Dissertations / Theses on the topic 'El Niño current Mathematical models'

In this thesis models for the simulation of transient phenomena in high voltage direct-current systems are developed. The new converter model is versatile and the solution algorithm is free from numerical oscillations. A new generic inverter control described in this thesis is based on a predictive approach. Steady-state and transient simulations of two-terminal and multi-terminal (i.e., a parallel converter system) high voltage direct-current systems are carried out using the new converter system model. Comparison between the two-terminal transient simulation results and the high voltage direct-current simulator outputs shows good agreement. An alternating-current/direct-current initialization procedure for the Electromagnetic Transients Program (EMTP) has been investigated and a novel initialization algorithm has been suggested in this thesis.
Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate

This thesis examines the cross-sectional and time series variation between commodities futures prices and current prices. The 'Theory of Storage' states that the difference between the two prices will be a function of two factors: The first is the cost of storing the commodity over the term of the futures contract (carrying costs). The second factor is the value of the convenience yield. The convenience yield is a concept which evolved from the theory of storage and is explained as the benefit which accrues to the individual or firm that holds the commodity in storage but does not accrue to the holder of the futures contract. It is generally assumed that the value of a commodity's convenience yield is decreasing in the aggregate inventory available and some indirect empirical support has been generated for this assumption, however, an economic model has not been provided which derives the result. There are two objectives of this thesis: The first is to provide a model of the convenience yield which explains the relationship between the level of inventories and the value of the convenience yield. The second objective is to empirically test the predictions of the model. The model provided shows the convenience yield to be decreasing in the level of aggregate inventory. In addition, the value of the convenience is found to be related to the time-series process of shocks to demand. An analogy is drawn between the convenience yield and an option with a stochastic exercise price. Using futures price data and aggregate inventory data, the empirical implications of the model are tested. The results support the hypothesis that a commodity's convenience yield is decreasing in aggregate inventory. Some evidence is also provided that the convenience yield is decreasing in the correlation between shocks to demand.
Business, Sauder School of
Finance, Division of
Graduate

The use of switched linear networks is prevalent in power processing systems such as dc-to-dc power converters. These converters provide regulated dc output power and are widely used in the power supplies for computers and peripherals. As with most systems where high performance is a criterion, the power converter is usually enclosed in a feedback loop. Feedback design usually entails the use of small signal (linear) models with linear feedback design techniques. There exists a large number of small signal models, but most of them are ad hoc. A describing function approach has previously been presented which determines the exact small signal frequency response of the converters. This method is now used to systematically derive circuit models for power converters. The aim of the work presented in this thesis is to model the nonlinear part of the switching converters which are simply the switches. This study is focused on the Pulse Width Modulation (PWM) class of converters. Majority of available models are not very accurate because they are averaged models. The aim of the new approach is to obtain accurate and simple model. A partial realization of this aim has been achieved; the new model is accurate over a very wide frequency range, but presently the parameters are rather complicated.

The solar corona is a highly conductive plasma which is dominated by the coronal magnetic field. Observations show that important solar phenomena like flares or the heating of the corona are driven by magnetic energy, probably through the process of magnetic reconnection. The release of magnetic energy by reconnection requires that non-ideal processes take place in contradiction to the high conductivity of the corona. One possibility to overcome this problem is to generate strong electrical currents in strongly localised regions. In this thesis we investigate how such localised currents can be formed by slow ideal evolution of topologically simple magnetic fields. To this purpose numerical simulations are carried out using an Eulerian and a Lagrangian MHD relaxation code. We first use a simple example (twisting of a uniform field) to investigate the advantages and disadvantages of both codes and to discover possible limitations for their application. We show that for the problems addressed in this thesis the Lagrangian code is more suited because it can resolve the localised current densities much better than the Eulerian code. We then focus in particular on magnetic fields containing a so-called Hyperbolic Flux Tube (HPT). A recently proposed analytical theory predicts that HFT's are sites where under certain conditions strong current build-up can be expected. We use our code to carry out a systematic parametric study of the dependence of current growth for a typical HFT configuration. We have also developed a completely new version of the analytical theory which is directly based on the set-up of our numerical simulations. We find that the simulations agree with the analytical prediction in a quantitative way but that the analytical theory underestimates the current growth quite substantially, probably by not taking into account the non-linear character of the full problem.

In this thesis we investigate current sheets in the solar corona. The well known 1D model for the tearing mode instability is presented, before progressing to 2D where we introduce a non-uniform resistivity. The effect this has on growth rates is investigated and we find that the inclusion of the non-uniform term in η cause a decrease in the growth rate of the dominant mode. Analytical approximations and numerical simulations are then used to model current sheet formation by considering two distinct experiments. First, a magnetic field is sheared in two directions, perpendicular to each other. A twisted current layer is formed and we find that as we increase grid resolution, the maximum current increases, the width of the current layer decreases and the total current in the layer is approximately constant. This, together with the residual Lorentz force calculated, suggests that a current sheet is trying to form. The current layer then starts to fragment. By considering the parallel electric field and calculating the perpendicular vorticity, we find evidence of reconnection. The resulting temperatures easily reach the required coronal values. The second set of simulations carried out model an initially straight magnetic field which is stressed by elliptical boundary motions. A highly twisted current layer is formed and analysis of the energetics, current structures, magnetic field and the resulting temperatures is carried out. Results are similar in nature to that of the shearing experiment.

