Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008. / Includes bibliographical references (leaves 83-84). / The importance of alternative energy development has been dramatically increased by the dwindling supplies of oil and gas, and our growing efforts to protect our environment. A variety of meaningful steps have been taken in order to come up with cleaner, healthier and more affordable energy alternatives. Wind energy is one of the most reliable energy alternatives for countries that have sufficiently large wind sources. Due to the presence of steady and strong winds, and the distance from coastline residential, the offshore wind farm has become highly attractive as an ideal energy crisis solution. Floating wind turbine systems are being considered as a key solution to make the offshore wind farm feasible from an economic viewpoint, and viable as an energy resource. This paper presents the design of a synthetic mooring system for spar buoy floating wind turbines functioning in shallow water depths. Nacelle acceleration, static and dynamic tensions on catenaries, the maximum tension acting on the anchors are considered as design performances, and a stochastic analysis method has been used to evaluate those quantities based on sea state spectral density functions. The performance at a 100-year hurricane condition is being defined as a limiting case, and a linear wave theory has been the most fundamental theory applied for the present analysis. / by Sungho Lee. / S.M.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 34). / The flight of birds inspired engineers like Leonard da Vinci and Wilbur and Orville Wright to design aircraft that mimic the behavior they observed. The success of the Wright brothers' first controllable aircraft ushered in an era of rapid advances in aviation technology leading to the airplanes of today. Despite these advances, airplanes possess many restrictions that prevent them from being as efficient as their nature-engineered counterparts. Researchers have thus returned to the methods of the earlier engineers in aviation and begun observing birds to look for ways to improve aircraft design. Two methods currently being researched to improve aircraft efficiency are morphing wings and perching. Morphing wings allow airplanes to change the shape of their wings to suit the needs of their mission. Perching is a landing maneuver that uses the nonlinear dynamics of stall to create the drag forces necessary to decelerate the aircraft. Experiments on these methods prove them viable for implementation in small scale aircraft such as remote-controlled planes and unmanned aerial vehicles. However, because of the complexities involved in both morphing wings and perching, further developments are necessary to achieve full implementation. / by Elvine Philip B. Pineda. / S.B.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1984. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by George Em Karniadakis. / M.S.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2003. / Includes bibliographical references (leaf 32). / Every year, automobile manufacturers strive to improve upon their existing designs. Every year, they make small adjustments and try to optimize their designs. Unfortunately, this 'optimization' is often a compromise between multiple components and thus the individual components are not working as well as they could be. Axiomatic Design is a methodology which attempts to avoid these relations between components. By fragmenting the assembly into smaller subcomponents, we can identify and sever these couplings. I used axiomatic design to help redesign a coupled automobile steering/suspension system. / by Beto Peliks. / S.B.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2003. / Includes bibliographical references (leaves 65-67). / by Janice S. Tan. / S.M.

Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 219-229). / This thesis investigates the non-linear regime of electrophoresis, in particular the variation of electrophoretic velocity with electric field at high field strengths. Known theoretical approaches to the problem accounting for ion steric effects, dielectric decrement effects and charge-induced thickening are consolidated, further developed and validated using numerical simulations. In doing so, the influences of the relative strengths of surface conductivity and bulk conductivity and of the relative importance of advection to diffusive transport in the electrolyte are both investigated. In addition, further light is shed on the dependence of electrophoretic mobility on the ionic and particle sizes, and on the relevant ionic diffusivities. / by Wai Hong Ronald Chan. / S.B.

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 56-58). / Cooling demands of advanced electronics are increasing rapidly, often exceeding capabilities of conventional thermal management techniques. Thin film evaporation has emerged as one of the most promising thermal management solutions. High heat transfer rates can be achieved in thin films of liquids due to a small conduction resistance through the film to the evaporating interface. In this thesis, we investigated evaporation from nanoporous membranes. The capillary wicking of the nanopores supplies liquid to the evaporating interface, passively maintaining the thin film. Different evaporation regimes were predicted through modeling and were demonstrated experimentally. Good agreement was shown between the predicted and observed transitions between regimes. Improved heat transfer performance was demonstrated in the pore level evaporation regime over other regimes, with heat transfer rates up to one order of magnitude larger for a given superheat in comparison to the flooding regime. An improved experimental setup for investigating thin film evaporation from nanopores was developed, where a biphilic membrane, i.e., a membrane with two wetting behaviors, was used for enhanced experimental control to allow characterization of the importance of different design parameters. This improved setup was then used to demonstrate the dependence of thin film evaporation on the location of the meniscus within the nanopores. This dependence on meniscus location within the pore was also shown to increase with increasing superheat. We observed a 46% reduction in heat transfer rates at a superheat of 15 °C for an L* of 14.67 compared to an L* of 2, where L* is the ratio of the depth of the meniscus within the pore to the pore radius. This work provides practical insights for the design of devices based on nanoporous evaporation. Heat transfer regimes can be predicted based on fluid supply conditions, evaporative heat flux, and membrane geometry. Furthermore, the biphilic membrane serves as a valuable experimental platform for testing the role of membrane geometry on heat transfer performance in the pore level evaporation regime. Future work will focus on demonstrating the importance of different parameters and using experimental results to either validate existing models for evaporation from nanopores or develop more suitable ones. / by Kyle Wilke. / S.M.

