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DSpace at MIT (104.280 recursos)

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Mechanical Engineering - Ph.D. / Sc.D.

Mostrando recursos 1 - 20 de 1.582

  1. The effect of cooling on boundary layer transition in a gas

    Kline, S. J. (Stephen Jay), 1922-
    by Stephen Jay Kline.

  2. Low-cost, high performance solar vapor generation

    Ni, George (George Wei)
    Sustainable access to energy and access to water are two of the defining technological problems that society currently faces. Threats of climate change and depletion of fossil fuel reserves are forcing a shift towards more renewable sources of energy, such as solar energy and others. At the same time, water resources are becoming scarcer, caused by unsustainable extraction of ground water resources. Current projections show that by 2025, the population of people living in water-stressed areas is expected to increase to 3.9 billion. Exacerbating this problem is continuing urbanization, which stresses local water supplies further. The two problems of energy...

  3. Development and validation of a novel framework for designing and optimizing passive prosthetic feet using lower leg trajectory

    Olesnavage, Kathryn M
    This thesis presents a novel framework to optimize the design of passive prosthetic feet to best replicate physiological lower leg trajectory under typical ground reaction forces. The goal of developing this framework is ultimately to design a low cost, mass manufacturable prosthetic foot for persons with amputations living in the developing world. Despite a vast body of literature on prosthetic foot design, there is a dearth of knowledge regarding how the mechanical characteristics of passive prosthetic feet affect their biomechanical performance. Without understanding this relationship, the design of a prosthetic foot cannot be optimized for peak performance as measured by...

  4. Probabilistic regional ocean predictions : stochastic fields and optimal planning

    Narayanan Subramani, Deepak
    The coastal ocean is a prime example of multiscale nonlinear fluid dynamics. Ocean fields in such regions are complex, with multiple spatial and temporal scales and nonstationary heterogeneous statistics. Due to the limited measurements, there are multiple sources of uncertainties, including the initial conditions, boundary conditions, forcing, parameters, and even the model parameterizations and equations themselves. To reduce uncertainties and allow long-duration measurements, the energy consumption of ocean observing platforms need to be optimized. Predicting the distributions of reachable regions, time-optimal paths, and risk-optimal paths in uncertain, strong and dynamic flows is also essential for their optimal and safe operations....

  5. Development of adsorption-based atmospheric water harvesting and thermal energy storage technologies

    Kim, Hyunho, Ph. D. Massachusetts Institute of Technology
    Sorption-based energy technologies can be used to efficiently harvest fresh water from the atmosphere and to store thermal energy for climate control. Recent advances in porous adsorbents such as metal-organic frameworks (MOFs), with high sorption capacity and low regeneration temperature requirements, allow us to efficiently utilize them for these applications. However, detailed experimental and theoretical frameworks for the use of advanced sorbents have not considered several important aspects (e.g., inter/intracrystalline diffusion kinetics). This thesis provides a generalized framework to select the materials, design, and develop sorption-based energy and water systems. First, this thesis presents the design and demonstration of a...

  6. Variational approach to solving the phonon Boltzmann transport equation for analyzing nanoscale thermal transport experiments

    Chiloyan, Vazrik
    Over time, technology has shrunk to smaller length scales, and as a result the heat transport in these systems has entered the nanoscale regime. With increasing computational speed and power consumption, there is a need to efficiently dissipate the heat generated for proper thermal management of computer chips. The ability to understand the physics of thermal transport in this regime is critical in order to model, engineer, and improve the performance of materials and devices. In the nanoscale regime, thermal transport is no longer diffusive, and the Fourier heat conduction equation, which we commonly utilize at the macroscale, fails to...

  7. Robust stability assessment for future power systems

    Nguyen, Hung Dinh
    Loss of stability in electrical power systems may eventually lead to blackouts which, despite being rare, are extremely costly. However, ensuring system stability is a non-trivial task for several reasons. First, power grids, by nature, are complex nonlinear dynamical systems, so assessing and maintaining system stability is challenging mainly due to the co-existence of multiple equilibria and the lack of global stability. Second, the systems are subject to various sources of uncertainties. For example, the renewable energy injections may vary depending on the weather conditions. Unfortunately, existing security assessment may not be sufficient to verify system stability in the presence...

