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 Flowfield Estimation and Vortex Stabilization near an Actuated Airfoil(2019) Gomez Berdugo, Daniel Fernando; Paley, Derek A; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Feedback control of unsteady flow structures is a challenging problem that is of interest for the creation of agile bio-inspired micro aerial vehicles. This thesis presents two separate results in the estimation and control of unsteady flow structures: the application of a principled estimation method that generates full flowfield estimates using data obtained from a limited number of pressure sensors, and the analysis of a nonlinear control system consisting of a single vortex in a freestream near an actuated cylinder and an airfoil. The estimation method is based on Dynamic Mode Decompositions (DMD), a data-driven algorithm that approximates a time series of data as a sum of modes that evolve linearly. DMD is used here to create a linear system that approximates the flow dynamics and pressure sensor output from Particle Image Velocimetry (PIV) and pressure measurements of the flowfield around the airfoil. A DMD Kalman Filter (DMD-KF) uses the pressure measurements to estimate the evolution of this linear system, and thus produce an approximation of the flowfield from the pressure data alone. The DMD-KF is implemented for experimental data from two different setups: a pitching cambered ellipse airfoil and a surging NACA 0012 airfoil. Filter performance is evaluated using the original flowfield PIV data, and compared with a DMD reconstruction. For control analysis, heaving and/or surging are used as input to stabilize the vortex position relative to the body. The closed-loop system utilizes a linear state-feedback control law. Conditions on the control gains to stabilize any of the equilibrium points are determined analytically for the cylinder case and numerically for the airfoil. Simulations of the open- and closed-loop systems illustrate the bifurcations that arise from varying the vortex strength, bound circulation and/or control gains.Item RESPONSE CONTROL IN NONLINEAR SYSTEMS WITH NOISE(2019) Agarwal, Vipin Kumar; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Noise is unavoidable and/or present in a wide variety of engineering systems. Although considered to be undesirable from certain viewpoints, it can play a useful role in influencing the behavior of nonlinear mechanical and structural systems that have multiple solutions in the form of equilibrium points, periodic solutions, and aperiodic (including chaotic) solutions. The aim of this dissertation work is to discover clues related to noise enabled steering or control for engendering desirable changes in system behavior. A combination of experimental, analytical, and numerical studies have been undertaken on the following: i) shifting of jump-up and jump-down frequencies leading to an eventual collapse of hysteresis observed in the response of a nonlinear oscillator, ii) influence of noise on the chaotic response of a nonlinear system, and iii) noise-induced escape route from a chaotic-attractor. Furthermore, a combination of analytical and numerical studies have been undertaken to understand an extended Jeffcott rotor-stator system and the influence of noise on the system dynamics. Additionally, this dissertation includes work on partial control of chaotic systems under the influence of noise, wherein the trajectories are confined inside a particular region (chaotic attractor) despite the presence of white noise. Maintaining chaotic behavior in systems in the presence of an external disturbance may be desirable and important for the dynamics of certain systems. The proposed algorithm has been shown to be effective for systems with different dimensions. The dissertation outcomes provide answers to the following fundamental questions: i) how can noise influence the long-time responses of mechanical and structural systems and ii) how can noise be used to steer a system response to avoid an undesirable dynamical state. These answers can serve as an important foundation for many industrial applications (e.g., applications with rotor-stator systems) as well.Item DYNAMICS OF LARGE SYSTEMS OF NONLINEARLY EVOLVING UNITS(2017) Lu, Zhixin; Ott, Edward; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The dynamics of large systems of many nonlinearly evolving units is a general research area that has great importance for many areas in science and technology, including biology, computation by artificial neural networks, statistical mechanics, flocking in animal groups, the dynamics of coupled neurons in the brain, and many others. While universal principles and techniques are largely lacking in this broad area of research, there is still one particular phenomenon that seems to be broadly applicable. In particular, this is the idea of emergence, by which is meant macroscopic behaviors that “emerge” from a large system of many “smaller or simpler entities such that ... large entities” [i.e., macroscopic behaviors] arise which “exhibit properties the smaller/simpler entities do not exhibit.” [1]. In this thesis we investigate mechanisms and manifestations of emergence in four dynamical systems consisting many nonlinearly evolving units. These four systems are as follows. (a) We first study the motion of a large ensemble of many noninteracting particles in a slowly changing Hamiltonian