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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    Inertial Parameter Identification of a Captured Payload Attached to a Robotic Manipulator on a Free-Flying Spacecraft
    (2024) Limparis, Nicholas Michael; Akin, David L; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The groundwork for the dynamics of a free-flyer with a manipulator has been laid out by Yoshida, Vafa and Dubowsky, and Papadopoulos and Moosovian with the Generalized Jacobian Matrix, Virtual Manipulator, and Barycentric Vector Approach respectively. The identification of parameters for a robot manipulator has also been approached for industrial robots as well as through adaptive control theory. What is proposed is a method for inertial parameter identification and verification for a spacecraft with an attached manipulator that is an extension of the ground-fixed Inverse Direct Dynamic Model to function for a free-flying spacecraft. This method for inertial parameter identification for a spacecraft-manipulator system with an attached client spacecraft, debris, or other grappled payload is developed in this thesis and is experimentally tested using results for a servicer and an "unknown" grappled payload using three separate test beds. The results of the experiments show that the proposed method is capable of identifying the inertial parameters of the servicer and the grappled payload.
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    On the Dynamics of Binary Asteroids Applied to DART Mission Target (65803) Didymos
    (2022) Agrusa, Harrison Fitzgerald; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    NASA’s Double Asteroid Redirection Test (DART) mission will be the first full-scale demonstration of a kinetic impactor for planetary defense. On September 26, 2022, the DART spacecraft is expected to impact Dimorphos, the secondary component of the Didymos binary asteroid system. The DART impact will reduce Dimorphos’s relative orbital velocity, shrinking both its semimajor axis and orbit period. The mutual orbit period will then be measured us- ing ground- and space-based observations in order to deduce the momentum transfer efficiency, which is an important parameter in planetary defense that has never been measured experimentally at a realistic scale. This thesis comprises a set of studies on the spin and orbital dynamics of the Didymos system conducted in support of the DART mission. Owing to the close proximity of Didymos and Dimorphos and their irregular shapes, the mutual dynamics are non-Keplerian and exhibit a high degree of spin-orbit coupling, which often requires the use of specialized numerical methods to model the system. First, we conducted a benchmarking and sensitivity study to identify the best simulation codes for future DART-supported studies and to understand how small perturbations in the initial conditions can affect the resulting dynamical evolution of the system. Then, we demonstrated that Dimorphos can enter a wide range of post-impact spin states, including possible chaotic non-principal axis rotation, depending on its shape and the amount of momentum transferred by the DART impact. We then explored the implications of an excited spin state, including the possibility of ongoing granular motion on Dimorphos’s surface resulting from the orbital perturbation induced by the DART impact. This thesis is focused predominantly on the dynamics of the Didymos binary. However, there are many other binary systems among the near-Earth asteroid population with similar physical and dynamical properties, making the results presented here relevant to the NEA binary population in general.
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    Influence of Noise on Response Localizations in Mechanical Oscillator Arrays
    (2022) Cilenti, Lautaro Daniel; Balachandran, Balakumar; Cameron, Maria; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The dynamics of mechanical systems such as turbomachinery and vibration energy harvesting systems (VEH) consisting of one or multiple cantilever structures is often modeled by arrays of periodically driven coupled nonlinear oscillators. It is known that such systems may have multiple stable vibration steady states. Some of these steady states are localized vibrations that are characterized by high amplitude vibrations of a subset of the system, with the rest of the system being in a state of either low amplitude vibrations or no vibrations. On one hand, these localized vibrations can be detrimental to mechanical integrity of turbomachinery, while on the other hand, the vibrations can be potentially desirable for increasing energy yield in VEHs. Transitions into or out of localized vibrations may occur under the influence of random factors. A combination of experimental and numerical studies has been performed in this dissertation to study the associated transition times and probability of transitions in these mechanical systems. The developments reported here include the following: (i) a numerical methodology based on the Path Integral Method to quantify the probability of transitions due to noise, (ii) a numerical methodology based on the Action Plot Method to quantify the quasipotential and most probable transition paths in nonlinear systems with periodic external excitations, and (iii) experimental evidence and stochastic simulations of the influence of noise on response localizations of rotating macro-scale cantilever structures. The methodology and results discussed in this dissertation provide insights relevant to the stochastic nonlinear dynamics community, and more broadly, designers of mechanical systems to avoid potentially undesirable stochastic nonlinear behavior.
