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‣ Phenomenology and kinematics of discrete plastic deformation events in amorphous silicon : atomistic simulation using the Stillinger-Weber potential

Demkowicz, Michael J. (Michael John), 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 64 p.; 3361803 bytes; 3361609 bytes; application/pdf; application/pdf
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The need to understand plastic deformation in amorphous covalently bonded materials arose from the unique mechanical properties of disordered intergranular layers in nc-TiN/a-Si₃N₄ ceramic composites. Silicon was chosen as a model disordered network solid for the purpose of conducting feasible atomistic computer simulations of plastic deformation. Amorphous silicon structures were created by melting and quenching using a molecular dynamics algorithm. These structured were plastically deformed by conjugate gradient static energy minimization. Atomic level analysis was carried out using appropriately generalized notions of stress and strain. Plastic deformation was found to occur in a series of discrete stress relaxations, each one of which was accompanied by a well localized atomic level rearrangement. The transforming regions were roughly ellipsoidal in shape and involved the cooperative motion 100-500 atoms spanning a length scale of 0.7-2.5nm. This length scale is large in comparison to the typical thickness of disordered intergranular layers in nanocrystalline ceramic composites, indicating that the plastic relaxation process in such intergranular layers cannot be the same as the one found in bulk amorphous covalent solids.; by Michael J. Demkowicz.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ Modeling and fabrication of self-assembling micron-scale rollup structures

Cybulski, James Stanley, 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 146 p.; 6820872 bytes; 6820679 bytes; application/pdf; application/pdf
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Self-assembling micron-scale structures based on standard photolithographic and thin film deposition techniques are investigated. Differences in residual stress between successive thin film layers causes the structures to roll up when an underlying sacrificial layer is removed. The primary structure of interest was a swiss-roll structure comprised of alternating layers of metal and insulator so that the self-assembled system forms an RCL electrical circuit with a well-defined resonant frequency. So-called nanoscroll structures, comprised of two swiss-rolls attached at a common end, were the most commonly observed fabrication result and are expected to have electrical properties very similar to swiss rolls. It has been predicted that such electrical properties lead to, a negative effective magnetic permeability for a narrow frequency band, potentially in the far infrared region. These structures thus can contribute a vital component ([mu]eff < 0) necessary for developing a LHM (left-handed material). Many successful materials combinations have been demonstrated. The layers in the recommended system are (from the bottom up) silicon dioxide, chrome, gold, and chrome. The smallest rollup diameter was achieved for a system of nickel on silicon dioxide and was measured to be 2 [mu]m. Processing conditions such as film thickness and releasing etchant were also optimized. The mechanical behavior of the films was modeled using standard beam theory modified for application to thin films and predicted stresses that are comparable to those found in literature. Other applications for these rollup structures were considered...

‣ Atomistic characterization of stress-driven configurational instability and its activation mechanisms

Zhu, Ting, 1971-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 156 p.; 8615198 bytes; 8615006 bytes; application/pdf; application/pdf
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Cleavage decohesion and shear dislocation nucleation are two basic modes of localized deformation in crystal lattices, which normally result from instability of the atomic configuration driven by mechanical forces. The critical state of instability and its thermal activation mechanisms can be quantitatively determined by analyzing the energetics of the lattice system. In this thesis, the unit processes of configurational instability of crystal lattices under various non-uniform structural and/or chemical environments are characterized by systematically probing the atomistic potential energy landscape of each system using the state of the art configurational space sampling schemes. The problems studied are homogeneous dislocation nucleation in a perfect crystal by nanoindentation, dislocation emission and cleavage decohesion at atomically sharp crack tips, and chemically-enhanced bond breaking in a wet silica nanorod. These processes are studied in a unified manner such that two important types of properties are determined: one is the athermal load at which the instability takes place instantaneously without the aid of thermal fluctuations, and the other is the stress-dependent activation energy used for an estimate of the kinetic rate of transition. Along the way...