A numerical study is made of the wintertime circulation within the Southeast Indian Ocean (SEIO). The downwelling favourable winds result in a continuous eastward Coastal Current (CC) extending from Cape Leeuwin to the eastern coast of Tasmania, where it forms a confluence with the south branch of the East Australian Current. An additional forcing mechanism for the CC is the Leeuwin Current in the western part of the domain. The study here is divided in two parts: (1) available data and the wintertime averaged results from the Ocean Circulation and Climate Advanced Model (OCCAM) are analysed to provide a first order description of the large-scale circulation; (2) a high resolution model (Princeton Ocean Model) is nested within OCCAM to examine the shelf-slope circulation within the eastern SEIO. The nested model is forced with climatological monthly average winds and several experiments were run to simulate the effects of surface fluxes of density, enhanced bottom friction and stronger winds. In summary, the shelf-slope circulation is governed by a surface south-eastward CC that carries around 2 Sv and reaches velocities of up to 50 cm/s, where the shelf is narrowest. The core of the current is generally constrained to the shelf-break region. Zonal winds and geostrophic control of the CC lead to a transport of 1 Sv through Bass Strait and a north-eastward jet that is directed into the strait between King Is. and Tasmania. Further south, the CC is poleward and known as the Zeehan Current (ZC). Between Cape Leeuwin and Tasmania and over the slope region, a westward current (the Flinders Current) is found at depths of 500-1000 m and has an associated transport of 5-7 Sv. The current is shown to result from a northward Sverdrup transport in the deep ocean. Meso-scale eddies are shown to result from baroclinic instability and have wavelengths of around 250 km and transports of 3-4 Sv, and can dominate the slope circulation. A comparison of the numerical results is also made with two current meter data sets and results show an interannual variability in the ZC strength, that is probably related to ENSO.

This research examines the role that modern portfolio theory (MPT) plays in current South Africa asset management practice in comparison to other portfolio management techniques and security evaluation methods. The purpose of asset management is to pool complementary financial market expertise, in order to generate returns in excess of the market return on the investments of the owners of financial resources that are entrusted to the firm, since the owners of financial resources might not be able to make superior investment decisions on their own. The research presents and discusses the literature pertaining to modern portfolio theory, traditional portfolio theory (fundamental and technical analyses), and behavioural finance theory. The implication of the efficient market hypothesis in relation to all the portfolio management theories is also presented and discussed. In line with a positivist paradigm, the survey research methodology, which combines both qualitative and quantitative aspects, was adopted. The instrument used for data collection was a questionnaire, which was found to be reliable and valid for this research. The questionnaire encompassed the Lickert scale to measure the data. The results of the analysis were interpreted using descriptive statistics. The results of this research suggest that modern portfolio theory does not play a significant role in the management of portfolios and security evaluation in South Africa. South African asset managers regard fundamental analysis as the most significant method of security evaluation in the management of portfolios. Technical analysis and econometric models are regarded as playing a moderate role and complement fundamental analysis whilst behavioural finance models play the least role. This research recommends an integrated portfolio management strategy that incorporates MPT, traditional portfolio theory and behavioural finance models to enhance investor value and protection.

This thesis considers the properties of MHS equilibria formed through non-resistive MHD relaxation of analytical non-potential magnetic field models, which contain two null points connected by a generic separator. Four types of analytical magnetic fields are formulated, with different forms of current. The magnetic field model which has a uniform current directed along the separator, is used through the rest of this thesis to form MHS equilibria and to study reconnection. This magnetic field, which is not force-free, embedded in a high-beta plasma, relaxes non-resistively using a 3D MHD code. The relaxation causes the field about the separator to collapse leading to a twisted current layer forming along the separator. The MHS equilibrium current layer slowly becomes stronger, longer, wider and thinner with time. Its properties, and the properties of the plasma, are found to depend on the initial parameters of the magnetic field, which control the geometry of the magnetic configuration. Such a MHS equilibria is used in a high plasma-beta reconnection experiment. An anomalous resistivity ensures that only the central strong current in the separator current layer is dissipated. The reconnection occurs in two phases characterised by fast and slow reconnection, respectively. Waves, launched from the diffusion site, communicate the loss of force balance at the current layer and set up flows in the system. The energy transport in this system is dominated by Ohmic dissipation. Several methods are presented which allow a low plasma-beta value to be approached in the single-separator model. One method is chosen and this model is relaxed non-resistively to form a MHS equilibrium. A twisted current layer grows along the separator, containing stronger current than in the high plasma-beta experiments, and has a local enhancement in pressure inside it. The growth rate of this current layer is similar to that found in the high plasma-beta experiments, however, the current layer becomes thinner and narrower over time.