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    INVESTIGATION OF COMPOUND ROTORCRAFT AEROMECHANICS THROUGH WIND-TUNNEL TESTING AND ANALYSIS
    (2022) Maurya, Shashank; Datta, Anubhav; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The aeromechanics of a slowed-rotor compound rotorcraft is investigated through wind-tunnel testing and comprehensive analysis. The emphasis is on a lift-offset wing compound with a hingeless rotor configuration. A new Maryland Compound Rig is developed and instrumented for wind-tunnel testing and an in-house rotor comprehensive code is modified and expanded for compound rotorcraft analysis. The compound rig consists of a lift compound model and a propeller model. The lift compound model consists of an interchangeable hub (articulated or hingeless), a fuselage, a half-wing of 70% rotor radius on the retreating side. The wing has a dedicated load cell and multiple attachment points relative to the rotor hub (16%R, 24%R, and 32%R and 5%R aft of the hub). The rotor diameter is 5.7-ft. The rotor has four blades with NACA 0012 airfoils with no twist and no taper. The wing incidence angle is variable between 0 to 12 degrees. The wing has a linearly varying thickness with symmetric airfoils NACA 0015 at the tip and NACA 0020 at the root. Sensors can measure rotor hub forces and moments, wing root forces and moments, blade pitch angles, structural loads (flap bending moment, lagbending moment, and torsional moment) at 25%R, pitch link loads, and hub vibratory loads. Wind tunnel tests are conducted up to advance ratio 0.7 for lift compound with half-wing at wing incidence angles of 4 and 8 degrees and compared with an isolated rotor. Hover tests are conducted up to tip Mach number of 0.5 to measure download penalty with the wing at various positions. The University of Maryland Advanced Rotorcraft Code (UMARC) is modified for compound rotorcraft analysis code. Aerodynamic models for the wing and the propeller are integrated. A recently developed Maryland Free Wake model is integrated, which can model the wake interaction between unequal and inharmonic speed rotor, wing, and propeller. The analysis is then validated with the test data. The validated analysis is used to analyze the US Army hypothetical full-scale aircraft. The compound rotorcraft is categorized into multiple configurations in a systematic manner to find the extreme limits of speed and efficiency of each. The key conclusions are: 1) slowing the rotor or compounding the configuration provide no benefit individually; they must be accomplished together, 2) Half-Wing is more beneficial if a lift-offset hingeless rotor is used, 3) hover download penalty is only 3% of net thrust, and this penalty can be predicted satisfactorily by free wake, 4) the main rotor wake interaction is more pronounced on the wing and less on the propeller, 5) the validated analysis indicates a speed of 240 knots may be possible with 20% RPM reduction along with a wing and propeller, if structural weights allow, and 6) the oscillatory and vibratory lag moments and in-plane hub loads may be significantly reduced by compounding.
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    EXPANDING THE TOOLKIT: STRUCTURE, DYNAMICS, AND DRUG INTERACTIONS OF THE “PRIMING LOOP” FROM HEPATITIS B VIRUS PRE-GENOMIC RNA BY SOLUTION NMR SPECTROSCOPY
    (2022) Olenginski, Lukasz Tyler; Dayie, Theodore K; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    RNAs are dynamic macromolecules that function as essential components of biological pathways that result in human disease, making them attractive therapeutic targets. Yet, RNA structural biology lags significantly behind that of proteins, limiting mechanistic understanding of RNA chemical biology. Fortunately, solution NMR spectroscopy can probe the structure, dynamics, and interactions of RNA in solution at atomic resolution, opening the door to their functional understanding. However, NMR analysis of RNA – with only four unique ribonucleotide building blocks – suffers from spectral crowding and broad linewidths, especially as RNAs grow in size. One effective strategy to overcome these challenges is to introduce NMR-active stable isotopes into RNA in an atom- and position-specific manner. Here, we outline the development of labeling technologies, their use in benefiting RNA dynamics measurements, and applications to study the structure, dynamics, and interactions of a conserved regulatory RNA stem-loop from hepatitis B virus that is critical for viral replication.
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    Modeling Viscoelastic Behavior Using Flexible Multibody Dynamics Formulations
    (2020) Nemani, Nishant; Bauchau, Olivier Prof.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Viscoelastic behavior is frequently observed in dynamical flexible multibody systems. In the simplest form it is manifested in one dimensional revolute and prismatic joints. Beyond which more complex force elements such as six degree of freedom flexible joints can also be found. Finally, beams, plates and shells are found to exhibit viscoelastic behavior too. In the past extensive work has been done on analyzing the dynamic response of three dimensional beams by performing cross-sectional analysis through finite element methods and subsequently solving the reduced beam problem. The approach is particularly relevant for the analysis of complex cross sections and helps improve computational efficiency significantly. A formulation which incorporates a viscoelastic model of the generalized Maxwell type with a solution of the three dimensional beam theory which gives an exact solution of static three dimensional elasticity problems is presented. Multiple examples incorporating the use of the aforementioned model in the context of viscoelastic beams and joints are presented. Shortcomings of the Kelvin-Voigt model, which is often used for flexible multibody systems, are underlined.
