Development of physics-based models and design optimization of power electronic conversion systems

The main objective for physics based modeling of the power converter components is to design the whole converter with respect to physical and operational constraints. Therefore, all the elements and components of the energy conversion system are modeled numerically and combined together to achieve the whole system behavioral model. Previously proposed high frequency (HF) models of power converters are based on circuit models that are only related to the parasitic inner parameters of the power devices and the connections between the components. This dissertation aims to obtain appropriate physics-based models for power conversion systems, which not only can represent the steady state behavior of the components, but also can predict their high frequency characteristics. The developed physics-based model would represent the physical device with a high level of accuracy in predicting its operating condition. The proposed physics-based model enables us to accurately develop components such as; effective EMI filters, switching algorithms and circuit topologies [7]. One of the applications of the developed modeling technique is design of new sets of topologies for high-frequency, high efficiency converters for variable speed drives. The main advantage of the modeling method, presented in this dissertation, is the practical design of an inverter for high power applications with the ability to overcome the blocking voltage limitations of available power semiconductor devices. Another advantage is selection of the best matching topology with inherent reduction of switching losses which can be utilized to improve the overall efficiency. The physics-based modeling approach, in this dissertation, makes it possible to design any power electronic conversion system to meet electromagnetic standards and design constraints. This includes physical characteristics such as; decreasing the size and weight of the package, optimized interactions with the neighboring components and higher power density. In addition, the electromagnetic behaviors and signatures can be evaluated including the study of conducted and radiated EMI interactions in addition to the design of attenuation measures and enclosures.

The possible selves method in evaluating youth development interventions: The use of Integrated Qualitative /Quantitative Data Analytic Strategies (IQ -DAS)

This study investigated the feasibility of using qualitative methods to provide empirical documentation of the long-term qualitative change in the life course trajectories of “at risk” youth in a school based positive youth development program (the Changing Lives Program—CLP). This work draws from life course theory for a developmental framework and from recent advances in the use of qualitative methods in general and a grounded theory approach in particular. Grounded theory provided a methodological framework for conceptualizing the use of qualitative methods for assessing qualitative life change. The study investigated the feasibility of using the Possible Selves Questionnaire-Qualitative Extension (PSQ-QE) for evaluating the impact of the program on qualitative change in participants' life trajectory relative to a non-intervention control group. Integrated Qualitative/Quantitative Data Analytic Strategies (IQ-DAS) that we have been developing a part of our program of research provided the data analytic framework for the study. ^ Change was evaluated in 85 at risk high school students in CLP high school counseling groups over three assessment periods (pre, post, and follow-up), and a non-intervention control group of 23 students over two assessment periods (pre and post). Intervention gains and maintenance and the extent to which these patterns of change were moderated by gender and ethnicity were evaluated using a mixed design Repeated Measures Multivariate Analysis of Variance (RMANOVA) in which Time (pre, post) was the within (repeated) factor and Condition, Gender, and Ethnicity the between group factors. The trends for the direction of qualitative change were positive from pre to post and maintained at the year-end follow-up. More important, the 3-way interaction for Time x Gender x Ethnicity was significant, Roy's Θ =. 205, F(2, 37) = 3.80, p <.032, indicating that the overall pattern of positive change was significantly moderated by gender and ethnicity. Thus, the findings also provided preliminary evidence for a positive impact of the youth development program on long-term change in life course trajectory, and were suggestive with respect to the issue of amenability to treatment, i.e., the identification of subgroups of individuals in a target population who are likely to be the most amenable or responsive to a treatment. ^

The investigation of photocatalysts and iron based materials in the oxidation and adsorption of toxic organic and chromium materials

The presences of heavy metals, organic contaminants and natural toxins in natural water bodies pose a serious threat to the environment and the health of living organisms. Therefore, there is a critical need to identify sustainable and environmentally friendly water treatment processes. In this dissertation, I focus on the fundamental studies of advanced oxidation processes and magnetic nano-materials as promising new technologies for water treatments. Advanced oxidation processes employ reactive oxygen species (ROS) which can lead to the mineralization of a number of pollutants and toxins. The rates of formation, steady-state concentrations, and kinetic parameters of hydroxyl radical and singlet oxygen produced by various TiO2 photocatalysts under UV or visible irradiations were measured using selective chemical probes. Hydroxyl radical is the dominant ROS, and its generation is dependent on experimental conditions. The optimal condition for generation of hydroxyl radical by of TiO2 coated glass microspheres is studied by response surface methodology, and the optimal conditions are applied for the degradation of dimethyl phthalate. Singlet oxygen (1O2) also plays an important role for advanced processes, so the degradation of microcystin-LR by rose bengal, an 1O2 sensitizer was studied. The measured bimolecular reaction rate constant between MC-LR and 1O2 is ∼ 106 M-1 s-1 based on competition kinetics with furfuryl alcohol. The typical adsorbent needs separation after the treatment, while magnetic iron oxides can be easily removed by a magnetic field. Maghemite and humic acid coated magnetite (HA-Fe3O4) were synthesized, characterized and applied for chromium(VI) removal. The adsorption of chromium(VI) by maghemite and HA-Fe3O4 follow a pseudo-second-order kinetic process. The adsorption of chromium(VI) by maghemite is accurately modeled using adsorption isotherms, and solution pH and presence of humic acid influence adsorption. Humic acid coated magnetite can adsorb and reduce chromium(VI) to non-toxic chromium (III), and the reaction is not highly dependent on solution pH. The functional groups associated with humic acid act as ligands lead to the Cr(III) complex via a coupled reduction-complexation mechanism. Extended X-ray absorption fine structure spectroscopy demonstrates the Cr(III) in the Cr-loaded HA-Fe 3O4 materials has six neighboring oxygen atoms in an octahedral geometry with average bond lengths of 1.98 Å.