The need for smaller and more efficient power supplies steadily grows. Many power supplies incorporate high-frequency dc-to-dc switching converters to meet these demands. Recently, a new class of switching converters has been introduced which can operate at very high frequencies to further reduce size and increase efficiency. They are called quasi-resonant converters. Previously, the dc characteristics of many of these converters had been determined, assuming ideal components and circuit operating conditions. However, as the frequency of operation increases, the circuit behavior becomes less ideal causing changes in the expected characteristics. This is because resistive losses, semiconductor junction capacitances, and other parasitic (undesirable) elements become more pronounced at higher frequencies.

This thesis investigates the effects of parasitic elements on the dc characteristics of several zero-current-switched, buck-derived quasi-resonant converters. For the quasi-resonant buck converter, it is demonstrated that for certain operating conditions the dc voltage gain can increase when parasitic losses are increased. Design guidelines are given for maximizing this converterâ s efficiency. Various forward quasi-resonant topologies are investigated, and the effects of parasitic elements on circuit operation are highlighted. A dc analysis is performed for the secondary-resonance forward converter, which has not previously been analyzed. This converter can operate either in full-wave or half-wave mode. Its dc voltage gain in full-wave mode is less sensitive to load variations than { other resonant forward topologies that only operate in ha1f-wave mode.

Master of Science

Paralyzed muscle fatigues more quickly than intact muscle. The reason for this difference is currently unknown. This work will bridge this gap in knowledge by evaluating the predictive abilities of higher-resolution closed-form mathematical models of muscle force and fatigue. Knowledge garnered from this effort will suggest possible mechanisms for the differences in fatiguability of muscle in different states of health. The hypothesis to be tested is that the concept missing from present models, and thus the present understanding of the physiology, is the dynamic behavior of divalent calcium (Ca2+) during induced muscle contraction. If the behavior of Ca2+ can be understood as a Riccati-Bass diffusion process, muscle force and low-frequency fatigue in paralyzed muscle can be more accurately predicted over the time course of response to neuromuscular electrical stimulation. The abilities of existing mathematical models to predict force and low-frequency fatigue are compared to the predictive abilities of new models that include the Riccati-Bass equation. There are several major findings of this study. First, it was found that the structure of the Conaway models better predicts force and low-frequency fatigue than do the Ding models. Second, the cross-bridge friction is the most influential factor in generating force in fresh muscle at frequencies greater than 5 pps. Finally, the calcium leak current is most influential in low-frequency fatigue in paralyzed muscle. It is concluded that the process of muscle fatigue occurs as calcium channel remodeling and inactivation of excitation-contraction coupling from ionic crowding accelerate with every additional contraction.

Ocean tides worldwide have exhibited secular changes in the past century, simultaneous with a global secular rise in mean sea level (MSL). The combination of these two factors contributes to higher water levels, and may increase threats to coastal regions and populations over the next century. Equally as important as these long-term changes are the short-term fluctuations in sea levels and tidal properties. These fluctuations may interact to yield locally extreme water level events, especially when combined with storm surge. This study, presented in three parts, examines the relationships between tidal anomalies and MSL anomalies on yearly and monthly timescales, with a goal of diagnosing dynamical factors that may influence the long-term evolution of tides in the Pacific Ocean. Correlations between yearly averaged properties are denoted tidal anomaly trends (TATs), and will be used to explore interannual behavior. Correlations of monthly averaged properties are denoted seasonal tidal anomaly trends (STATs), and are used to examine seasonal behavior. Four tidal constituents are analyzed: the two largest semidiurnal (twice daily) constituents, M2 and S2, and the two largest diurnal (once daily) constituents, K1 and O1.

Part I surveys TATs and STATs at 153 Pacific Ocean tide gauges, and discusses regional patterns within the entire Pacific Ocean. TATs with statistically significant relations between MSL and amplitudes (A-TATs) are seen at 89% of all gauges; 92 gauges for M2, 66 for S2, 82 for K1, and 59 for O1. TATs with statistically significant relations between tidal phase (the relative timing of high water of the tide) and MSL (P-TATs) are observed at 55 gauges for M2, 47 for S2, 42 for K1, and 61 for O1. Significant seasonal variations (STATs) are observed at about a third of all gauges, with the largest concentration in Southeast Asia. The effect of combined A-TATs was also considered. At selected stations, observed tidal sensitivity with MSL was extrapolated forward in time to the predicted sea level in 2100. Results suggest that stations with large positive combined A-TATs produce total water levels that are greater than those predicted by an increase in MSL alone, increasing the chances of high-water events.

Part II examines the mechanisms behind the yearly (TAT) variability in the Western Tropical Pacific Ocean. Significant amplitude TATs are found at more than half of 26 gauges for each of the two strongest tidal constituents, K1 (diurnal) and M2 (semidiurnal). For the lesser constituents analyzed (O1 and S2), significant trends are observed at ten gauges.

