It is useful to study amorphous cellulose as most reactions happen in the non-crystalline regions first and at the edge of crystalline regions. In this study, amorphous thin films of cotton linter cellulose with various thicknesses were spincoated on silicon wafers from cellulose solutions in dimethyl sulfoxide / ionic liquid mixtures. Optical microscopy and atomic force microscopy indicated that the morphology of as-cast films was sensitive to the film preparation conditions. A sample preparation protocol with low humidity system was developed to achieve featureless smooth films over multiple length scales from nanometers to tens of microns. X-ray reflectivity, X-ray diffraction, Fourier transform infrared spectroscopy and high resolution sum-frequency generation vibrational spectroscopy were utilized to confirm that there were no crystalline regions in the films. One- and three- layer models were used to analyze the X-ray reflectivity data to obtain information about roughness, density and interfacial roughness as a function of film thickness from 10-100nm. Stability tests of the thin films were conducted under harsh conditions including hot water, acid and alkali solutions. The stability of thin films of cellulose blended with the synthetic polymer, polyacrylonitrile, was also investigated. The blend thin films improved the etching resistance to alkali solutions and retained the stability in hot water and acid solutions compared to the pure cellulose films.Item Stability and Change of Cortisol Reactivity to a Laboratory Stressor from Early to Middle Childhood(2015) Leppert, Katherine A.; Dougherty, Lea R; Psychology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study examined the stability and change of children’s cortisol reactivity to a laboratory stressor from early to middle childhood and moderators of change. Ninety-six children completed stress-inducing laboratory tasks and provided five salivary cortisol samples at preschool age (T1; M = 49.88 months, SD = 9.51 months) and three years later (T2; M = 87.44 months, SD = 11.42 months). At T1, parents completed clinical interviews assessing child and parent psychopathology. Cortisol reactivity patterns significantly changed from decreasing to increasing reactivity from early to middle childhood. Moreover, preschool psychopathology moderated this change. Children with fewer preschool psychiatric symptoms demonstrated more stable reactivity patterns, whereas children with preschool psychiatric comorbidity demonstrated more unstable reactivity patterns across assessments. Findings suggest a developmental shift from decreasing to increasing cortisol reactivity from early to middle childhood, and highlight early preschool psychopathology as a moderator of change in cortisol reactivity.Item COOPERATIVE NETWORKING AND RELATED ISSUES: STABILITY, ENERGY HARVESTING, AND NEIGHBOR DISCOVERY(2013) Jeon, Jeongho; Ephremides, Anthony; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation deals with various newly emerging topics in the context of cooperative networking. The first part is about the cognitive radio. To guarantee the performance of high priority users, it is important to know the activity of the high priority communication system but the knowledge is usually imperfect due to randomness in the observed signal. In such a context, the stability property of cognitive radio systems in the presence of sensing errors is studied. General guidelines on controlling the operating point of the sensing device over its receiver operating characteristics are also given. We then consider the hybrid of different modes of operation for cognitive radio systems with time-varying connectivity. The random connectivity gives additional chances that can be utilized by the low priority communication system. The second part of this dissertation is about the random access. We are specifically interested in the scenario when the nodes are harvesting energy from the environment. For such a system, we accurately assess the effect of limited, but renewable, energy availability on the stability region. The effect of finite capacity batteries is also studied. We next consider the exploitation of diversity amongst users under random access framework. That is, each user adapts its transmission probability based on the local channel state information in a decentralized manner. The impact of imperfect channel state information on the stability region is investigated. Furthermore, it is compared to the class of stationary scheduling policies that make centralized decisions based on the channel state feedback. The backpressure policy for cross-layer control of wireless multi-hop networks is known to be throughput-optimal for i.i.d. arrivals. The third part of this dissertation is about the backpressure-based control for networks with time-correlated arrivals that may exhibit long-range dependency. It is shown that the original backpressure policy is still throughput-optimal but with increased average network delay. The case when the arrival rate vector is possibly outside the stability region is also studied by augmenting the backpressure policy with the flow control mechanism. Lastly, the problem of neighbor discovery in a wireless sensor network is dealt. We first introduce the realistic effect of physical layer considerations in the evaluation of the performance of logical discovery algorithms by incorporating physical layer parameters. Secondly, given the lack of knowledge of the number of neighbors along with the lack of knowledge of the individual signal parameters, we adopt the viewpoint of random set theory to the problem of detecting the transmitting neighbors. Random set theory is a generalization of standard probability theory by assigning sets, rather than values, to