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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1978. / Microfiche copy available in Archives and Science. / Bibliography: leaves 104-111. / by Nathan Hawley. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN. / Bibliography: leaves 138-141. / by James Douglas Walker. / M.S.

Yes / Laboratory experiments have been carried out to investigate wave damping over the seabed, in which the excess pore pressure and free surface elevations are synchronously measured for examining the wave-induced soil dynamics and wave kinematics. Two types of soil, namely fine sand and silt, are tested to examine the role of soil in the wave damping. Observation of experiments shows that (i) soil liquefaction takes place for some tests with silty bed and soil particles suspend into the water layer when the bed is made of silt; (ii) sand ripples can be generated for experiments with sand bed. Measurements reveal that the wave damping greatly depends on the soil dynamic responses to wave loading and the wave damping mechanism over the silty seabed differs from that over the sand bed. On the one hand, the wave damping rate is greatly increased, when soil liquefaction occurs in the silty bed. On the other hand, the presence of sand ripples generated by oscillatory flow in the sand bed experiments also increases the wave damping to some extent. Furthermore, experimental results show that soil particle suspension in the silt bed test contributes to the wave damping. Theoretical analysis is presented to enhance discussions on the wave damping. The theoretical calculations demonstrate that the wave damping is mainly induced by the shear stress in the boundary layer for the cases when no liquefaction occurs. While for the cases when soil liquefaction takes place, the viscous flow in the liquefied layer contributes most towards to the wave damping. / the National Science Fund for Distinguished Young Scholars (Grant No. 51425901), the National Key Research and Development Program of China (2017YFC1404200), the Marine Renewable Energy Research Project of State Oceanic Administration (GHME2015GC01), and the 111 Project (Grant No. B12032)

Interactive software systems have both functional and user interface components. User interface design and development requires specialized usability engineering (UE) knowledge, training, and experience in topics such as psychology, cognition, specialized design guidelines, and task analysis. The design and development of a functional core requires specialized software engineering (SE) knowledge, training, and experience in topics such as algorithms, data structures, software architectures, calling structures, and database management. Given that the user interface and the functional core are two closely coupled components of an interactive software system, with each constraining the design of the other, there is a need for the SE and UE life cycles to be connected to support communication among roles between the two development life cycles. Additionally, there is a corresponding need for appropriate computer science curricula to train the SE and UE roles about the connections between the two processes. In this dissertation, we connected the SE and UE life cycles by creating the Ripple project development environment which fosters communication between the SE and UE roles and by creating a graduate-level cross-pollinated SE-UE joint course offering, with student teams spanning the two classes, to educate students about the intricacies of interactive-software development. Using this joint course we simulated different conditions of interactive-software development (i.e. with different types of project constraints and role playing) and assigned different teams to these conditions. As part of semester-long class projects these teams developed prototype systems for a real client using their assigned development condition. Two of the total of eight teams in this study used the Ripple framework. As part of this experimental course offering, various instruments were employed throughout the semester to assess the effectiveness of a framework like Ripple and to investigate candidate factors that impact the quality of product and process of interactive-software systems. The study highlighted the importance of communication among the SE and UE roles and exemplified the need for the two roles to respect each other and to have the willingness to work with one another. Also, there appears to exist an inherent conflict of interest when the same people play both UE and SE roles as they seem to choose user interface features that are easy to implement and not necessarily easy to use by system's target users. Regarding pedagogy, students in this study indicated that this joint SE-UE course was more useful in learning about interactive-software development and that it provided a better learning experience than traditional SE-only or UE-only courses. / Ph. D.

Microinverter has become a new trend for photovoltaic (PV) grid-tie systems due to its advantages which include greater energy harvest, simplified system installation, enhanced safety, and flexible expansion. Since an individual microinverter system is typically attached to the back of a PV module, it is desirable that it has a long lifespan that can match PV modules, which routinely warrant 25 years of operation. In order to increase the life expectancy and improve the long-term reliability, electrolytic capacitors must be avoided in microinverters because they have been identified as an unreliable component. One solution to avoid electrolytic capacitors in microinverters is using a two-stage architecture, where the high voltage direct current (DC) bus can work as a double line ripple buffer. For two-stage electrolyte-free microinverters, a high boost ratio dc-dc converter is required to increase the low PV module voltage to a high DC bus voltage required to run the inverter at the second stage. New high boost ratio dc-dc converter topologies using the hybrid transformer concept are presented in this dissertation. The proposed converters have improved magnetic and device utilization. Combine these features with the converter's reduced switching losses which results in a low cost, simple structure system with high efficiency. Using the California Energy Commission (CEC) efficiency standards a 250 W prototype was tested achieving an overall system efficiency of 97.3%. The power inversion stage of electrolyte-free microinverters requires a high efficiency grid-tie inverter. A transformerless inverter topology with low electro-magnetic interference (EMI) and leakage current is presented. It has the ability to use modern superjunction MOSFETs in conjunction with zero-reverse-recovery silicon carbide (SiC) diodes to achieve ultrahigh efficiency. The performance of the topology was experimentally verified with a tested CEC efficiency of 98.6%. Due to the relatively low energy density of film capacitors compared to electrolytic counterparts, less capacitance is used on the DC bus in order to lower the cost and reduce the volume of electrolyte-free microinverters. The reduced capacitance leads to high double line ripple voltage oscillation on DC bus. If the double line oscillation propagates back into the PV module, the maximum power point tracking (MPPT) performance would be compromised. A control method which prevents the double line oscillation from going to the PV modules, thus improving the MPPT performance was proposed. Finally, a control technique using a single microcontroller with low sampling frequency was presented to effectively eliminate electrolyte capacitors in two-stage microinverters without any added penalties. The effectiveness of this control technique was validated both by simulation and experimental results. / Ph. D.