Part III analyzes the seasonal behavior of tides (STATs) at twenty tide gauges in the Southeast Asian waters, which exhibit variation by 10 – 30% of mean tidal amplitudes. A barotropic ocean tide model that considers the seasonal effects of MSL, stratification, and geostrophic and Ekman velocity is used to explain the observed seasonal variability in tides due to variations in monsoon-influenced climate forcing, with successful results at about half of all gauges. The observed changes in tides are best explained by the influence of non-tidal velocities (geostrophic and Ekman), though the effect of changing stratification is also an important secondary causative mechanism.

From the results of these surveys and investigations, it is concluded that short-term fluctuations in MSL and tidal properties at multiple time scales may be as important in determining the state of future water levels as the long-term trends. Global explanations for the observed tidal behavior have not been found in this study; however, significant regional explanations are found at the yearly time scale in the Solomon Sea, and at the seasonal time scale in Southeast Asia. It is likely that tidal sensitivity to annual and seasonal variations in MSL at other locations also are driven by locally specific processes, rather than factors with basin-wide coherence. (Abstract shortened by ProQuest.)

Краснов, Р. В. Підвищення надійності електродвигунів компресорів електропоїздів постійного струму : авт. дис. к. т. н.: 05.22.09 / Р. В. Краснов ; Дніпропетр. нац. ун-т залізн. трансп. ім. акад. В. Лазаряна. - Д., 2012.
UK: АНОТАЦІЯ Дисертація присвячена підвищенню надійності електродвигунів компресорів електрорухомого складу шляхом забезпечення нормованих значень нагрівання ізоляції в реальних умовах експлуатації і удосконалення системи захисту якірно-го кола електродвигуна від надмірних струмів, які можуть викликати нагрівання ізоляції більше допустимих значень. Пропонується, на відміну від попередніх досліджень, процес пуску розгля-дати таким, що складається з двох етапів. В роботі проведено дослідження по визначенню значень струмів на кожному з цих етапів і впливу тривалості етапів на нагрівання ізоляції. Для теоретичних досліджень автором запропонована ма-тематична модель, адекватність якої підтверджена тим, що різниця між параме-трами, які отримані з допомогою цієї моделі та експериментально (шляхом ос-цилографування), знаходиться в межах до 5 %. Отримані залежності тривалості першого та другого етапів пуску, а також значень струму якоря на цих етапах при різних умовах: різних значеннях на-пруги в контактній мережі, температурі навколишнього середовища, тиску в пневмомережі та значенням моменту на валу двигуна після закінчення пуску. Експериментально та теоретично доведено, що в окремих випадках захист електродвигуна з допомогою теплового реле ТРВ-8,5 не забезпечується. З допомогою теплової моделі досліджені процеси нагрівання ізоляції обмо-тки якоря під час пуску та подальшої роботи електродвигуна компресора при різних можливих режимах та умовах. Результати досліджень, що отримані за допомогою математичної та тепло-вої моделей використані для програмування нового пристрою захисту електро-двигуна компресора від перенавантажень. Запропонований пристрій захисту безвідмовно протягом декількох років працює на реальному електропоїзді. RU: АННОТАЦИЯ Диссертация посвящена повышению надежности электродвигателей ком-прессоров электроподвижного состава путём обеспечения рациональных значе-ний нагревания изоляции в реальных условиях эксплуатации и усовершенство-вания системы защиты якорной цепи электродвигателя от чрезмерных токов, которые могут вызвать нагревание изоляции больше нормированных значений. Проведен анализ выхода из строя электродвигателей компрессоров на электропоездах постоянного тока серии ЭР-1, 2 по причине пробоя изоляции обмоток якоря и полюсов. Как показывает обзор литературы по теме диссерта-ционной работы, проведенный автором, до теперешнего времени влияние про-цесса пуска на нагревание изоляции обмоток якоря и полюсов в полной мере не исследовано, особенно это относится к рассмотрению процесса пуска как тако-вого. Автор, в отличие от предыдущих исследований, предлагает процесс пуска рассматривать состоящим из двух этапов. Первый этап пуска – напряжение на электродвигатель подано, а его вал не вращается в течении некоторого времени. Второй этап – вал электродвигателя трогается и начинает набирать обороты до установившегося значения угловой скорости. В работе проведено исследование по определению значений токов на каж-дом из этих этапов и влиянию длительности этапов на нагревание изоляции. Для этого вначале проведено экспериментальное исследование процессов пуска электродвигателя компрессора на реальном электропоезде путем осциллогра-фирования этого процесса с помощью современного измерительного комплекса с исользованием датчиков напряжения (тип LEM CV4-6000) и тока (тип LEM LT100-8/SP30). Для теоретических исследований автором предложена математическая мо-дель, адекватность которой подтверждена тем, что расхождение параметров, полученных путем осциллографирования и с помощью математической модели находится в пределах до 5 %. Для получения численных значений при исследо-вании процессов пуска на указанных двух этапах с помощью математической модели используется метод Рунге-Кутта-Фелберга. Получены зависимости тока якоря на первом и втором этапах пуска при разных условиях: разных значениях напряжения в контактной сети, температу-ры окружающей среды, давления в пневматической сети. Получены также зависимости длительности первого и второго этапов пус-ка от указанных условий. Установлено, что в реальных условиях