‣ Fluid and structural modeling of the disease-free and atherosclerotic human carotid bifurcation

Younis, Hesham F. (Hesham Farouk)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 165 leaves; 16277364 bytes; 16277118 bytes; application/pdf; application/pdf
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Cardiovascular disease is the leading cause of death in the United States. It claims more lives each year than the next 7 leading causes of death combined. Atherosclerosis is a major cause of cardiovascular disease and is an inflammatory process characterized by intimal thickening of arteries and plaque build up. Atherosclerosis can also damage the arterial wall and in turn cause the formation of a thrombus that can partially or totally occlude the diseased artery. If the actual vessel is the carotid artery, this often leads to a stroke. The focus of this study is to analyze the blood flow and arterial wall motion in the carotid bifurcation, a frequently diseased site of the cerebral circulation. Using ultrasound velocity measurements and magnetic resonance imaging (MRI) data of normal volunteers and patients in vivo, realistic blood velocity profiles and wall anatomy geometries are generated and imported into a commercial finite element package, ADINA (Automatic Dynamic Incremental Nonlinear Analysis, Watertown, MA) for computational analysis. Using three types of 3D transient finite element analyses: structure only, rigid walled fluid only and fully coupled fluid-structure interaction, areas of low fluid shear stress and high mechanical strain are identified. These results are compared with experimental evidence collected from the literature as well as histological data of plaque gathered from imaged patients after endarterectomy to identify potential correlations with regions of inflammation.; (cont.) In normal volunteers...

‣ Cost modeling and design for manufacturing guidelines for advanced composite fabrication

Haffner, Sascha M. (Sascha Marcel), 1968-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 2 v. (493 p.); 66703561 bytes; 66703320 bytes; application/pdf; application/pdf
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Experience shows that the majority of costs are committed during the early stages of the development process. Presently, many cost estimation methods are available to the public for metal processing, but there are almost none (excluding proprietary) for advanced composite materials. Therefore, the central objective of this thesis is to provide a comprehensive overview of the costs of common composite production technologies such as Hand Layup, Resin Transfer Molding, Automated Tow Placement, Pultrusion, Forming, and Assembly. The work includes information on investment costs for production equipment and tooling as well as estimation guidelines for labor and material. Designers are presented with Design for Manufacturing guidelines (DFM) explaining how process selection and part design can lead to potential cost saving opportunities. Process based or technical cost models are well suited to quantify manufacturing costs and relate them to part design features, such as size and shape complexity. These physically based scaling principles can be easily adapted to changes in process technology and thereby reducing data requirements. In order to identify all relevant cost drivers, a detailed process plan is compiled for each composite manufacturing method. These processes can include up to 50 process steps and a total of 270 cost equations are used to calculate the cost contribution of each.; (cont.) A number of case studies conducted in concert with our industrial sponsors clearly identifies the best point of each production process and for example help to explain the economic benefits of co-curing versus mechanical assembly. Users can therefore study the economic consequences of design changes in detail and consequently highlight any favorable design/process combinations. To further facilitate the comparison of process performance and to promote the feedback from industry all of the models are available on the Internet at http://web.mit.edu/lmp/www/composites/costmodel/.; by Sascha M. Haffner.; Thesis (Ph. D.)--Massachusetts Institute of Technology...

‣ Pick-and-place nanoassembly

Hubert, Brian N., 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 149 leaves; 35868206 bytes; 35867964 bytes; application/pdf; application/pdf
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A new all-additive method for direct fabrication of nanometer-scale planar and multilayer structures using the probe tip of an atomic force microscope (AFM) and a material reservoir is proposed. The process, which is called Pick-and-Place NanoAssembly, enables true "pick-and-place" retrieval and deposition of materials with a wide range of electrical, chemical, and mechanical properties. The silicon tip of an AFM is used to discretely pick up molecules from a reservoir, transfer them to a construction zone, and then weld them to a surface. Unlike the prior art, this assembly method offers high-resolution direct patterning of a variety of materials, many of which are not amenable to patterning using current probe-based or conventional lithography methods. Metal nanoparticles, polymers, inks, solvents, and organics have been deposited onto a variety of substrates with resolutions approaching 1 million dots per inch (1 trillion dots per square inch). Lines of nanoparticles have been deposited with line widths of less than 17 nm. These materials can be assembled using reservoirs of viscous liquids, non-viscous liquids, and soft solids. Deposited volumes span a range of 10 orders of magnitude from 10-24 to 10-14 liters. Structures with dimensions of 60 to 100 nm are common.; he patterning process is capable of creating structures with height-to-width aspect ratios of better than 1-to-2...

