A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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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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    Silicon-based terahertz waveguides
    (2015) Li, Shanshan; Murphy, Thomas E.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this thesis, we present the design, fabrication and application of two types of silicon-based terahertz waveguides. The first is anisotropically etched highly doped silicon surface for supporting terahertz plasmonic guided wave. We demonstrate propagation of terahertz waves confined to a semiconductor surface that is periodically corrugated with subwavelength structures. We observe that the grating structure creates resonant modes that are confined near the surface. The degree of confinement and frequency of the resonant mode is found to be related to the pitch and depth of structures. The second is silicon dielectric ridge waveguide used to confine terahertz pulses and study silicon's terahertz intensity-dependent absorption. We observe that the absorption saturates under strong terahertz fields. By comparing the response between lightly-doped and intrinsic silicon waveguides, we confirm the role of hot carriers in this saturable absorption. We introduce a nonlinear dynamical model of Drude conductivity that, when incorporated into a wave propagation equation, predicts a comparable field-induced transparency and elucidates the physical mechanisms underlying this nonlinear effect. The results are numerically confirmed by Monte Carlo simulations of the Boltzmann transport equation, coupled with split-step nonlinear wave propagation.
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    Nonlinear Fluid-Structure Interactions in Flapping Wing Systems
    (2013) Fitzgerald, Timothy; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This work relates to fluid-structure interactions in the context of flapping wing systems. System models of flapping flight are explored by using a coupling scheme to provide communication between a fluid model and a structural model describing a flexible wing. The constructed computational models serve as a tool for investigating complex fluid-structure interactions and characterizing them. Primary goals of this work are construction of models to understand nonlinear phenomena associated with the flexible flapping wing systems, and explore means and methods to enhance their performance characteristics. Several system analysis tools are employed to characterize the coupled fluid-structure system dynamics, including proper orthogonal decomposition, dimension calculations, time histories, and frequency spectra. Results obtained from two-dimensional simulations conducted for a combination of a two-link structural system and a fluid system are presented and discussed. Comparisons are made between the use of direct numerical simulation and the unsteady vortex lattice method as the fluid model in this coupled dynamical system. To enable three-dimensional studies, a novel solid model is formulated from continuum mechanics for geometrically exact finite elements. A new partitioned fluid-structure interaction algorithm based on the Generalized-α method is formulated and implemented in a large scale fluids solver inside the FLASH framework. Consistent boundary conditions are also formulated by using Lagrangian particles. Several examples demonstrating the effectiveness of the methods and implementation are shown, in particular, for flapping flight at low Reynolds numbers. Unique experiments have also been undertaken to determine the first few natural frequencies and mode shapes associated with hawkmoth wings. The computational framework developed in this dissertation and the research findings can be used as a basis to understand the role of flexibility in flapping wing systems, further explore the complex dynamics of flapping wing systems, and also develop design schemes that might make use of nonlinear phenomena for performance enhancement.
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    Characterization of Linear Electro-Optic Effect of Poled Organic Thin Films
    (2008-02-29) Park, Dong Hun; Lee, Chi H; Herman, Warren N; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The goal of this thesis is to re-evaluate both Teng-Man and attenuated total reflection (ATR) methods for measuring the linear electro-optic (EO) coefficients of poled organic thin films based on a multilayer structure containing a transparent conducting oxide (TCO) layer. The linear EO properties are often characterized using the Teng-Man reflection method. However, it has been reported that experimental error can result from ignoring multiple reflections and that an accurate determination of the EO effect could be achieved only by a numerical calculation that applies anisotropic Fresnel equations to the multilayer structure. We present new closed-form expressions for analysis of Teng-Man measurements of the EO coefficients of poled polymer thin films. These expressions account for multiple reflection effects using a rigorous analysis of the multilayered structure for varying angles of incidence. The analysis based on plane waves is applicable to both transparent and absorptive films and takes into account the properties of the TCO electrode layer and buffer layers. Methods for fitting data are presented and the error introduced by ignoring the TCO layer and multiple reflections is discussed. We also discuss the effect of Gaussian beam optics and the suitability of a thick z-cut LiNbO3 crystal as a reference to validate the Teng-Man measurement. Simply taking the metal electrode off the Teng-Man sample makes it feasible to use the ATR method using a metal-coated prism. This technique has the capability of measuring anisotropic indices of refraction along with film thicknesses. In addition, it enables measurement of r13 and r33 separately without an assumption for the ratio of r13 to r33 as required in the Teng-Man method. We have found that the ATR analysis based on a three-layer waveguide structure (air/film/substrate) can produce a large error especially when the film supports a single guided mode and the ATR analysis based on a multilayer structure containing a TCO layer gives you a more reliable estimation. We discuss the error introduced by using the three-layer waveguide structure and compare to using the multilayer structure. Finally, we discuss the characterization of the optical property of TCO's using ellipsometeric analysis, which is required for both the rigorous Teng-Man and ATR analysis. Representative experimental results showing that the result from the ATR method based on the multilayer structure shows a good agreement with that from the rigorous Teng-Man analysis are presented. We have measured a very high linear electro-optic coefficient (r33=350 pm/V) from a NLO film (AJ-TTE-II, synthesized by Alex Jen's group at University of Washington) at 1310 nm wavelength, which is ~12 times higher than the best inorganic electro-optic crystal LiNbO3.
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    LEAD ZIRCONATE TITANATE THIN FILMS FOR PIEZOELECTRIC ACTUATION AND SENSING OF MEMS RESONATORS
    (2005-12-07) Piekarski, Brett; DeVoe, Donald; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This research is focused on examining the potential benefits and limitations of applying sol-gel lead zirconate titanate (PZT) piezoelectric thin films to on-chip piezoelectrically driven RF microelectromechanical system (MEMS) resonators in the low frequency (LF) to very high frequency (VHF) frequency range. MEMS fabrication methods are presented for fabricating PZT-based MEMS resonator structures along with investigations into the resultant thin film residual stresses and material properties, and their impact on resonator frequency, beam curvature, and resonant mode shape. The PZT, silicon dioxide (SiO2), platinum (Pt), and silicon nitride (Si3N4) thin film material properties are characterized and validated by wafer bow, cantilever resonance, cantilever thermal-induced tip deflection and finite element modeling (FEM) techniques. The performance of the fabricated PZT-based MEMS resonators are presented and compared to previously demonstrated zinc oxide (ZnO) based resonators as well as to electrostatically based MEMS resonator designs. Resonators with frequency response peaks of greater than 25 dB, quality factors up to 4700, and resonant frequencies up to 10 MHz are demonstrated along with a discussion of their advantages and disadvantages for use as MEMS resonators. Nonlinear resonator response is also investigated in relation to the onset of classic Duffing behavior, beam buckling and mode coupling. Fabrication techniques, operating conditions, and design rules are presented to minimize or eliminate nonlinear resonator response.