эксплуатации длительность первого этапа пуска составляет от 19 до 74 мс, однако, несмотря на относительно малое значение, токи, возникающие на первом этапе, существенно влияют на эффек-тивное значение тока за всё время пуска и соответственно существенно влияют на нагревание изоляции. Так увеличение длительности первого этапа пуска до 30 % по сравнению с пуском электродвигателя при номинальном напряжении в контактной сети и нормальных внешних условиях увеличивает эквивалентный ток за всё время пуска до 9,1 %. Установлены зависимости между длительностью первого этапа и значени-ем момента на валу двигателя после окончания пуска. Экспериментальные и теоретические исследования показали, что в отдель-ных случаях защита электродвигателя с помощью теплового реле ТРВ-8,5 не обеспечивается. С помощью тепловой модели исследованы процессы нагревания изоляции обмотки якоря во время пуска и дальнейшей работы электродвигателя компрес-сора при номинальной нагрузке на его валу и при бо́льших нагрузках, что воз-можно при реальных условиях эксплуатации. Доказано, что в эксплуатации возможны режимы работы, при которых из – за перегрева изоляции срок её службы снижается до 4,8 лет, что соответствует статистическим данным. Установлена зависимость времени, за которое протекание тока определён-ного значения приводит к максимально допустимому нормированному нагре-ванию изоляции обмоток якоря от этого тока при наиболее неблагоприятной температуре окружающей среды – +40 ˚С. Эта зависимость использована для программирования предложенного автором нового устройства защиты якорных цепей на базе современных микроконтроллеров. Предложенное автором устройство защиты от перегрузок электродвигателя компрессора безотказно в течение нескольких лет работает на реальном элек-тропоезде. Проведено технико – экономическое обоснование внедрения результатов работы. EN: ABSTRACT The dissertation paper is devoted to improving the reliability of electromotor compressors of rolling stock by providing normative values of heat insulation under real operating conditions and improve protection anchor range from excessive elec-tric currents that can cause heat insulation more acceptable values. Proposed, in contrast to previous studies, the process of starting to consider as being composed of two stages. In this paper a study to determine the values of currents at each of these phases and the influence of the duration of the stages of heat insulation. For theoretical investigations of the author to proposed mathematical model, whose adequacy is confirmed that the difference between the parameters ob-tained using this model and experimentally (by oscillography) is within 5%. Dependences of the duration of the first and second stages of start-up and current values of anchor on these stages under different conditions: different values of voltage in the contact system, ambient temperature, air pressure in the pneumatic system and the value of the moment on the shaft of the electric motor after starting. Experimentally and theoretically shown that in some cases protection of the electric motor with thermal relay ТРВ-8, 5 is not provided. With the thermal model the processes of heat insulation of armature winding during start-up and subsequent operation of the electric motor of compressor for various possible modes and conditions are investigated. The research results, obtained by using mathematical and thermal models, used for programming a new protection device of electric motor compressor from overload. The proposed protection device flawlessly for several years working on real electric rolling stock.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in ful lment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 2015
During a cloud to ground lightning event a charge centre within the storm cloud is discharged. The discharge of a charge centre within the cloud leads to a change in the electric eld radiated by the charge centre. It is theoretically possible to infer the lightning current from the derivative of the electric eld. It is only possible to infer the lightning current from the electric eld data where the noise is comparatively much smaller than the electric eld data. The changing electric elds for a lightning event that occurred on the 3rd January 2015 13:15:13 were recorded by a at plate electric eld sensor with a passive integrator. The oscilloscope used to capture the electric eld data has a relatively large measurement noise and a low resolution. A low pass digital lter was applied to the recorded electric eld data to reduce the e ects from the high frequency noise. The lightning strokes were recorded by the South African Lightning Detection Network. The Lightning Detection Network data is used to obtain the distance of the lightning event from the sensor, to scale the inferred lightning current. The Lightning Detection Network also provides a lightning peak current measurement to compare to the peak current inferred from the electric eld data. The lightning stroke current was inferred from the electric eld recording for various bandwidths of the low pass lter. Inconsistent changes to the inferred lightning stroke current as the lter bandwidth is changed shows that the frequency components for each stroke di ers. The peak stroke current was not constant for any lter bandwidth range implying that the measurement noise is relatively too large. The case study presented demonstrates that with a relatively large noise magnitude (3 to 4 discrete steps of the digital recording) compared to the electric eld signal (21 discrete steps) it is di cult to accurately infer the lightning current from the electric elds recorded.
MT2017