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 93-96). / The primary contributions of this thesis include the first stages of development of a 2D, finite-volume, non-hydrostatic, [sigma]-coordinate code and beginning to apply the Dynamically Orthogonal field equations to study the sensitivity of internal tides to perturbations in the density field. First, we ensure that the 2D Finite Volume (2DFV) code that we use can accurately capture non-hydrostatic internal tides since these dynamics have not yet been carefully evaluated for accuracy in this framework. We find that, for low-aspect ratio topographies, the z-coordinate mesh in the 2DFV code produces numerical artifacts near the bathymetry. To ameliorate these stair-casing effects, and to develop the framework towards a moving mesh with free-surface dynamics, we have begun to implement a non-hydrostatic [sigma]-coordinate framework which significantly improves the representation of the internal tides for low-aspect ratio topographies. Finally we investigate the applicability of stochastic density perturbations in an internal tide field. We utilize the Dynamically Orthogonal field equations for this investigation because they achieve substantial model order reduction over ensemble Monte-Carlo methods. / by Sydney Glass Sroka. / S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 161-170). / Gas-solid fluidized bed reactors find many applications in the energy, chemical and biomedical industries because of their high heat and mass transfer. Their design and optimization continue to be a challenge; the dependence of mixing dynamics on operating conditions and the impact of the reactor scale are poorly understood. This is compounded by severe limitations on diagnostics in their harsh operating environment. Computational Fluid Dynamics (CFD) will play a pivotal role in advancing this technology since it enables unrestricted access inside the reactor, enhancing fundamental understanding of several coupled phenomena and their interactions at various scales. The thesis is focused on the bubbling fluidization of Geldart B particles (typically 103<Ar<105 and 100<Re<102) for application to biomass gasification reactors. 3D CFD simulations are conducted using the Two-Fluid framework, representing the solids phase as a continuum and optimally balancing fidelity and computational needs. To ensure accuracy, critical submodels accounting for unresolved particle-scale interactions (solids stress tensor, particle-gas drag force and particle-wall slip condition) are identified, and computational efficiency gains are achieved through optimal choice of coordinate system, domain discretization and grid resolution. The modeling framework is validated by comparing predictions with experimental measurements spanning a wide range of operating conditions and diagnostic techniques. We performed some of the first fine-grid 3D simulations of intermediate sized beds (30-70 cm diameter) and identified bubbling dynamics and solids circulation as key metrics for characterizing the fluidization hydrodynamics accurately. Towards this end, we developed Multiphase-flow Statistics using 3D Detection and Tracking Algorithm (MS3DATA) to detect and track bubbles using time and spatially resolved data. Our tools are employed to quantify the effect of reactor size on fluidization hydrodynamics. Under similar operating conditions, significantly larger bubbles are observed in small lab-scale beds (~10-15 cm diameter) because of flow confinement. However, the mechanism for bubble coalescence and growth is consistent across reactor scales: small bubbles are formed near the gas distributor, coalesce and rise laterally towards the bed center forming slugs. The transition to slugging also marks a shift in the dominant solids circulation pattern and is dependent on the bed geometry and excess gas velocity. The analysis conclusively demonstrates that (a) the hydrodynamics are independent of walls only when bubbles are much smaller than reactor dimensions and (b) within the regime of interest, a 50 cm diameter pilot reactor is representative of larger scales. Mixing dynamics are quantified by examining the gas and solids flow-field in and around bubbles. Bubble rise velocity is proportional to the square root of its diameter, while gas flow sufficiently far (~30 particle diameters) depends only on the particle properties. Meanwhile, voidage distribution in the vicinity of bubbles results in higher local permeability to gas flow causing (a) preferential bubble pathways, as bubbles are propelled towards areas already frequented by bubbles and (b) gas bypass or throughflow, as low resistance networks are created for interstitial dense-phase gas. Under typical bubbling conditions of Geldart B particles, throughflow almost short circuits through the reactor (residence time is 2-3x shorter than global average) and can reach 30-40% of the total gas flow with detrimental effects on solids mixing and fuel conversion. A predictive