‣ An electrothermally-actuated bistable MEMS relay for power applications; An electrothermally-actuated bistable microelectromechanical system relay for power applications

Qiu, Jin, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 92 p.; 14958112 bytes; 14957870 bytes; application/pdf; application/pdf
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This thesis first develops a bistable mechanism that does not rely on internal stress or hinges for its bistability, which is then combined with transient electrothermal actuation and contact structure to develop a MEMS relay for power switching. The relay components, fabricated by a through etch of a silicon wafer using deep reactive ion etch (DRIE), move laterally in the plane of the wafer. The synthesis, analysis, design, fabrication and testing of the relay are all described in this thesis. The bistable mechanism comprises two parallel cosine curved clamped-clamped beams that are also clamped together at their centers. If designed properly it exhibits a second stable deflected shape that is nearly a mirror image of its as-fabricated shape mirrored through the centerline connecting its clamped ends. Both theoretical and finite-element analyses are applied to the modeling, design and optimization of the force-displacement characteristic of the curved beam mechanism. The analyses results agree well with experimental measurement after accounting for fabrication variations. If fabricated properly, the mechanism is mechanically robust; some mechanisms have been switched between their bistable states one million times without sign of failure. The bistable mechanism is combined with two contacts to form a crossbar relay. The contact structure is designed with flat shape and additional compliance; and proper metalization process is developed for its DRIE etched sidewalls; both contributing to a more reliable and low resistance contact. Because of bistability this relay requires no actuation power in either its on or off states. Therefore...

‣ Experimental study of the atomization process for viscous liquids by meniscus perturbation and micro air jet

Lee, Heejin, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 79 leaves; 4107287 bytes; 4115587 bytes; application/pdf; application/pdf
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An experimental study was performed to understand the atomization process in the specialized printing method which consists of the high frequency oscillating motion of a needle and a micro air jet. Highly viscous liquids, such as glycerin and acrylic paint were successfully printed with this printing method within the line width of 1 mm. High-speed imaging technique was used to observe the evolution and mechanical perturbation of a liquid meniscus which is coated on the oscillating needle. Various parameters which affect print patterns were identified to understand and control the atomization process occurring at the oscillating needle. For time-controlled atomization or printing, a pulse air jet system was implemented to print liquids only when it is demanded, and it was shown that the period of atomization can be controlled by the air jet on-and-off. The inertial coating process was studied to explain the dynamic meniscus profile, compared with static meniscus. Kinematic analysis of the needle motion was performed, which shows that the needle motion is a sinusoidal one undergoing inertial coating. Liquid sheet breakup mechanism in the presence of the air stream was also studied in conjunction with the principle of the air- blast atomizer. Performing as a printing device or a droplet generator...

‣ Force-induced calcium concentration change and focal adhesion translocation : effects of force amplitude and frequency

Mack, Peter J. (Peter Joseph), 1980-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 66 p.; 4223125 bytes; 4229466 bytes; application/pdf; application/pdf
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Vascular endothelial cells rapidly sense and transduce external forces into biological signals through a process known as mechanotransduction. Numerous biological processes are involved in mechanotransduction, including calcium signaling and activation of focal adhesion sites, but little is known about how cells initially sense changes in the external mechanical environment. In order to examine the rapid mechanosensing thresholds involved with mechanotransduction, calcium concentration changes and focal adhesion site translocations were observed with fluorescent microscopy by labeling intracellular calcium with Fluo-3 calcium dye and by infecting cells with GFP-paxillin fusion proteins. Monitoring calcium concentration changes proved unreliable for determining mechanotransduction thresholds, while a non-graded, time dependent ([similar to] minutes) steady load threshold for mechanotransduction was established between 0.90 and 1.45 nN for focal adhesion site activation. Activation was greatest near the point of forcing (< 7.5 [mu]m), indicating that shear forces imposed on the apical cell membrane transmit non-uniformly to the basal cell surface and that focal adhesion sites may function as individual mechanosensors responding to local levels of force. Results from a while applying nN-level magnetic trap shear forces to the cell apex via integrin-linked magnetic beads. Both biological responses were monitored continuum...