by Yeung Wai Lung.
Thesis (M.Phil.)--Chinese University of Hong Kong, 2002.
Includes bibliographical references (leaves 120-122).
Text in English; abstracts in English and Chinese.
by Yeung Wai Lung.
Abstract --- p.i
Abstract (Chinese version) --- p.ii
Acknowledgements --- p.iii
Contents --- p.iv
List of figures --- p.vii
List of tables --- p.xi
Chapter 1 --- Introduction --- p.1
Chapter 2 --- The El Nino ´ؤ Southern Oscillation Phenomenon --- p.3
Chapter 2.1 --- Introduction to El Nino - Southern Oscillation --- p.3
Chapter 2.2 --- The mean conditions of the Tropical Ocean --- p.4
Chapter 2.3 --- Life Cycle of ENSO --- p.8
Chapter 2.4 --- Understanding ENSO --- p.12
Chapter 3 --- ENSO prediction schemes --- p.18
Chapter 3.1 --- Introduction --- p.18
Chapter 3.2 --- Statistical prediction models --- p.19
Chapter 3.2.1 --- Introduction --- p.19
Chapter 3.2.2 --- Atmospheric models --- p.19
Chapter 3.2.3 --- Oceanic models --- p.22
Chapter 3.3 --- Coupled ocean-atmosphere models --- p.24
Chapter 3.3.1 --- Introduction --- p.24
Chapter 3.3.2 --- Intermediate coupled models --- p.25
Chapter 3.3.3 --- Hybrid coupled models --- p.28
Chapter 3.3.4 --- Coupled general circulation models --- p.30
Chapter 3.4 --- Skill sensitivities --- p.36
Chapter 3.4.1 --- Introduction --- p.36
Chapter 3.4.2 --- Dependence of skill on season --- p.36
Chapter 3.4.3 --- Dependence of skill on decade --- p.37
Chapter 3.4.4 --- Dependence of skill on the Phase of the ENSO cycle --- p.38
Chapter 4 --- The ENSO model --- p.40
Chapter 4.1 --- Introduction --- p.40
Chapter 4.2 --- Models formalism --- p.40
Chapter 4.2.1 --- Oceanic model --- p.40
Chapter 4.2.2 --- Atmospheric model --- p.43
Chapter 4.2.3 --- Coupling of atmospheric and oceanic models --- p.44
Chapter 4.3 --- Numerical implementation --- p.45
Chapter 4.3.1 --- Oceanic model --- p.45
Chapter 4.3.2 --- Atmospheric model --- p.49
Chapter 4.3.3 --- Averaging and interpolation during coupling --- p.52
Chapter 4.4 --- Standard response of the model --- p.54
Chapter 4.5 --- Limitation of the model --- p.57
Chapter 5 --- Behavior of the ENSO model --- p.59
Chapter 5.1 --- Introduction --- p.59
Chapter 5.2 --- Numerical results of the model --- p.60
Chapter 5.3 --- Effects of the parameters --- p.62
Chapter 5.3.1 --- Introduction --- p.62
Chapter 5.3.2 --- Coupling constant parameters --- p.62
Chapter 5.3.3 --- Rayleigh friction --- p.66
Chapter 5.3.4 --- The oceanic Kelvin wave speed --- p.69
Chapter 5.3.5 --- The ocean basin length --- p.69
Chapter 5.3.6 --- Reflections at the western boundary --- p.74
Chapter 5.4 --- Modification of the model background state --- p.76
Chapter 5.4.1 --- Introduction --- p.76
Chapter 5.4.2 --- The results of the modified model --- p.77
Chapter 5.4.3 --- Sensitivity of the western boundary of the modified model --- p.81
Chapter 5.4.4 --- Parameters sensitivities of the modified model --- p.83
Chapter 5.5 --- Conclusion and discussion --- p.87
Appendix A --- p.91
Appendix B --- p.112
Bibliography --- p.120

A simple atmosphere and ocean model of relevance to El Nino and Southern Oscillation (ENSO) is discussed. Both the atmosphere and ocean models are two layer, three dimensional, linear and baroclinic, and generally follow the Oregon State University coupled general circulation model. However, the parameterization differs considerably from previous work in the treatment of the atmospheric latent heat release. This new parameterization follows the formula used in the theory of conditional instability of the second kind (CISK). In this the latent heat release is proportional to the low level convergence. Utilizing the "Comprehensive Ocean Atmosphere Data Set" (COADS), which contains all oceanic and atmospheric surface variables over the global ocean from 1946-1979, experimental model results are discussed for determination of the validity of the parameterizations. In particular, the years 1957, 1965 and 1972 in which El Nino events occurred are examined. The parameterization is deemed to be realistic, and should permit simulation of the El Nino upon coupling the two models.
Graduation date: 1988

