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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Item Experimental Investigations of Capillary Effects on Nonlinear Free-Surface Waves(2010) Diorio, James Daniel; Duncan, James H; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents the results of three experiments on various aspects of the effects of surface tension on nonlinear free-surface waves. The first two experiments focus on capillary effects on the breaking of short-wavelength gravity waves, a problem of interest in areas of physical oceanography and remote sensing. The third experiment is concerned with the bifurcation of solitary capillary-gravity waves, a problem that is relevant in the study of nonlinear, dispersive wave systems. In the first set of experiments, streamwise profile measurements were made of spilling breakers at the point of incipient breaking. Both wind-waves and mechanically generated waves were investigated in this study, with gravity wavelengths in the range of 10--120 cm. Although it has been previously argued that the crest shape is dependent only on the surface tension, the results reported herein are to the contrary as several geometrical parameters used to describe the crest change significantly with the wavelength. However, the non-dimensional crest shape is self-similar, with two-shape parameters that depend on a measure of the local wave slope. This self-similarity persists over the entire range of wavelengths and breaker conditions measured, indicating a universal behavior in the near-crest dynamics that is independent of the method used to generate the wave. The measured wave slope is found to be related to the wave growth rate and phase-speed prior to breaking, a result that contributes towards the development of a breaking criterion for unsteady capillary-gravity waves. The second set of experiments examines the cross-stream surface structure in the turbulent breaking zone generated by short-wavelength breakers. Waves in this study were generated using a mechanical wedge and ranged in wavelength from 80--120 cm. To isolate the effects of surface tension on the flow, the important experimental parameters were adjusted to produce Froude-scaled, dispersively-focused wave packets. The results show the development of ``quasi''-2D streamwise ripples along with smaller cross-stream ripples that grow as breaking develops and can become comparable in amplitude to the streamwise ripples for larger breakers. It is found that the amplitude of the cross-stream surface ripples scale as $\bar{&lambda}^3$, where $\bar{&lambda}$ is the average wavelength of the wave packet. The cross-stream ripple activity appears to be highest in the ``troughs'' of the larger streamwise ripples, with the appearance of persistent ``scar''-like features. Based on these observations, a simple model for the coupling between the vorticity and capillary structure in the breaking zone is conjectured. The third set of experiments focuses on the generation of capillary-gravity waves by a pressure source moving near the minimum phase speed cmin. Near this minimum, nonlinear capillary-gravity solitary waves, or ``lumps'', have been shown to exist theoretically. We identify an abrupt transition to a wave-like state that features a localized solitary wave that trails the pressure forcing. This trailing wave is steady, fully localized in 3D, elongated in the cross-stream relative to the streamwise direction, and has a one-to-one relationship between height and phase speed. All of these characterisitics are commensurate with the freely propogating ``lumps'' computed by previous authors, and a quantitative comparison between these previous numerical calculations and the current experiments is presented. At speeds closer to cmin, a new time-dependent state is observed that can qualitatively be described by the shedding of solitary depressions from the tips of a ``V''-shaped pattern. These results are discussed in conjunction with a new theoretical model for these waves that employs nonlinear and viscous effects, both of which are crucial in capturing the salient features of the surface response. While discussed in the context of water waves, these results have applicaiton to other wave systems where nonlinear and dispersive effects are important.Item Physics of Breaking Bow Waves: A Parametric Investigation using a 2D+T Wave Maker(2009) Maxeiner, Eric; Duncan, James H; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A mechanical 2-dimensional wave maker with a flexible surface was used to create waves similar to those formed at the bow of a moving ship. Utilizing the 2D+T approximation, the wave maker was programmed so that its deformable wave board creates a time sequence of shapes that simulate the line of intersection between one side of the hull of a slender ship model moving at constant speed and an imaginary vertical plane oriented normal to the ship model track. Instead of trying to simulate a particular ship hull, however, the wave maker simulates a parametric set of flat plate motions that contain components of typical bow shapes. The resulting surface waves were measured using a cinematic laser-induced fluorescence technique and the resulting wave profiles were analyzed. A tremendous variation of wave shapes was observed. A variety of wave characteristics including the peak contact point height, peak wave height, wave crest speed and plunging jet thickness distribution were measured and related to the corresponding wave maker motion parameters. Despite the complexity of the wave maker motions, it was observed that wave maker velocity and acceleration along the water line were the wave maker parameters with the strongest influence on many of the measured wave characteristics. Additional analysis reveals that the initial acceleration of the wave maker affects some wave characteristics, especially those related to plunging jet behavior, but does not significantly affect the overall size and shape of the wave. It was also observed that the behavior of wave formation and breaking ranged between two distinct modes. The first mode consists of an overdriven wave that contains a pronounced vertical jet along the face of the wave maker. The overdriven wave breaks close to the wave maker, before a wave crest has fully formed. The second mode is a more slowly developing wave that breaks further away from the wave maker. The developing waves do not contain the pronounced vertical jet observed in overdriven waves. The two modes appear to be related to the initial wave maker acceleration and amount of water displaced by the wave maker.Item Modeling of a High Energy Density Propulsion System Based on the Combustion of Aluminum and Steam(2007-12-13) Eagle, W. Ethan; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents a thermodynamic analysis of a novel Rankine cycle aluminum/steam combustion power system being developed for use in Unmanned Underwater Vehicles (UUVs). The analysis is performed using a system modeling tool developed by the NASA Glenn Research Center called Numerical Propulsion System Solver (NPSS). Thermodynamic models of the individual components are created and linked together in NPSS, which then solves the system by enforcing mass and energy conservation. Design and off-design conditions are simulated and predicted performance is compared with predictions made by two other research groups. The simulations predict that this power system could provide at least five-fold increases in range and endurance for the US Navy's 'Sea Horse' UUV. A rudimentary sensitivity analysis is used to identify the factors which most strongly influence the performance of the design. Lastly, recommendations for future work and possible model improvements are discussed.