Universiẗat, Diss., 2006--Karlsruhe.

Thesis (M.S.)--Ohio University, August, 1981. / Title from PDF t.p.

Thesis (M.S.)--Ohio University, August, 1988. / Title from PDF t.p.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaves 156-159). Also available via the Internet.

Diss. (sammanfattning) Uppsala : Uppsala universitet, 2006. / Härtill 6 uppsatser. Med sammanfattning på svenska.

The vortex-induced oscillation of a circular cylinder is modelled by a non-linear system with two degrees of freedom. The periodic lift acting on the cylinder due to the vortex-street wake is represented by a self-excited oscillator, which is coupled to the cylinder motion. Approximate solutions and stability criteria are presented which are valid over restricted intervals. Changes to the form of the coupled-oscillator model and its approximate solution are examined in order to improve the comparison between predicted model and experimental results. The changes are motivated by the study of experimental evidence, and by comparison with the known properties of similar systems of non-linear equations. Significant improvement in the coupled-oscillator model performance is obtained through the inclusion of an effective structural damping term which is dependent on wind speed and cylinder displacement. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

Multiphase interleaving buck converters are widely used in today's industrial point-of-load (POL) converters, especially the microprocessor voltage regulators (VRs). The issue of today's multiphase interleaving buck converters is the conflict between the high efficiency and the fast transient in the phase inductor design. In 2000, P. Wong proposed the multiphase coupledinductor buck converter to solve this issue. With the phase inductors coupled together, the coupled-inductor worked as a nonlinear inductor due to the phase-shifted switching network, and the coupled-inductor has different equivalent inductances during steady-state and transient. One the one hand, the steady state inductance is increased due to coupling and the efficiency of the multiphase coupled-inductor buck converter is increased; on the other hand, the transient inductance is reduced and the transient performance of the multiphase coupled-inductor buck is improved. After that, many researches have investigated the multiphase coupled-inductor buck converters in different aspects. However, there are still many challenges in this area: the comprehensive analysis of the converter, the alternative coupled inductor structures with the good performance, the current sensing of converter and the light-load efficiency improvement. They are investigated in this dissertation. The comprehensive analysis of the multiphase coupled-inductor buck converter is investigated. The n-phase (n>2) coupled-inductor buck converter with the duty cycle D>1/n hasn't been analyzed before. In this dissertation, the multiphase coupled-inductor buck converter is systematically analyzed for any phase number and any duty cycle condition. The asymmetric multiphase coupled-inductor buck converter is also analyzed. The existing coupled-inductor has a long winding path issue. In low-voltage, high-current applications, the short winding path is preferred because the winding loss dominates the inductor total loss and a short winding path can greatly reduce the winding loss. To solve this long winding path issue, several twisted-core coupled-inductors are proposed. The twisted-core coupled-inductor has such a severe 3D fringing effect that the conventional reluctance modeling method gives a poor result, unacceptable from the design point of view. By applying and extending Sullivan's space cutting method to the twisted core coupled inductor, a precise reluctance model of the twisted-core coupled-inductor is proposed. The reluctance model gives designers the intuition of the twisted-core coupled-inductors and facilitates the design of the twisted-core coupled-inductors. The design using this reluctance model shows good correlation between the design requirement and the design result. The developed space cutting method can also be used in other complex magnetic structures with the strong fringing effect. Today, more and more POL converters are integrated and the bottleneck of the integrated POL converters is the large inductor size. Different coupled-inductor structures are proposed to reduce the large inductor size and to improve the power density of the integrated POL converter. The investigation is based on the low temperature co-fire ceramic (LTCC) process. It is found that the side-by-side-winding coupled-inductor structure achieves a smaller footprint and size. With the two-segment B-H curve approximation, the proposed coupled-inductor structure can be easily modeled and designed. The designed coupled-inductor prototype reduces the magnetic size by half. Accordingly, the LTCC integrated coupled-inductor POL converter doubles the power density compared to its non-coupled-inductor POL counterpart and an amazing 500W/in³ power density is achieved. In a multiphase coupled-inductor converter, there are several coupled-inductor setups. For example, for a six-phase coupled-inductor converter, three two-phase coupled inductors, two three-phase coupled-inductors and one six-phase coupled inductors can be used. Different coupled-inductor setups are investigated and it is found that there is a diminishing return effect for both the steady-state efficiency improvement and the transient performance improvement when the coupling phase number increases. The conventional DCR current sensing method is a very popular current sensing method for today's multiphase non-coupled-inductor buck converters. Unfortunately, this current sensing method doesn't work for the multiphase coupled-inductor buck converter. To solve this issue, two novel DCR current sensing methods are proposed for the multiphase coupled-inductor buck converter. Although the multiphase coupled-inductor buck converters have shown a lot of benefits, they have a low efficiency under light-load working in DCM. Since the DCM operation of the multiphase coupled-inductor buck converter has never been investigated, they are analyzed in detail and the reason for the low efficiency is identified. It is found that there are more-than-one DCM modes for the multiphase coupled-inductor buck converter: DCM1, DCM2 …, and DCMn. In the DCM2, DCM3 …, and DCMn modes, the phase-currents reach zero-current more-than-once during one switching period, which causes the low efficiency of the multiphase coupledinductor buck converter in the light load. With the understanding of the low efficiency issue, the burst-in-DCM1-mode control method is proposed to improve the light load efficiency of the multiphase coupled-inductor buck converter. Experimental results prove the proposed solution. / Ph. D.

M.Sc. / The understanding of the classification and origin of geological systems is facilitated by the acquisition of accurate and precise analytical data. New instrumentation is rapidly developed for the preparation and quantitative analysis of geological materials using smaller amounts of sample, with lower limits of detection and faster analysis times. The development of new methodologies is crucial for the effective utilisation of new instrumentation. This study was conceptualised because of the relationship between the Department of Geology and the Central Analytical Facility of the Faculty of Science, University of Johannesburg. There is a high demand for accurate and precise chemical data for various lithologies and the availability of high-end analytical equipment, but little practical expertise to utilise this equipment to its full potential. The study is centred round the analysis of four common sedimentary lithological groups that are routinely studied by the Department of Geology namely carbonate rocks, shales, iron ore and manganese ore. A large literature base exists for the decomposition and analysis of geological materials. The bulk of this literature is centred round more established and conventional methods of sample preparation and analysis. The use of microwave digestion instrumentation and methodologies in recent times has revolutionised sample decomposition with shorter decomposition times, smaller sample masses required for decomposition, lower loss of analytes to volatilisation and creating a safer laboratory environment for analysts to work in. Inductively coupled plasma optical emission spectroscopy (ICP-OES) is a more mature method of sample analysis, being commercialised in the mid 1970’s, while inductively coupled plasma mass spectroscopy (ICP-MS) is a newer technique, v being commercialised as of the mid 1980’s. These two techniques are multi-element techniques, with low limits of detection and fast analysis times for a plethora of analytes, over seventy elements for ICP-OES and over eighty elements for ICP-MS. Samples from all four lithological groups were successfully digested in a microwave digestion unit with varying combinations of nitric acid (HNO3), hydrochloric acid (HCl), perchloric acid (HClO4) and hydrofluoric acid (HF) and various digestion programs. Accurate and repeatable methods of analysis were developed for iron, manganese, calcium and potassium for all four lithologies with ICP-OES; aluminium was successfully determined for shale, iron and manganese ore with ICP-OES. Titanium, sodium, arsenic, barium, bismuth, chromium, copper, molybdenum, scandium, strontium and vanadium were determined for all four lithotypes with acceptable accuracy and repeatability using ICP-MS.

No description available.

No description available.