‣ Printed electronics and micro-electromechanical systems; Printed electronics and MEMS systems

Wilhelm, Eric Jamesson, 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 121 p.; 7170783 bytes; 7170783 bytes; application/pdf; application/pdf
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Current electronics and micro-electromechanical systems (MEMS) manufacture is optimized for the production of very high-volume parts on a limited range of substrates. These processes are long, consume large amounts of resources, and require expensive machines and facilities, but yield excellent products. Cheaper, faster printing processes are beginning to emerge with the ability to economically produce low or high-volume electronics and MEMS on flexible substrates. This thesis describes the theoretical and practical design of a suite of printing processes including liquid embossing and offset liquid embossing (OLE). These printing techniques have created resistors, capacitors, and thin-film transistors without etching, vacuum deposition, or high temperatures. Here, the fabrication of all-printed electrostatic actuators is described In liquid embossing a polydimethylsiloxane (PDMS) stamp with bas-relief features is brought into intimate contact with a thin liquid film such as a metal or semi-conductor nanoparticle colloid, spin-on-glass, or polymer to create patterns as small as 100 nm. A simulation of liquid embossing was developed by coupling fluid flow in a thin liquid film to the diffusion of solvent into a PDMS stamp. The model accurately predicts real aspects of the printing process including the time required to stamp and usable stamp geometries. OLE was designed to address some of the limitations of liquid embossing. In OLE the patterned liquid film is transferred to a different substrate...

‣ Design of compliant mechanisms for attenuation of unidirectional vibrations in rotational systems

Szczesny, Spencer E., 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 148 leaves; 29023846 bytes; 30773270 bytes; application/pdf; application/pdf
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The purpose of this research was to generate the knowledge required to design compliant mechanisms that (1) attenuate undesired small-motion angular vibrations in rotational power transmission systems and (2) preserve the desired transmission of large-motion torque/angle inputs. This thesis investigates the design of vibration attenuating compliant mechanisms that are directly integrated into the load path of rotational systems. These devices enable designers to attenuate the amplitude of undesirable vibrations while simultaneously optimizing the transmission of torque inputs. The design, modeling, fabrication and experimental validation of two Compliant Vibration Attenuator (CVA) concepts will be presented. The first device, the Small Amplitude Vibration Isolator (SAVI), is a non-linear compliant device that isolates a resonating or non-resonating rotational system from vibrations by acting as a mechanical lowpass filter. The second device, the Damping Vibration Link (DVL) utilizes compliance and damping to attenuate undesired vibrations due to resonance. A linear lumped parameter model was created in Matlab® to simulate the static and dynamic characteristics of rotational power transmission systems. This model enables one to determine the dynamic characteristics of a system for a given set of inputs...

‣ Design of Si/SiC hybrid structures for elevated temperature micro-turbomachinery

Moon, Hyung-Soo, 1969-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 229 leaves; 11574627 bytes; 11574337 bytes; application/pdf; application/pdf
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thermal softening behavior at temperatures above 900 K. This thermal softening behavior limits the turbine inlet temperature, which in turn significantly degrades the overall engine efficiency. Previous studies have shown that hybrid structures of silicon and silicon carbide have good potential for improved engine performance. Detailed design of Si/SiC hybrid structures for high temperature micro-turbomachinery, however, has been hampered by the relatively poor performance of single crystal Si at elevated temperatures and high stresses and by the unavailability of accurate material properties data for both Si and SiC at the temperatures of interest. From previous work, the critical structures and materials issues to be resolved, in order to proceed with the design of high temperature Si/SiC hybrid structures, were identified as follows: 1. the safety margin of the Si/SiC hybrid structures based on the upper yield strength of Si 2. reliable estimation of the service life of the Si/SiC hybrid structures 3. structural instabilities caused by the combination of stress concentrations and strain softening. In the course of this thesis, these issues provided the key motivations of the work, and have been substantially resolved. As a first step...

‣ Controlling fuel and diluent gas flow for a diesel engine operating in the fuel rich low-temperature-combustion mode

Lopez, David M
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 39 p.
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The flow of a diluent gas supplied to a motoring engine was controlled at a diluent to air mass flow ratios of 10%, 30%, 50%, and 70%. This arrangement was a significant set up for running the engine in the Low-Temperature Combustion mode. The engine used was a 436 cc Yanmar Diesel engine, driven at constant 2200 rpm by a 10 hp AC powered dynamometer. Intake air flow was measured by a model FMA-903-V Air Velocity Transducer by Omega Engineering, Inc., and the diluent gas flow was both measured and controlled by a model FMA-2613A Mass Flow Controller, also by Omega Engineering, Inc. Both were connected to a computer through a National Instruments USB-6211 data acquisition hub, and the signals from both were processed in real time through National Instruments' LabView 8.2 software. The diluent gas used was nitrogen. The flow controller was found to have reasonable flow precision but poor flow accuracy at many of the flow rates encountered during this experiment, with a minimum steady state error of 3.7% for a flow rate of 207.4 Standard Liters Per Minute (SLPM), the highest flow studied, and a maximum error of 97.4% at 53.8 SLPM, the lowest flow studied.; (cont.) The substantial error at low flow rates stems from the rated lower flow limit of the controller of 250 SLPM. A relation describing the amount of steady state error present was determined empirically...