random outcomes and it has been applied to multi-user detection problem when the set of transmitters are unknown and dynamically changing.Item Can self-control change substantially over time?: Rethinking the nature and role of self-control in Gottfredson and Hirschi's general theory of crime(2011) Na, Chong Min; Paternoster, Raymond; Criminology and Criminal Justice; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The primary goal of this study is to verify if the changing level of structural and situational `sensitivity' to costs and benefits associated with deviant behaviors (e.g., Hirschi (2004) and Gottfredson (2006)'s redefined self-control, Tittle, Ward, and Grasmick's (2004) "desire to exercise self-control," Wikström and Treiber's (2007) "situationally-based" self-control) is associated with the changing level of more general `ability' to measure costs and benefits within individuals (e.g., Gottfredson and Hirschi's (1990) trait-like self-control, Tittle, Ward, and Grasmick's (2004) "capacity for self-control," Wikström and Treiber's (2007) "executive capability"). More importantly, to better disentangle the causal mechanisms underlying stability and change in offending behaviors over time, This study examines how low self-control as one of the constituent elements of offending propensity changes over time in the general population and across different study groups using both a hierarchical linear model (HLM) and a second-order latent growth model (LGM). Then, structural equation modeling (SEM) is employed to examine the on-going processes of cumulative advantage and disadvantage by more explicitly testing the bidirectional relationship of key theoretical constructs (e.g., self-control vs. social control/bond) over time. In contrast to the Gottfredson and Hirschi's prediction, this study found meaningful differences in the growth pattern of self-control among individuals in the population in general and especially across different study groups. Interestingly, the changing level of social control/bond triggered by experimental conditions accounted for the between-group difference observed. The same pattern persists when different analytic techniques and model specifications are applied to test the same research hypotheses, which suggests that the results are not an artifact of measurement error, model specification, or statistical methods. Most of all, this study was able to better disentangle the `long-term' relationship between self- and social control variables, which is found to be more dynamic and bidirectional than previously hypothesized.Item Network flow optimization and distributed control algorithms(2006-11-21) Chen, Huigang; Baras, John S; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis concerns the problem of designing distributed algorithms for achieving efficient and fair bandwidth allocations in a resource constrained network. This problem is fundamental to the design of transmission protocols for communication networks, since the fluid models of popular protocols such as TCP and Proportional Fair Controller can be viewed as distributed algorithms which solve the network flow optimization problems corresponding to some fairness criteria. Because of the convexity of the optimization problem as well as its decoupling structure, there exist classical dual algorithm and primal/dual algorithm which are both distributed. However, the main difficulty is the possible instability of the dynamics of these algorithms caused by transmission delays. We use customized Lyapunov-Krasovskii functionals to obtain the stability conditions for these algorithms in networks with heterogeneous time-varying delays. There are two main features of our results. The first is that these stability conditions can be enforced by a small amount of information exchange among relevant users and links. The second is that these stability conditions only depend on the upper bound of delays, not on the rate of delay variations. We further our discussion on scalable algorithms with minimum information to maintain stability. We present a design methodology for such algorithms and prove the global stability of our scalable controllers by the use of Zames-Falb multipliers. Next we extend this method to design the first scalable and globally stable algorithm for the joint multipath routing and flow optimization problem. We achieve this by adding additional delays to different paths for all users. Lastly we discuss the joint single path routing and flow optimization problem, which is a NP hard problem. We show bounded price of anarchy for combined flow and routing game for simple networks and show for many-user networks, simple Nash algorithm leads to approximate optimum of the problem.Item Vehicle Path Optimization of Emergency Lane Change Maneuvers for Vehicle Simulation(2005-08-23) O'Hara, Steven Robert; Schultz, Gregory A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Driver-based handling tests, such as the Double Lane Change (DLC) maneuver are subjective in nature and depend largely on driver skill and road conditions. They also suffer from poor repeatability. Implementation of these tests on hardware-in-the-loop simulators can also produce subjective results if the steer profiles are not systematically generated. This research produced a vehicle path optimization model that generated optimal paths for handling tests based on minimizing the maximum curvature during the maneuver. This approach lessened the dynamics of the vehicle and increased the chances of successful test results at given speeds. Excel's Solver was used for the optimization. The model results were compared to field test and hardware-in-the-loop test results, showing potential for reductions in lateral acceleration and vehicle side-slip.