For the purpose of investigating the fundamental nature of the interannual oscillation observed in the tropical Pacific in conjunction with El Nino/Southern Oscillation (ENSO), the Comprehensive Ocean Atmosphere Data Set was analyzed. Based upon this analysis, an "intermediate" tropical atmosphere-ocean coupled model was developed. For reason of their particular importance to atmosphere-ocean interactions, moisture processes such as condensation-convergence, evaporation-sea surface temperature and evaporation-wind feedbacks were given special emphasis in both the data analysis and modeling processes. The atmospheric component of the model is based on the first baroclinic mode, which is driven by the atmospheric internal heating. The oceanic model consists of two layers with an imbedded oceanic mixed-layer, by which SST is predicted. The present study has demonstrated that despite the structural simplicity of the oceanic model, it is capable of simulating mean oceanic circulation. In preparation for coupling, individual models were first tested with the use of appropriate time-dependent boundary conditions specified from the composite ENSO data. Both model simulations reproduced the major features associated with the ENSO events. Coupling was performed following the imposition of wind stress anomalies over the western Pacific for a given time period and the removal of all external forcing for a period of eight years thereafter. The coupled responses simulated during the the first two-year period provided reasonable simulations of the following ENSO-like features: the appearance of warm sea-surface temperature anomalies in the central Pacific, deepening/shallowing of mixed-layers in the eastern/western Pacific, weakening of the trades in the central Pacific, strengthening/weakening of the North Equatorial Counter Current/South Equatorial Currents, and enhanced convective activities around the dateline. The long-term coupled integration showed a pattern of interannual oscillation over a period of approximately three years. The results obtained from this coupling study have illustrated (1) that the necessary condition for the interannual oscillation is the interaction between the atmosphere and the ocean and (2) that it is likely that oceanic wave dynamics plays a crucial role in the determination of the growth and decay of ENSO events.
Graduation date: 1991

Switch-mode power supplies (SMPS's) not only convert energy, they also consume it. Typical operational efficiencies are approximately 25 to 60% for linear power supplies, and approximately 50-90% for switching power supplies. This means that products whose end-use electronics are dc, such as televisions and DVD players, could consume 50% less power when operating if the power supply were upgraded from 40% efficiency to 80% efficiency. Savings can occur not only from using SMPS's instead of linear power supplies, but also from specifying highly efficient switching power supplies. In many cases, efficiencies are still lagging to keep costs down, since the power consumption is considered to be relatively low (40W-700W range). Over time, however, efficiency improvement strategies will pay back based on the cost of energy. Therefore three common flyback converter topologies have been studied through this thesis in the Low (15W), Medium (40W), and High (150W) Power levels. Efficiency analysis on the three power level topologies showed that the greatest opportunity for efficiency improvement existed in the 40W (medium power) topology. Efficiency improvement and measurement approaches are investigated and an optimized medium-power flyback converter is proposed and implemented resulting in an efficiency improvement from 57.8% to 83.6%.
Graduation date: 2004

The drying of sugar is the last step in the recovery of solid sugar from sugar-cane. To ensure that the sugar can be transported and stored, the final moisture content leaving the sugar mill must be carefully controlled. Data spanning periods of normal plant operation were collected at the Tongaat-Hulett Ltd Darnall sugar mill. These measurements were reconciled to achieve instantaneous mass and energy balances across the sugar dryer. Using these measurements, a general model has been developed to simulate the sugar drying. It includes ten compartments through which the sugar and drying air flow, with a mass and energy balance in each compartment. It was assumed that a "film" around the sugar crystal is supersaturated, and that crystallisation is still occurring. A sorption isotherm determining the equilibrium moisture content of the sugar, at which point mass transfer ceases, was included. The model has been matched to process measurements by adjusting the heat and mass transfer coefficients. A Dynamic Matrix Controller was developed and tested off-line on the model, using the reconciled measurement sequences. The controller manipulated the inlet air temperature in order to control the exit sugar moisture content. The model predictive control format successfully dealt with the large process dead-time (5 minutes).
Thesis (M.Sc.Eng.)-University of Natal, Durban, 2001.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in ful lment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 2015
This research investigates the e ects of the frequency components of the lightning First Short Stroke (FSS) on the current pathway through human tissues using frequency domain analysis. A Double Exponential Function (DEF) is developed to model the FSS with frequency components in the range 10 Hz 100 kHz. Human tissues are simulated using Finite Element Analysis (FEA) in COMSOL and comprises of two types of models: Single Layer Cylindrical Model (SLCM) and Multi-layered Cylindrical Model (MLCM). The SLCM models 54 human tissues independently and the MLCM models the human leg with ve tissue layers: bone marrow, cortical bone, muscle, blood and fat. Three aspects are analysed: current density, complex impedance and power dissipation. From the SLCM results, aqueous tissues have the lowest impedances and tissue heat dissipation is proportional to tissue impedance. Results from the MLCM show that 85% of the FSS current ows through muscle, 11% ows through blood, 3:5% through fat and the rest through cortical bone and bone marrow. From the results, frequency dependent equivalent circuit models consisting of resistors and capacitors connected in series are proposed. The simulation results are correlated with three main clinical symptoms of lightning injuries: neurological, cardiovascular and external burns. The results of this work are applicable to the analysis of High Voltage (HV) injuries at power frequencies.
MT2017

Indiana University-Purdue University Indianapolis (IUPUI)
This research introduces a new design of a converter modified from SEPIC converter (Single end primary inductive converter), capable of generating desired voltage levels and polarities. The new switching converter topology allows for boost and buck of the input voltage theoretically achieving infinite positive and negative voltage levels. The proposed topology utilizes single high frequency switch to perform the power conversion which simplifies the design of the gate driver, but meanwhile, it still retains the ability to provide a wide range of output voltage. Mathematical modeling of the converter and computer simulations are validated by experimental data. To verify its performance a prototype was designed and built. It is experimentally proven that the circuit can generate a desired voltage in the range of voltages up to ±170 V, delivering 480 Watts of power to a resistive load.