‣ Using the non-intrusive load monitor for shipboard supervisory control; Using the NILM for shipboard supervisory control

Bennett, Patrick Lawrence
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 92 leaves
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Field studies have demonstrated that it is possible to evaluate the state of many shipboard systems by analyzing the power drawn by electromechanical actuators [1], [2], [3], [4], [5]. One device that can perform such an analysis is the non-intrusive load monitor (NILM). This thesis investigates the use of the NILM as a supervisory control system in the engineering plant of gas-turbine-powered vessel. Field tests demonstrate that the NILM can potentially reduce overall sensor count if used in a supervisory control system. To demonstrate the NILM's capabilities in supervisory control systems, experiments are being conducted at the U.S. Navy's Land-Based Engineering Site (LBES) in Philadelphia, Pennsylvania. Following a brief description of the LBES facility and the NILM itself, this thesis presents testing procedures and methodology with results obtained during the extensive field studies. This thesis also describes the on-going efforts to further demonstrate and develop the NILM's capabilities in supervisory control systems.; by Patrick Lawrence Bennett.; Thesis (Nav. E. and S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.; Includes bibliographical references (leaves 78-80).

‣ Complexity theory in axiomatic design

Lee, Tae-Sik, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 182 p.; 8552231 bytes; 8552039 bytes; application/pdf; application/pdf
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During the last couple of decades, the term complexity has been commonly found in use in many fields of science, sometimes as a measurable quantity with a rigorous but narrow definition and other times as merely an ad hoc label. With an emphasis on pragmatic engineering applications, this thesis investigates the complexity concept defined in axiomatic design theory to avoid vague use of the term 'complexity' in engineering system design, to provide deeper insight into possible causes of complexity, and to develop a systematic approach to complexity reduction. The complexity concept in axiomatic design theory is defined as a measure of uncertainty in achieving a desired set of functional requirements. In this thesis, it is revisited to refine its definition. Four different types of complexity are identified in axiomatic design complexity theory: time-independent real complexity, time-independent imaginary complexity, time-dependent combinatorial complexity and time-dependent periodic complexity. Time-independent real complexity is equivalent to the information content, which is a measure of a probability of achieving functional requirements. Time-independent imaginary complexity is defined as the uncertainty due to ignorance of the interactions between functional requirements and design parameters. Time-dependent complexity consists of combinatorial complexity and periodic complexity...

‣ Lubricant oil consumption effects on diesel exhaust ash emissions using a sulfur dioxide trace technique and thermogravimetry

Plumley, Michael J
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 131 p.; 8832895 bytes; 8838361 bytes; application/pdf; application/pdf
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A detailed experimental study was conducted targeting lubricant consumption effects on ,diesel exhaust ash levels using a model year 2002 5.9L diesel engine, high and low Sulfur commercial lubricants, and clean diesel fuels. Regulatory decreases in allowable particulate matter emissions for on road diesel engines are driving industry to develop diesel particulate filters to trap and combust particulate. Remaining ash not combusted in this process clogs filters requiring engine down time and additional cleaning expenses. Recent reductions in fuel Sulfur and ash levels have also made lubricant consumption a significant relative contributor to particulate and ash generation. The goal of this study, a detailed understanding of lubricant contribution to particulate formation and ash transport, is required to enhance future filter design. 'The use of ultra clean fuels enhances accuracy of the Sulfur Dioxide tracer technique for estimating lubricant consumption and increases the relative contribution of lubricant to particulate emission. Results indicate the subject engine lubricant consumption is typical of others reported in literature. Particulate matter emission increases were measured after switching from a relatively low Sulfur, low sulfated ash oil to a high Sulfur...