  8. Dynamic soaring beyond biomimetics: control of an albatross-inspired wind-powered system

    Bousquet, Gabriel D
    Albatrosses extract their propulsive energy from horizontal winds in a maneuver called dynamic soaring, and travel impressive distance (5000 km/week) by "riding the winds". Accordingly, for albatrosses flight is barely more strenuous than rest. While thermal soaring, exploited by birds of prey and sports gliders, consists of simply remaining in updrafts, extracting energy from horizontal winds necessitates redistributing momentum across the wind shear layer, by means of an intricate and dynamic flight manoeuver. Historically, dynamic soaring has been described as a sequence of half-turns connecting upwind climbs and downwind dives through the surface shear layer. Relaxing the half-turn hypothesis, this...

  9. Dynamics, stability and scaling of turbulent methane oxy-combustion

    Chakroun, Nadim Walid
    Carbon capture and storage (CCS) is an important strategy for reducing CO₂ emissions, with oxy-fuel combustion being one of the most promising technologies because of it is high efficiency and low cost. In oxy-combustion, CH₄/O₂/CO₂ mixtures burn at low temperatures (~~1700 K), high pressures (~~40 bar), where laminar burning velocities are about 7 times lower than in traditional CH₄ /Air mixtures. Thus oxy-fuel combustors are more prone to blowoff and dynamic instabilities. In this thesis we examine turbulent oxy-combustion flame stabilization physics at the large and small scales using experimental studies and numerical simulations. Experimental measurements are used to establish...

  10. Precision control of continuous microcontact printing

    Bageant, Maia R. (Maia Reynolds)
    This work focuses on the development of experimental equipment enabling the scale-up of microcontact printing for industrial use. An examination of existing experimental microcontact printing hardware and identification of its deficiencies are given, and the design and implementation of improvements are detailed. In particular, these improvements focus on the enabling of closed-loop force control of the printing process by the establishment of a deterministic computational platform and additional sensing. An understanding and rationale for the overall control design of the microcontact printing process is developed. Though the goal is to control the compression of each individual microscale feature on the...

  11. Aerodynamic drag on deformable and active structures in high Reynolds number conditions

    Guttag, Mark A. (Mark Andrew)
    We explore the effect of topography on the aerodynamic behavior of deformable structures, at high Reynolds numbers. We first introduce a novel mechanism to control the aerodynamic drag on cylinders, in the critical Reynolds number regime. We created both axially grooved and dimpled specimens, comprised of latex membranes stretched over rigid acrylic skeletons. By decreasing the internal pressure of the specimens, the latex stretched inward thus changing the shape of the surface. Using a combination of finite element simulations and precision mechanical experiments, we characterized the relationship between the mechanical deformation in the membrane and pneumatic loading. Wind tunnel experiments...

  12. A phase field model for the gallium permeation of aluminum grain boundaries

    Aggarwal, Raghav
    Interfaces, such as grain boundaries, solid-liquid interfaces and solid-solid heterophase interfaces, are important features found in materials. Material properties such as fracture toughness, corrosion susceptibility and high temperature creep are influenced by grain boundary physics. The structure of grain boundaries affects their properties. In this thesis, we have developed a predictive model for a particular grain boundary structure-property relationship: the permeation of liquid gallium through aluminum grain boundaries. Liquid gallium is known to permeate through aluminum grain boundaries. The reduction in interface energy by the replacement of one Al-Al grain boundary interface with two Al-Ga interfaces drives the permeation. The...

  13. A leg exoskeleton simulator for design, sensing and control development

    Kuan, Jiun-Yih
    Leg exoskeletons have been developed in an effort to augment human locomotion for over a century. However, only two portable leg exoskeletal devices have shown a significant decrease in walking metabolism [35, 11], not to mention, no device that has shown effective assistance and biomimetic behavior across different walking speeds and terrains. This thesis aims to build a Leg Exoskeleton Simulator to effectively search the space of potential prosthetic and orthotic design, control, and sensing strategies so as to find the best means to improve human locomotion through wearable electromechanical technology and modern bionics, enabling rapid advancement of the human-machine...

