Theses and Dissertations from UMD

Permanent URI for this communityhttp://hdl.handle.net/1903/2

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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2016 - 20192

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    NONLINEARITY BASED ACOUSTIC WAVE REDIRECTION
    (2019) Telly, Saliou Binet; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Advances in metamaterials technology have revealed novel opportunities for achieving peculiar material properties amenable to the control of wave propagation paths for various applications not realizable with conventional materials. Some prominent examples are schemes for electromagnetic and acoustic cloaking and focusing devices. In the classical approach to the formulations of these devices, one exploits a change of physical coordinates to achieve a desired wave behavior within a finite space. Such a change can be interpreted as a transformation of material properties when the field equations of interest are invariant to coordinate trans- formations. To date, this intuitive approach has led to the formulation of various two-dimensional devices for wave redirection, including infinite circular and square cloaks for both electromagnetic and acoustic fields. For acoustic fields, however, the transformation approach is constrained to fluid-like metamaterials amenable to the propagation of longitudinal waves only. Complications arise with solid materials because of their inherent ability to sustain the propagation of both longitudinal and transverse waves, which refract differently in linear materials due to dissimilar propagation speeds. In this dissertation, the author first seeks a sequence of transformations that may be used for cloaking two-dimensional airfoil sections through the classical transformation method. For the sought sequence, the author takes advantage of the mapping properties of the Joukowsky Transformation, and the closely related Kaman-Trefftz Transformation, which are two well-known complex mappings that have classically been used in the study of the aerodynamics of airfoil sections. Next, the author explores wave redirection mechanisms that may take advantage of nonlinear wave phenomena in elastic solid materials for acoustic wave redirection. Starting from the classical nonlinear Murnaghan model, a hyper-elastic material is formulated to realize couplings between shear and compressional modes that could lead to a more suitable refractive behavior for acoustic wave redirection in a solid metamaterial. The formulated model is studied by using perturbation and approximation techniques. The findings of this dissertation can be useful for redirecting and manipulating acoustic waves for practical applications.
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    Interaction of Acoustic Waves with a Laminar Line-Flame
    (2016) Friedman, Adam Neal; Stoliarov, Stanislav I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A systematic study was conducted to elucidate the effects of acoustic perturbations on laminar diffusion line-flames and the conditions required to cause acoustically-driven extinction. Flames were produced from the fuels n-pentane, n-hexane, n-heptane, n-octane, and JP-8, using fuel-laden wicks. The wicks were housed inside of a burner whose geometry produced flames that approximated a two dimensional flame sheet. The acoustics utilized ranged in frequency between 30-50 Hz and acoustic pressures between 5-50 Pa. The unperturbed mass loss rate and flame height of the alkanes were studied, and they were found to scale in a linear manner consistent with Burke-Schumann. The mass loss rate of hexane-fueled flames experiencing acoustic perturbations was then studied. It was found that the strongest influence on the mass loss rate was the magnitude of oscillatory air movement experienced by the flame. Finally, acoustic perturbations were imposed on flames using all fuels to determine acoustic extinction criterion. Using the data collected, a model was developed which characterized the acoustic conditions required to cause flame extinction. The model was based on the ratio of an acoustic Nusselt Number to the Spalding B Number of the fuel, and it was found that at the minimum speaker power required to cause extinction this ratio was a constant. Furthermore, it was found that at conditions where the ratio was below this constant, a flame could still exist; at conditions where the ratio was greater than or equal to this constant, flame extinction always occurred.