The G20 leaders’ forum was created at a time when the global financial system was on a precipice. Credit markets were freezing. Stock markets were collapsing. Rolling failures across financial institutions were shattering economic confidence. In their first communique, leaders concluded that a root cause of the crisis was insufficient macroeconomic cooperation. It was only through improved cooperation, they warned, that a global depression could be averted. The thesis explores the impact of macroeconomic policy cooperation within the G20 since 2008 and what it means for the challenges facing the world today. It poses four central questions. Does the G20 influence the domestic macroeconomic policies of its members? What are the economic benefits of macroeconomic cooperation and under what circumstances do they arise? Does the G20 produce political benefits that encourage cooperation? Has the G20 been successful in its attempts at macroeconomic cooperation in the past? The thesis attempts to answer these questions using data analysis, a new G20-version of a general equilibrium model and the results from in-depth interviews with 63 leaders, central banks governors, finance ministers and officials from across all G20 countries, including Kevin Rudd, Janet Yellen, Haruhiko Kuroda, Ben Bernanke, Jack Lew, Mark Carney, Phil Lowe and 56 others. The thesis looks at the six areas that make up the G20’s history on macroeconomic cooperation since 2008 – a chronology of the G20’s first 10 years as a forum for leaders -and their relevance to the challenges facing the world in 2018: the economic and political benefits of coordinating fiscal stimulus (Chapter 3), fixing the inadequacies in the world’s crisis-fighting mechanisms (Chapter 4), helping countries to reduce debt and deficits (Chapter 5), strengthening monetary policy cooperation (Chapter 6), reducing global current account imbalances (Chapter 7) and promoting coordinated structural reform (Chapter 8). Chapter 9 summarises the findings of the thesis. Fundamentally, the thesis demonstrates the benefits and influence of global macroeconomic policy cooperation and the cost of abandoning it. The thesis shows that the G20 does influence domestic macroeconomic policies, that the benefits from cooperation can be substantial, that the G20 can help politicians undertake difficult reforms and that the G20 has a strong record of success and remains a critical part of global economic governance. Using the interview results, the thesis builds a new framework on the influence of global forums on domestic policies. It explores how this influence varies depending on the form of cooperation (such as information sharing), the transmission mechanism of influence (such as generating peer pressure) and the variables that determine the strength of that influence (such as a crisis). Using the general equilibrium modelling results, it explores when there is a case for cooperation and the benefits that can flow from it. The gains to GDP, for example, can be 120 per cent larger if fiscal stimulus is coordinated than if it is not. It shows that coordination can help reduce global current account imbalances, can help countries to reduce debt and deficits, can help prevent currency wars and protectionism and, by coordinating macroeconomic resources through international crisis-response institutions, can prevent crises occurring and respond faster when they do. Finally, the thesis uses the results from each chapter to review the overall success of the G20 on macroeconomic cooperation, including where it has succeeded, where it has fallen short and what factors influence its success. It shows that the G20 has had a good track record in achieving its goals and that criticisms of the G20 often overlook its shift from crisis response to focusing on more difficult structural and institutional issues. The thesis concludes by presenting a forward-looking agenda for the G20 to improve macroeconomic cooperation in the future, particularly in countering the current backlash against globalisation and multilateral cooperation while addressing some of its root causes.

Indiana University-Purdue University Indianapolis (IUPUI)
The substantial increase demand for electrical energy requires high efficient apparatus dealing with energy conversion. Several technologies have been suggested to implement power supplies with higher efficiency, such as multilevel and interleaved converters. This thesis proposes an energy conversion unit with an optimized number of output voltage levels per number of switches nL=nS. The proposed five-level four-switch per phase converter has nL=nS=5/4 which is by far the best relationship among the converters presented in technical literature. A comprehensive literature review on existing five-level converter topologies is done to compare the proposed topology with conventional multilevel converters. The most important characteristics of the proposed configuration are: (i) reduced number of semiconductor devices, while keeping a high number of levels at the output converter side, (ii) only one DC source without any need to balance capacitor voltages, (iii) high efficiency, (iv) there is no dead-time requirement for the converters operation, (v) leg isolation procedure with lower stress for the DC-link capacitor. Single-phase and three-phase version of the proposed converter is presented in this thesis. Details regarding the operation of the configuration and modulation strategy are presented, as well as the comparison between the proposed converter and the conventional ones. Simulated results are presented to validate the theoretical expectations. In addition a fault tolerant converter based on proposed topology for micro-grid systems is presented. A hybrid pulse-width-modulation for the pre-fault operation and transition from the pre-fault to post-fault operation will be discussed. Selected steady-state and transient results are demonstrated to validate the theoretical modeling.