Physics-based modeling of power system components for the evaluation of low-frequency radiated electromagnetic fields

The low-frequency electromagnetic compatibility (EMC) is an increasingly important aspect in the design of practical systems to ensure the functional safety and reliability of complex products. The opportunities for using numerical techniques to predict and analyze system's EMC are therefore of considerable interest in many industries. As the first phase of study, a proper model, including all the details of the component, was required. Therefore, the advances in EMC modeling were studied with classifying analytical and numerical models. The selected model was finite element (FE) modeling, coupled with the distributed network method, to generate the model of the converter's components and obtain the frequency behavioral model of the converter. The method has the ability to reveal the behavior of parasitic elements and higher resonances, which have critical impacts in studying EMI problems. For the EMC and signature studies of the machine drives, the equivalent source modeling was studied. Considering the details of the multi-machine environment, including actual models, some innovation in equivalent source modeling was performed to decrease the simulation time dramatically. Several models were designed in this study and the voltage current cube model and wire model have the best result. The GA-based PSO method is used as the optimization process. Superposition and suppression of the fields in coupling the components were also studied and verified. The simulation time of the equivalent model is 80-100 times lower than the detailed model. All tests were verified experimentally. As the application of EMC and signature study, the fault diagnosis and condition monitoring of an induction motor drive was developed using radiated fields. In addition to experimental tests, the 3DFE analysis was coupled with circuit-based software to implement the incipient fault cases. The identification was implemented using ANN for seventy various faulty cases. The simulation results were verified experimentally. Finally, the identification of the types of power components were implemented. The results show that it is possible to identify the type of components, as well as the faulty components, by comparing the amplitudes of their stray field harmonics. The identification using the stray fields is nondestructive and can be used for the setups that cannot go offline and be dismantled

The maintenance of tracking : the role of organizational culture

Modern comprehensive high schools do not formally track students into different programs, but schools offer different curricular sequences with important and stratified consequences for students' post-secondary education. This study used qualitative methodology to examine how schools' organizational cultures influence the maintenance of tracking practices in four comprehensive high schools in Miami. The methodology included long-term participant observation in each of the four schools, unstructured and semi-structured interviews and the collection of written documents produced by the district. A framework based on the concepts of environment, mission,, information, strategy, and leadership was used to analyze the data. It was found that school cultures shared deeply held beliefs that regard ability as a fixed trait. This prevented schools from providing access to information about the consequences of course selection to the majority of the student body, with the exception of those students defined as "college bound." State and County level policies that reward achievement in standardized tests combined with school overcrowding, resulted in organizational cultures that favored the adoption of strategies stressing efficiency, as opposed to a challenging education for all students. Only one of the four schools in the study had a policy requiring students to attempt courses that were more challenging. The practice was resented by both teachers and counselors, since it was perceived as interfering with other goals of the institution, i.e.: graduating students in four years. The culture of the schools stressed college as the only legitimate post-secondary option; consequently, the majority of counselors did not encourage students-even those already defined as "not college material"-to consider other alternatives, such as vocational education. The elimination of formal tracks in these comprehensive high schools resulted in the school culture lacking a clear mission in regards to non-college bound students. Findings are discussed in relation to current theoretical explanations for educational policy and equality of opportunity.

An investigation of the effects of a school-based intervention on feelings of personal expressiveness in multi-problem adolescent youth

This study was conducted to determine the effects of the Changing Lives Program intervention on troubled adolescents' feelings of personal expressiveness, believed to be one domain of positive identity development. Forty-three intervention and twenty nonintervention comparison control participants were given a battery of pre-, post-, and follow-up assessments including the Personally Expressive Questionnaire (Waterman, 1995), which was used to derive participants' feelings of personal expressiveness scores. Using Repeated Measures Analysis of Multivariate Analysis (RMANOVA), a significant four-way interaction of Time X Condition X Gender X Ethnicity was found relative to the Control, Roy's Ɵ = .166, F(2,47) = 3.899, p < .027 indicating that intervention participants' feelings of personal expressiveness did increase significantly relative to the control group. Furthermore, the results suggest differential outcomes based on ethnicity, suggesting the need for future study with respect to specificity of effects and mechanisms of identity formation in differing ethnic subgroups.