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    DYNAMICS OF GLOBAL SURFACE WATER 1999 - PRESENT
    (2021) Pickens, Amy; Hansen, Matthew C; Geography; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Inland surface waters are critical to life, supplying fresh water and habitat, but are constantly in flux. There have been considerable advances in surface water monitoring over the last decade, though the extent of surface water has not been well-quantified per international reporting standards. Global characterizations of change have been primarily bi-temporal. This is problematic due to significant areas with multi-year cycles of wet and dry periods or anomalous high water or drought years. Many areas also exhibit strong seasonal fluctuations, such as floodplains and other natural wetlands. This dissertation aims to characterize open surface water extent dynamics by employing all of the Landsat archive 1999-present, and to report area estimates with associated uncertainty measures as required by policy guidelines. From 1999 to 2018, the extent of permanent water (in liquid or ice state) was 2.93 (standard error ±0.09) million km2, representing only 60.82 (±1.93)% of the total area that had water for some duration of the period. The unidirectional loss and gain areas were relatively small, accounting for only 1.10 (±0.23)% and 2.87 (±0.58)% of total water area, respectively. The area that transitioned multiple times between water and land states on an annual scale was over four times larger (19.74 (±2.16)%), totaling 0.95 (±0.10) million km2, establishing the need to evaluate the time-series from the entire period to assess change dynamics. From a seasonal perspective, June has over double the amount of open surface water as January, with 3.91 (±0.19) million km2 and 1.59 (±0.21) million km2, respectively. This is due to the vast network of lakes and rivers across the high-latitudes of the northern hemisphere that freeze over during the winter, with a maximum extent of ice over areas of permanent and seasonal water in February, totaling 2.49 (±0.25) million km2. This is the first global study to estimate the areas of extent and change with associated uncertainty measures and evaluate the seasonal dynamics of surface water and ice in a combined analysis. The methods developed here provide a framework for continuing to evaluate past trends and monitoring current dynamics of surface water and ice.
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    PHYSICS-BASED AND DATA-DRIVEN MODELING OF HYBRID ROBOT MOVEMENT ON SOFT TERRAIN
    (2020) Wang, Guanjin; Balachandran, Balakumar; Riaz, Amir; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Navigating an unmapped environment is one of the ten biggest challenges facing the robotics community. Wheeled robots can move fast on flat surfaces but suffer from loss of traction and immobility on soft ground. However, legged machines have superior mobility over wheeled locomotion when they are in motion over flowable ground or a terrain with obstacles but can only move at relatively low speeds on flat surfaces. A question to answer is as follows: If legged and wheeled locomotion are combined, can the resulting hybrid leg-wheel locomotion enable fast movement in any terrain condition? To investigate the rich physics during dynamic interactions between a robot and a granular terrain, a physics-based computational framework based on the smoothed particle hydrodynamics (SPH) method has been developed and validated by using experimental results for single robot appendage interaction with the granular system. This framework has been extended and coupled with a multi-body simulator to model different robot configurations. Encouraging agreement is found amongst the numerical, theoretical, and experimental results, for a wide range of robot leg configurations, such as curvature and shape. Real-time navigation in a challenging terrain requires fast prediction of the dynamic response of the robot, which is useful for terrain identification and robot gait adaption. Therefore, a data-driven modeling framework has also been developed for the fast estimation of the slippage and sinkage of robots. The data-driven model leverages the high-quality data generated from the offline physics-based simulation for the training of a deep neural network constructed from long short-term memory (LSTM) cells. The results are expected to form a good basis for online robot navigation and exploration in unknown and complex terrains.