system that undergoes a separatrix crossing. In such systems, we find that separatrix-crossing induces a counterintuitive effect. Specifically, numerical simulation of two sets of densely sprinkled initial conditions on two energy curves appears to suggest that the two energy curves, one originally enclosing the other, seemingly interchange their positions. This, however, is topologically forbidden. We resolve this paradox by introducing a numerical simulation method we call “robust” and study its consequences. (b) We next study the collective dynamics of oscillatory pacemaker neurons in Suprachiasmatic Nucleus (SCN), which, through synchrony, govern the circadian rhythm of mammals. We start from a high-dimensional description of the many coupled oscillatory neuronal units within the SCN. This description is based on a forced Kuramoto model. We then reduce the system dimensionality by using the Ott Antonsen Ansatz and obtain a low-dimensional macroscopic description. Using this reduced macroscopic system, we explain the east-west asymmetry of jet-lag recovery and discus the consequences of our findings. (c) Thirdly, we study neuron firing in integrate-and-fire neural networks. We build a discrete-state/discrete-time model with both excitatory and inhibitory neurons and find a phase transition between avalanching dynamics and ceaseless firing dynamics. Power-law firing avalanche size/duration distributions are observed at critical parameter values. Furthermore, in this critical regime we find the same power law exponents as those observed from experiments and previous, more restricted, simulation studies. We also employ a mean-field method and show that inhibitory neurons in this system promote robustness of the criticality (i.e., an enhanced range of system parameter where power-law avalanche statistics applies). (d) Lastly, we study the dynamics of “reservoir computing networks” (RCN’s), which is a recurrent neural network (RNN) scheme for machine learning. The ad- vantage of RCN’s over traditional RNN’s is that the training is done only on the output layer, usually via a simple least-square method. We show that RCN’s are very effective for inferring unmeasured state variables of dynamical systems whose system state is only partially measured. Using the examples of the Lorenz system and the Rossler system we demonstrate the potential of an RCN to perform as an universal model-free “observer”.Item A Continuum Model for Flocking: Obstacle Avoidance, Equilibrium, and Stability(2010) Mecholsky, Nicholas Alexander; Ott, Edward; Antonsen, Jr., Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The modeling and investigation of the dynamics and configurations of animal groups is a subject of growing attention. In this dissertation, we present a partial-differential-equation based continuum model of flocking and use it to investigate several properties of group dynamics and equilibrium. We analyze the reaction of a flock to an obstacle or an attacking predator. We show that the flock response is in the form of density disturbances that resemble Mach cones whose configuration is determined by the anisotropic propagation of waves through the flock. We investigate the effect of a flock `pressure' and pairwise repulsion on an equilibrium density distribution. We investigate both linear and nonlinear pressures, look at the convergence to a ‘cold’ (T → 0) equilibrium solution, and find regions of parameter space where different models produce the same equilibrium. Finally, we analyze the stability of an equilibrium density distribution to long-wavelength perturbations. Analytic results for the stability of a constant density solution as well as stability regimes for constant density solutions to the equilibrium equations are presented.Item Oscillations of Microscale Composite Structures with Applications to Microresonators(2006-05-04) Li, He; Balachandran, Balakumar; Mote, Clayton Daniel; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Free and forced oscillations of piezoelectric microelectromechanical resonators fabricated as clamped-clamped laminate structures are studied in this dissertation. Piezoelectric actuation is used to excite these structures on the input side, and piezoelectric sensing is carried out on the output side. A refined mechanics model is developed for composite beam structures and used for studying the nonlinear transverse vibrations of the microresonators. The model accounts for longitudinal extension due to transverse displacements, distributed actuation, and stepwise axially varying properties. Assuming a buckling induced non-flat equilibrium position, an approximation for the static equilibrium position is determined and the free vibration problem is solved. For weak damping and weak forcing, the method of multiple scales is used to obtain an approximate frequency-response solution. Following this work, a complete solution for the pre-buckling, critical-buckling, and post-buckling problem for axially elastic beams is developed from the nonlinear model. The analytical predictions are compared with experimental data and they are found to be in good agreement. The present work provides a means for determining the spatial and temporal response of microresonators and it can be used as a design tool for many microelectromechanical systems (MEMS). The present work provides the first evidence for buckling influenced dynamics in microresonators.