Development and evaluation of a geographic information system based method to estimate flooding susceptibility in an area of Broward County, Florida

The objective of this study was to develop a GIS-based multi-class index overlay model to determine areas susceptible to inland flooding during extreme precipitation events in Broward County, Florida. Data layers used in the method include Airborne Laser Terrain Mapper (ALTM) elevation data, excess precipitation depth determined through performing a Soil Conservation Service (SCS) Curve Number (CN) analysis, and the slope of the terrain. The method includes a calibration procedure that uses "weights and scores" criteria obtained from Hurricane Irene (1999) records, a reported 100-year precipitation event, Doppler radar data and documented flooding locations. Results are displayed in maps of Eastern Broward County depicting types of flooding scenarios for a 100-year, 24-hour storm based on the soil saturation conditions. As expected the results of the multi-class index overlay analysis showed that an increase for the potential of inland flooding could be expected when a higher antecedent moisture condition is experienced. The proposed method proves to have some potential as a predictive tool for flooding susceptibility based on a relatively simple approach.

Social Network Structure as a Critical Success Condition for Open Source Software Project Communities

In recent years, a surprising new phenomenon has emerged in which globally-distributed online communities collaborate to create useful and sophisticated computer software. These open source software groups are comprised of generally unaffiliated individuals and organizations who work in a seemingly chaotic fashion and who participate on a voluntary basis without direct financial incentive. The purpose of this research is to investigate the relationship between the social network structure of these intriguing groups and their level of output and activity, where social network structure is defined as 1) closure or connectedness within the group, 2) bridging ties which extend outside of the group, and 3) leader centrality within the group. Based on well-tested theories of social capital and centrality in teams, propositions were formulated which suggest that social network structures associated with successful open source software project communities will exhibit high levels of bridging and moderate levels of closure and leader centrality. The research setting was the SourceForge hosting organization and a study population of 143 project communities was identified. Independent variables included measures of closure and leader centrality defined over conversational ties, along with measures of bridging defined over membership ties. Dependent variables included source code commits and software releases for community output, and software downloads and project site page views for community activity. A cross-sectional study design was used and archival data were extracted and aggregated for the two-year period following the first release of project software. The resulting compiled variables were analyzed using multiple linear and quadratic regressions, controlling for group size and conversational volume. Contrary to theory-based expectations, the surprising results showed that successful project groups exhibited low levels of closure and that the levels of bridging and leader centrality were not important factors of success. These findings suggest that the creation and use of open source software may represent a fundamentally new socio-technical development process which disrupts the team paradigm and which triggers the need for building new theories of collaborative development. These new theories could point towards the broader application of open source methods for the creation of knowledge-based products other than software.

Risk Based Post Closure Care Analysis for Florida Landfills

Subtitle D of the Resource Conservation and Recovery Act (RCRA) requires a post closure period of 30 years for non hazardous wastes in landfills. Post closure care (PCC) activities under Subtitle D include leachate collection and treatment, groundwater monitoring, inspection and maintenance of the final cover, and monitoring to ensure that landfill gas does not migrate off site or into on site buildings. The decision to reduce PCC duration requires exploration of a performance based methodology to Florida landfills. PCC should be based on whether the landfill is a threat to human health or the environment. Historically no risk based procedure has been available to establish an early end to PCC. Landfill stability depends on a number of factors that include variables that relate to operations both before and after the closure of a landfill cell. Therefore, PCC decisions should be based on location specific factors, operational factors, design factors, post closure performance, end use, and risk analysis. The question of appropriate PCC period for Florida’s landfills requires in depth case studies focusing on the analysis of the performance data from closed landfills in Florida. Based on data availability, Davie Landfill was identified as case study site for a case by case analysis of landfill stability. The performance based PCC decision system developed by Geosyntec Consultants was used for the assessment of site conditions to project PCC needs. The available data for leachate and gas quantity and quality, ground water quality, and cap conditions were evaluated. The quality and quantity data for leachate and gas were analyzed to project the levels of pollutants in leachate and groundwater in reference to maximum contaminant level (MCL). In addition, the projected amount of gas quantity was estimated. A set of contaminants (including metals and organics) were identified as contaminants detected in groundwater for health risk assessment. These contaminants were selected based on their detection frequency and levels in leachate and ground water; and their historical and projected trends. During the evaluations a range of discrepancies and problems that related to the collection and documentation were encountered and possible solutions made. Based on the results of PCC performance integrated with risk assessment, projection of future PCC monitoring needs and sustainable waste management options were identified. According to these results, landfill gas monitoring can be terminated, leachate and groundwater monitoring for parameters above MCL and surveying of the cap integrity should be continued. The parameters which cause longer monitoring periods can be eliminated for the future sustainable landfills. As a conclusion, 30 year PCC period can be reduced for some of the landfill components based on their potential impacts to human health and environment (HH&E).