Mechanical Engineering Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2795

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Subject: search.filters.subject.Engineering, Automotive

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Now showing 1 - 5 of 5
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    Virtual Reality Modeling of a Car Suspension with Active Control Capability
    (2009) Smoker, Jason James; Baz, Amr M.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis presents the evolution of a full car model into virtual reality environment to visually demonstrate the dynamics of a car resulting from various inputs controlled both passively and actively. The model is a seven degree of freedom system that can be configured to be excited by either a bump or harmonic input. Active controls available to the system include the well known Linear Quadratic Regulator (LQR) as well as a new Nonlinear Energy Absorber (NEA) which utilizes both nonlinear springs and nonlinear damper. The mathematics of the plant, the kinematics of the system, and the visual specifications of the scene are integrated into a three-dimensional environment where the user can be immersed in the environment and witness in real-time the response of a specific configuration. This project was developed with the mindset that dynamic models of systems can be better understood through visual realization and interaction.
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    STUDY OF CONDENSATION OF REFRIGERANTS IN MICRO-CHANNELS FOR DEVELOPMENT OF FUTURE COMPACT MICRO-CHANNEL CONDENSERS
    (2008) Chowdhury, Sourav; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Mini- and micro- channel technology has gained considerable ground in the recent years in industry and is favored due to its several advantages stemming from its high surface to volume ratio and high values of proof pressure it can withstand. Micro-channel technology has paved the way to development of highly compact heat exchangers with low cost and mass penalties. In the present work, the issues related to the sizing of compact micro-channel condensers have been explored. The considered designs encompass both the conventional and MEMS fabrication techniques. In case of MEMS-fabricated micro-channel condenser, wet etching of the micro-channel structures, followed by bonding of two such wafers with silicon nitride layers at the interface was attempted. It was concluded that the silicon nitride bonding requires great care in terms of high degree of surface flatness and absence of roughness and also high degree of surface purity and thus cannot be recommended for mass fabrication. Following this investigation, a carefully prepared experimental setup and test micro-channel with hydraulic diameter 700 microns and aspect ratio 7:1 was fabricated and overall heat transfer and pressure drop aspects of two condensing refrigerants, R134a and R245fa were studied at a variety of test conditions. To the best of author's knowledge, so far no data has been reported in the literature on condensation in such high aspect ratio micro-channels. Most of the published experimental works on condensation of refrigerants are concerning conventional hydraulic diameter channels (> 3mm) and only recently some experimental data has been reported in the sub-millimeter scale channels for which the surface tension and viscosity effects play a dominant role and the effect of gravity is diminished. It is found that both experimental data and empirically-derived correlations tend to under-predict the present data by an average of 25%. The reason for this deviation could be because a high aspect ratio channel tends to collect the condensate in the corners of its cross-section leaving only a thin liquid film on the flat side surfaces for better heat transfer than in circular or low aspect ratio channels.
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    Transformation Plans for Optimizing Military Vehicle Testing
    (2007-05-15) Hoy, Timothy W; Herrmann, Jeffrey W; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The U.S. Army Aberdeen Test Center is a leading Department of Defense developmental test center and test range. A majority of the testing conducted at the Aberdeen Test Center is automotive in nature. Due to recent conflicts around the world, the U.S. Armed Services need to field new armored systems rapidly. The rapid deployment of automotive systems has caused the Department of Defense test community and the Aberdeen Test Center in particular to reevaluate and redefine traditional test plans and practices in order to maximize the amount of valid and pertinent data obtained from shortened test schedules. As a result, this thesis studies new transformation plans to provide ways to optimize military test plans. These transformation plans take into account existing military vehicle data from multiple sources including the Aberdeen Test Center's automotive road courses. These transformation plans are not only useful for shortened military tests, but can also be easily employed in developing test plans for private industry customers as well as long term test projects. The benefits in all cases are the same: an optimized test plan for automotive endurance operations.
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    VEHICLE HANDLING, STABILITY, AND BIFURCAITON ANALYSIS FOR NONLINEAR VEHICLE MODELS
    (2005-12-13) Nguyen, Vincent; Schultz, Gregory A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Vehicle handling, stability, and bifurcation of equilibrium conditions were studied using a state vector approach. The research provided a framework for an improved method of vehicle handling assessment that included non-linear regions of performance and transient behavior. Vehicle models under pure lateral slip, constant velocity, and constant front steer were developed. Four-wheel, two-axle vehicle models were evolved from simpler models and were extended to include vehicle roll dynamics and lateral load transfer effects. Nonlinearities stem from tire force characteristics that include for tire force saturation. Bifurcations were studied by quasi-static variations of vehicle speed and front steer angle. System models were expanded, assessing overall stability, including vehicle behavior outside normal operating ranges. Nonlinear models of understeering, oversteering, and neutral steering vehicles were created and analyzed. Domains of attraction for stable equilibrium were discussed along with physical interpretations of results from the system analysis.
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    WAVE PROPAGATION IN RODS, SHELLS, AND ROTATING SHAFTS WITH NON-UNIFORM GEOMETRY
    (2004-03-16) Toso, Mario; Baz, Amr; Mechanical Engineering
    The propagation of waves in rods, shells and rotating shafts with variable thickness is studied through numerical models and experimental measurements. All numerical models are formulated using the Transfer Matrix approach, which accurately reproduces the dynamic behavior and wave propagation characteristics of the considered structures at each frequency. The numerical predictions show that exponential and linear thickness profiles generate a cut-off frequency, below which waves do not propagate along the structure. Hence, the considered rods and shells are capable of filtering out low frequency and they behave as high-pass mechanical filters. The filtering capabilities of the considered class of rods and shells are investigated for different types of profiles. Furthermore, the effect introduced by using periodicity and changing the material properties of the structure in a functionally graded manner is investigated. The effect of linear profiles is practically evaluated by determining both the frequency and time response for excitations applied at one side of the structure. These results are compared to uniform profiles through the Wavelet Transform (WT), which visualizes the structure vibrational energy simultaneously in both the time and frequency domain. The agreement between the theoretical and experimental results validates the numerical models and demonstrates the effectiveness of the proposed design configurations in attenuating the propagation of waves especially in the low-frequency range. The filtering characteristics are also investigated for rotating shafts with tapered and stepped geometry. It is found out that rotation at a constant speed does not significantly modify the flexural wave propagation characteristics of the system. Also, the interest is extended to studying the Campbell diagrams of tapered and periodically stepped profiles. Experiments on the propagation of vibration from a gearbox through rotating shafts prove that tapered and periodic profiles can effectively redistribute the energy spectrum by confining the propagation to specific frequency bands. Such characteristics become more evident when the shaft is provided with active periodic piezoelectric inserts. The effectiveness of the constant axial loads and feedback control on the shaft performance is determined and compared to the alternative passive periodic treatments.