  14. Sensing and control for fishlike propulsion in unsteady environments

    Gao, Amy (Amy Ruiming)
    Fish are equipped with a unique and elaborate flow sensing system, the lateral line, that enables them to reconstruct the near-field three-dimensional flow around their bodies, and hence effect precise control for optimal propulsion and to achieve energy recovery from vortical flows. This is a capability that is not available to engineered underwater systems today. A paradigm example lies in the ability of fish to save energy when swimming in schools, through extracting energy from vortices generated by other fish. For a single fish modeled as an undulating, foil-shaped body at Reynolds number Re=5000, swimming directly behind another fish results...

  15. Effects of engine operating conditions on catalyst light-off and combustion variability

    Baron, Jan H
    Greenhouse gas targets for passenger car internal combustion engines as well as increasingly stringent emissions legislation around the globe require innovative development approaches for future engine generations. At the same time, trade-offs can limit important design parameters necessitating fundamental understanding of the parameters involved. Toxic tail pipe emissions of gasoline engines are often associated with the efficiency of three-way catalytic converters and the time they take to reach light-off temperature, since a well-functioning converter reduces the tail pipe emissions by up to 99 %. Previous research suggests that exhaust gas back pressure could potentially improve converter light-off, though a thorough...

  16. Towards first-principles prediction of electrocatalytic activity of ABO₃ perovskites

    Rong, Xi, Ph. D. Massachusetts Institute of Technology
    The design of efficient, stable, and inexpensive catalysts such as ABO 3 perovskites for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is crucial for the development of electrochemical energy conversion devices such as water electrolysis, fuel cells, and metal-air batteries. In order to enable high-throughput computation and screening for optimal catalysts, a deeper understanding of composition-structure-activity relationships is required. Currently, this endeavor is hindered by the complexity of the oxide surface structure and challenges in atomic-scale experimental characterization under in operando conditions. In this thesis, we address these issues by employing density functional theory and classical...

  17. Characterization and science of an aluminum fuel treatment process

    Slocum, Jonathan Thurston
    Presented is an inexpensive and highly effective method of activating bulk aluminum allowing it to react with water producing hydrogen gas and steam. The extreme energy density of the aluminum-water reaction, twice that of diesel fuel and forty-five times that of lithium-ion batteries, makes this new fuel a promising safe alternative to high-pressure hydrogen storage. Aluminum is the most abundant metal in the earth's crust and has long been recognized as a potential fuel source. The challenge however is disrupting the protective oxide layer in a cost effective and safe manner. In this work, I show that by exposing aluminum...

  18. Low-frequency, low-amplitude MEMS vibration energy harvesting

    Xu, Ruize, Ph. D. Massachusetts Institute of Technology
    Vibration energy harvesters work effectively only when the operating conditions match with the available vibration source. Typical resonating MEMS structures cannot be used with low-frequency, low-amplitude and unpredictable nature of ambient vibrations. Bi-stable nonlinear oscillator based energy harvesters are developed for lowering the operating frequency while widening the bandwidth, and are realized at MEMS scale for the first time. This design concept does not rely on the resonance of the MEMS structure but operates with the large snapping motion of the beam at very low frequencies when proper conditions are provided to overcome the energy barrier between the two energy...

  19. Investigating group behavioral quantization of oceanic fish with Continental-shelf scale ocean-acoustic sensing

    Yi, Dong Hoon
    The recent fish population decline due to increased human presence has led to calls for predictive methods to help reverse or stabilize the situation. It has been difficult, however, to establish such methods primarily due to the technical obstacles in observing fish populations in natural habitats. Here we use acoustics to observe the ocean environment and study fish behavior during the critical spawning period over continental-shelf scales. Fish are known to be one of the main sources of strong natural returns in the continental-shelf environment, and so identified as a major source of clutter for wide-area undersea surveillance. The first...

  20. Disrupting dynamic f-actin enhances skeletal muscle contraction due to mechanical softening

    Sivathanu, Vivek
    Skeletal muscle wasting disorders such as sarcopenia affect the daily mobility of millions of aging people globally due to decreased muscle mass and decreased muscle efficiency. In this study we discover a novel target to improve the efficiency of skeletal muscle by targeting the dynamic f-actin cytoskeleton. Using two model systems, an ex-vivo mouse muscle model and a novel in vitro optogenetic skeletal muscle micro-tissue model, we show that disruption of the dynamic f-actin cytoskeleton using small molecule actin dynamics inhibitors leads to a persistent 2-fold improvement in muscle active contractility. We explored possible drawbacks of f-actin disruption, including loss...

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