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    Dynamics in Metal Halide Perovskites for Optoelectronics
    (2020) Howard, John Michael; Leite, Marina S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A diverse portfolio of renewable energy technologies is required to limit global warming to less than 2 ◦C. Of the possible emissions-free options, photovoltaic (PV) technologies can be most widely deployed, given the abundance of the solar resource compared. As with all power generation sources, PV adoption is predicated on the availability of technology solutions that are both inexpensive and highly efficient. One solar cell material, the metal halide perovskites (MHP), may provide the ideal combination, with > 25% efficiency devices within the first decade since their invention fabricated through simple spin coating. Despite the unprecedented rise in MHP performance, stability remains a critical challenge with the most stable devices at the 1-year benchmark compared to the >25-year lifetime of Si-based PV. Further progress concerning enduring power output will require a fundamental understanding of the impact of environmental stressors (light, temperature, bias, oxygen, and water) on the basic physical processes governing solar cell operation. Therefore, my dissertation elucidates the interplay between the ambient environment and MHP composition on both the optical and electrical behavior using in situ methods. The first part of my thesis elucidates the time-dependent optical and elec- tronic response of different MHP compositions using different in situ microscopy techniques. I capture the transient photovoltage of both Br- and I-containing per- ovskites for different photon energies using heterodyne Kelvin probe measurements. My measurements demonstrate that the voltage rise (light ON) is 104× faster than the subsequent decay (light OFF). Uniquely, the decay time for the residual voltage depends on the excitation wavelength, but only for the MAPbBr3 thin film. Next, I spatially and temporally resolve the relationship between radiative recombination and relative humidity (rH) for multi-cation films. The time-dependent photolumi- nescence (PL) indicates that the Cs-Br ratio impacts the magnitude of light emission hysteresis across an rH cycle. Further, I establish the existence of a repeatable and reversible ≈25× PL gain for multiple moisture cycles up to 70% rH. The second part of my thesis establishes the ability of machine learning (ML) models to predict the time-dependent behavior of perovskite material properties. I collect a comprehensive set of humidity-dependent PL data for both MAPbBr3 and MAPbI3 perovskites. I then use that data to train recurrent neural networks to forecast light emission based on only the recorded rH values. Using Echo State Networks, I achieve a normalized root-mean-squared error of <11% for both compositions for a 12+ h prediction win- dow. Further, I use a Long Short Term Memory network to predict the PL from a degrading sample, achieving <5% error. My in situ measurements and predictive ML models provide a powerful framework for identifying structure-property rela- tionships and can help accelerate the development of long-term stable perovskite materials.
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    Wind Tunnel Test on Slowed Rotor Aeroechanics at High Advance Ratios
    (2020) Wang, Xing; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In forward flight, slowing down a rotor alleviates compressibility effects on the advancing side, extending the cruise speed limit and inducing high advance ratio flight regime. To investigate the aerodynamic phenomena at high advance ratios and provide data for the validation of analysis tools, a series of wind tunnel tests were conducted progressively with a 33.5-in radius, 4-bladed Mach-scaled rotor in the Glenn L. Martin Wind Tunnel. In the first stage of the research, a wind tunnel test was carried out at high advance ratios with highly similar, non-instrumented blades and on-hub control angle measurements, in order to gain a baseline performance and control dataset with minimum error due to blade structural dissimilarity and pitch angle discrepancy. The tests were conducted at advance ratios of 0.3 to 0.9, and a parametric study on shaft tilt was conducted at $0^\circ$ and $\pm 4^\circ$ shaft tilt angles. The test data were then compared with those of previous tests and with the predictions of the in-house comprehensive analysis UMARC. The airload results were investigated using comprehensive analysis to gain insights on the influences of advance ratio and shaft tilt angle on rotor performance and hub vibratory loads. Results indicate that the thrust benefit from backward shaft tilt is dependent on the change in the inflow condition and the induced angle of attack increment, and the reverse flow region at high advance ratios is the major contributor to changes in shaft torque and horizontal force. In the second stage of the research, the rotor blades were instrumented with pressure sensors and strain gauges at 30\% radius, and pressure data were acquired to calculate the sectional airloads by surface integration up to an advance ratio of 0.8. The test results of blade airloads and structural loads were compared with the predictions of comprehensive analysis (UMARC and PrasadUM) and CFD/CSD coupled analysis (PrasadUM/HAMSTR). The focus was on the data correlation between experimental pressure, airload and structural load data and the CFD/CSD predicted results at various collective and shaft tilt settings. Overall, the data correlation was found satisfactory, and the study provided some insights into the aerodynamic mechanisms that affect the rotor airload and performance, in particular the mechanisms of backward shaft tilt, hub/shaft wake and the formation of dynamic stall in the reverse flow region. The next stage focused on hingeless rotor with lift offset. Previous wind tunnel tests have shown that an articulated rotor trimmed to zero hub moment generates limited thrust at high advance ratios, because the advancing side needs to be trimmed against the retreating side with significant reverse flow, in which the rotor is ineffective in generating thrust. Therefore, a hingeless rotor that allows the advancing side to generate more thrust can be rewarding in overall thrust potential. Wind tunnel tests were conducted up to an advance ratio of 0.7 to investigate the behavior of hingeless rotors at high advance ratios with lift offsets. Performance, control angles, hub vibratory loads and blade structural loads were compared with comprehensive analysis predictions from UMARC, plus the wing performance predictions from AVL. The results demonstrate that a hingeless rotor with lift offset is more efficient in generating thrust and exhibits higher lift-to-drag ratio at high advance ratios. The blade structural load level is significantly higher compared to an articulated rotor, especially for 2/rev flap bending moment, which can pose a critical structural constraint on the rotor.