model, based on the local bubble-induced micromixing driven by solids upflow around the bubble nose and wake regions and downflow along its sides, has been developed for scaling the gross solids circulation by integrating over the bubble size and spatial distribution. Moreover, reduced models coupling mixing rates (from simulations) and gasification kinetics have been developed and used to analyze thermochemical conversion of biomass. Our CFD approach will be employed for reactive particulate systems in complex reactor geometry, while utilizing discrete element methods to further the fundamental understanding of the hydrodynamics-chemical kinetics coupling, and develop submodels for the continuum framework applications. / by Akhilesh Bakshi. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 241-251). / Offshore energy machines have great potential: higher capacity factors, more available space, and lower visual impacts than onshore machines. This thesis investigates how combining a wave energy converter (WEC) with a floating wind turbine (FWT) may produce offshore renewable energy cost savings. Attaching the WEC to the FWT greatly reduces the WEC's steel frame, mooring lines, electric transmission lines, and siting/permitting costs, which may comprise 56% of a standalone WEC's cost. A 5 MW FWT currently requires up to 1700 tons of platform steel and 5700 tons of ballast concrete for stabilization in the ocean. This required material may be reduced if the WEC stabilizes the FWT. This thesis addresses several challenges to designing a combined FWT-WEC. First, parameter sweeps for optimizing ocean machine performance are limited by high dimensionalities and nonlinearities, including power takeoff control and wave viscous forcing, which normally require computationally expensive time-domain simulations. This thesis develops a statistical linearization approach to rapidly compute machine dynamics statistics while accounting for nonlinearities in the frequency domain. It is verified that the statistical linearization method may capture significant dynamics effects that are neglected by the traditional Taylor series linearization approach, while computing the results approximately 100 times faster than time domain simulations. Using Morison's equation for wave viscosity and quasi-steady blade-element/momentum theory for rotor aerodynamics, we find that viscous effects and nonlinear aerodynamics may increase the FWT motion and tower stress by up to 15% in some wind-sea states compared the the Taylor series linearized system. Second, the WEC must stabilize rather than destabilize the FWT. This thesis investigates the dynamics statistics of dierent FWT-WEC configurations using a long wavelength, structurally coupled model. It is shown that simultaneous targeted energy transfer from both the FWT and waves to the WEC when the WEC and FWT are linked by a tuned spring is unlikely. That being said, this thesis considers heave-mode oscillating water column WEC's that are linked to the FWT platform by 4-bar linkages, so that the FWT and WEC's are uncoupled for small heave motions and rigidly coupled in all other degrees of freedom. It is shown that this configuration allows the WEC to move with a large amplitude in its energy harvesting degree of freedom, and therefore harvest a significant amount of power without significantly increasing the FWT motion in the same direction. In the rigidly-connected modes, the WEC inertial resistance to motion must be greater than the wave forcing, as these properties are transmitted to the FWT. Third, the WEC requires power robustness in dierent sea states. Typical WEC's require control schemes to maintain good power performance when the ocean wave dominant frequency differs from the WEC resonant frequency. This thesis introduces a nonlinearity into the WEC design that passively increases power adaptability in dierent sea states. While the optimized nonlinear WEC requires 57% more steel than the optimized linear WEC, the nonlinear WEC produces 72% more power on average, resulting in a 3% lower levelized cost of energy. Further optimization of the nonlinear WEC may find improved performance. This thesis determines that attaching a single linear hinged floating spar oscillating water column to the FWT reduces the levelized cost of energy from $0.31/kWh for the standalone system to $0.27/kWh (13%) without changing stress on the FWT tower. Attaching a single nonlinear hinged floating spar oscillating water column to the FWT reduces the levelized cost of energy to $0.26/kWh (16%) and reduces the lifetime equivalent fatigue stress on the FWT tower from 32.4 MPa to 31 MPa (5%). A 6-unit array of the nonlinear WEC's encircling the FWT platform may generate an average of 400 kW while reducing the FWT tower stress by over 50%. In wave tank experiments, the response statistics of four dierent combined FWT-WEC configurations are measured, verifying the FWT-WEC dynamics model. / by Jocelyn Maxine Kluger. / Ph. D.