‣ Shape optimization theory and applications in hydrodynamics

Geçer, Onur
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 89 leaves; 3800871 bytes; 3804538 bytes; application/pdf; application/pdf
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The Lagrange multiplier theorem and optimal control theory are applied to a continuous shape optimization problem for reducing the wave resistance of a submerged body translating at a steady forward velocity well below a free surface. In the latter approach, when the constraint formed by the boundary conditions and the Laplace's governing equation is adjoined to the objective functional to construct the Lagrangian, the dependence of the state on the control is disconnected and they are treated as independent variables; whereas in the first approach, dependences are preserved for the application of Lagrange multiplier theorem. Both methods are observed to yield identical solutions and adjoint equations. Two alternative ways are considered for determining the variation of the objective functional with respect to the state variable which is required to solve the adjoint equation defined on the body boundary. Comparison of these two ways also revealed identical solutions. Finally, a free surface boundary is included in the optimization problem and its effect on the submerged body shape optimization problem is considered.; (cont.) Noting that the analytical solution to the local optimization problem holds for any initial body geometry, it is therefore concluded that the above study will provide theoretical background for an efficient hydrodynamic shape optimization module to be coupled with up-to-date flow solvers currently available such as SWAN.; by Onur Geçer.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ Ultrasonic wave propagation in thick, layered composites containing degraded interfaces

Small, Peter D. (Peter David)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 94 p.; 3589791 bytes; 3593659 bytes; application/pdf; application/pdf
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The ultrasonic wave propagation of thick, layered composites containing degraded bonds is investigated. A theoretical one-dimensional model of three attenuative viscoelastic layers containing two imperfect interfaces is introduced. Elastic material properties and measured 'values of ultrasonic phase velocity and attenuation are used to represent E-glass and vinyl ester resin fiber-reinforced plastic (FRP) laminate, syntactic foam, and resin putty materials in the model. The ultrasonic phase velocity in all three materials is shown to be essentially constant in the range of 1.0 to 5.0 megahertz (MHz). The attenuation in all three materials is constant or slightly increasing in the range 1.0 to 3.0 MHz. Numerical simulation of the model via the mass- spring-dashpot lattice model reveals the importance of the input signal shape, wave speed, and layer thickness on obtaining non-overlapping, distinct return signals in pulse-echo ultrasonic nondestructive evaluation. The effect of the interface contact quality on the reflection and transmission coefficients of degraded interfaces is observed in both the simulated and theoretical results.; by Peter D. Small.; Thesis (Nav. E.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering; and...

‣ A strip theory approximation for wave forces on submerged vehicles in finite depth water

Rybka Jan A. (Jan Andrzej)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 84 leaves; 4176082 bytes; 4179542 bytes; application/pdf; application/pdf
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Autonomous Underwater Vehicles (AUV's) are becoming of increasing use in shallow waters for oceanographic data collection, coastal mapping, and military operations such as mine surveillance along enemy coastlines. Currently the control of AUV's in shallow water is very limited, largely due to a lack of knowledge of vessel forces in shallow water, especially in the presence of surface wave effects. The limitations of current control systems do not afford enough confidence to operate the vehicles in very close proximity to shore or in large waves because the control in the horizontal plane is not adequately reliable enough to prevent bottoming and free surface broaching. Current control system parameters are altered through trial and error to enable reasonable vehicle behavior in shallow water but the control of the vehicle is limited because a thorough understanding of wave forces on these vehicles is non existent. The development of a good analytical tool which adequately models wave forces and moments on an AUV in shallow water waves will enable the development of control systems which will be better able to maneuver the vehicle in shallower water and larger waves than the conditions in which AUV's are currently used.; (cont.) The purpose of this thesis is to further develop...

‣ Analysis and validation of an artifact resistant design for oxygen saturation measurement using photo pletyhsmographic ring sensors

Shaltis, Phillip Andrew
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 103 leaves; 5438181 bytes; 5450148 bytes; application/pdf; application/pdf
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Recent advances in continuous noninvasive health monitoring technologies provide clinicians and researchers with a previously unrealistic opportunity for closely tracking the developments and treatments of various pathologies both within and outside of a clinical setting. At the same time, miniaturized, wireless communication technologies have greatly enhanced the transmission of sensor data while reducing the size requirements for traditional, wearable sensors. The synergism of these innovations has led to the development of the Ring Sensor, a miniaturized, telemetric, photo plethysmograph sensor for continuous health monitoring. Previous work on the Ring Sensor has led to significant power savings in regards to data acquisition and transmission. Additionally, early long-term monitoring tests have indicated that the Ring Sensor is capable of acquiring a reliable waveform nearly 30% of the time. However, the utility of the Ring Sensor has remained somewhat limited. This thesis addresses several of the remaining issues associated with the Ring Sensor. The main design consideration associated with the Ring Sensor is achieving minimal power consumption while maintaining high signal quality. To this end, significant effort has been channeled to the development of an appropriate motion artifact model...