Les nanofils magnétiques suscitent un intérêt considérable depuis une quinzaine d'années en raison de leur utilisation potentielle pour la spintronique. Leur utilisation potentielle dans des dispositifs exige une description détaillée des états magnétiques locaux des nanofils. Dans cette thèse, j'ai étudié qualitativement et quantitativement les états magnétiques à l'état rémanent de nanofils magnétiques par holographie électronique (EH) et simulations micromagnétiques. Une analyse détaillée a été réalisée sur deux types de nanofils : multicouches Co/Cu et nanofils FeCoCu à diamètre modulé. Les deux systèmes ont été synthétisés par électrodéposition dans des membranes. La combinaison des caractérisations magnétiques, structurales et chimiques locales obtenues dans un TEM avec des simulations micromagnétiques ont permis une description complète de ces systèmes. Pour les nanofils multicouches Co / Cu, j'ai analysé l'influence des épaisseurs de cobalt et de cuivre ou de la structure cristalline de Co sur la configuration magnétique de nanofils isolés. Après l'application d'un champs de saturation dans des directions parallèle et perpendiculaire à l'axe des nanofils, j'ai étudié les configurations magnétiques pour les épaisseurs de Co / Cu suivantes : 25nm / 15nm, 25nm / 45nm, 50nm / 50nm et 100nm / 100nm. Trois configurations principales à la rémanence ont été trouvées : (i) un couplage antiparallèle entre les couches Co, (ii) une structure mono-domaine et (iii) un état vortex. Dans les nanofils Co (25 nm) / Cu (15 nm), en fonction de la direction du champ de saturation, les couches de Co peuvent présenter soit un couplage antiparallèle (champ de saturation perpendiculaire) ou un couplage de type vortex (champ de saturation en parallèle) avec un coeur aligné parallèlement à l'axe du fil. Cependant, 10% des nanofils étudié présente un état mono-domaine quel que soit le champ de saturation parallèle et perpendiculaire. Dans le cas Co (50 nm) / Cu (50 nm) et Co (25 nm) / Cu (45 nm), l'épaisseur plus grande de Cu séparant les couches ferromagnétiques réduit l'interaction magnétique entre des couches de Co voisines. L'état rémanent est donc formé de la combinaison de couches de Co monodomaines orientés perpendiculairement à l'axe du fil et de certains états vortex. Enfin pour la configuration Co (100 nm) / Cu (100 nm), un état monodomaine est observé quel que soit la direction du champ appliqué lors de la saturation. Toutes ces configurations magnétiques ont été déterminées et simulées à l'aide des calculs micromagnétiques jusqu'à ce qu'un accord quantitatif avec les résultats expérimentaux aient été obtenus. J'ai ainsi pu expliquer l'apparition et la stabilité de ces configurations en fonction des principaux paramètres magnétiques tels que l'échange, la valeur et la direction de l'anisotropie et l'aimantation. La comparaison entre les simulations et les résultats expérimentaux ont ainsi servi à déterminer précisément la valeur de ces paramètres. Dans les nanofils FeCoCu à diamètre modulé, une description détaillée de l'influence de la géométrie sur la configuration locale de spins a été réalisée. Les expériences d'holographie électronique montrent une structure magnétique monodomaines avec l'aimantation alignée longitudinalement. Cependant, nous avons trouvé grâce à des simulations micromagnétiques que cette configuration monodomaine est fortement affectée par la variation locale du diamètre. L'étude en particulier du champ de fuite mais aussi du champ démagnétisant à l'intérieur des nanofils a mis en évidence le rôle prépondérant des charges magnétiques aux zones de variation de diamètre. De plus l'aimantation présente une structure plus compliquée qu'un simple alignement le long de l'axe du fil. Enfin les résultats que j'ai obtenus ont abouti à une interprétation différente d'expériences précédentes en MFM. / Magnetic nanowires have raised significant interest in the last 15 years due to their potential use for spintronics. Technical achievements require a detailed description of the local magnetic states inside the nanowires at the remnant state. In this thesis, I performed quantitative and qualitative studies of the remnant magnetic states on magnetic nanowires by Electron Holography (EH) experiments and micromagnetic simulations. A detailed investigation was carried out on two types of nanowires: multilayered Co/Cu and diameter-modulated FeCoCu nanowires. Both systems were grown by template-based synthesis using electrodeposition process. The combination of local magnetic, structural and chemical characterizations obtained in a TEM with micromagnetic simulations brought a complete description of the systems. In the multilayered Co/Cu nanowires, I analysed how different factors such as the Co and Cu thicknesses or the Co crystal structure define the remnant magnetic configuration into isolated nanowires. After applying saturation fields along directions either parallel or perpendicular to the NW axis, I studied multilayered Co/Cu nanowires with the following relative Co/Cu thickness layers: 25nm/15nm, 25nm/45nm, 50nm/50nm, and 100nm/100nm. Three main remnant configurations were found: (i) antiparallel coupling between Co layers, (ii) mono-domain-like state and (iii) vortex state. In the Co(25 nm)/Cu(15 nm) nanowires, depending on the direction of the saturation field, the Co layers can present either an antiparallel coupling (perpendicular saturation field) or vortex coupling (parallel saturation field) with their core aligned parallel to the wire axis. However, 10% of the nanowires studied present a mono-domain-like state that remains for both parallel and perpendicular saturation fields. In the Co(50 nm)/Cu(50 nm) and Co(25 nm)/Cu(45 nm) nanowires, a larger Cu thickness separating the ferromagnetic layers reduces the magnetic interaction between neighbouring Co layers. The remnant state is hence formed by the combination of monodomain Co layers oriented perpendicularly to the wire axis and some tilted vortex states. Finally for the Co(100 nm)/Cu(100 nm) nanowires a monodomain-like state is found no matters the direction of the saturation field. All these magnetic configurations were determined and simulated using micromagnetic calculations until a quantitative agreement with experimental results has been obtained. I was able to explain the appearance and stability of these configurations according to the main magnetic parameters such as exchange, value and direction of the anisotropy and magnetization. The comparison between simulations and experimental results were used to precisely determine the value of these parameters. In the diameter-modulated cylindrical FeCoCu nanowires, a detailed description of the geometry-induced effect on the local spin configuration was performed. EH experiments seem to reveal that the wires present a remnant single-domain magnetic state with the spins longitudinally aligned. However, we found through micromagnetic simulations that such apparent single-domain state is strongly affected by the local variation of the diameter. The study of the leakage field and the demagnetizing field inside the nanowire highlighted the leading role of magnetic charges in modulated areas. The magnetization presents a more complicated structure than a simple alignment along the wire axis. Finally my results have led to a new interpretation of previous MFM experiments.

We extend the concept of central configurations to the N-body problem in spaces of nonzero constant curvature. Based on the work of Florin Diacu on relative equilib- ria of the curved N-body problem and the work of Smale on general relative equilibria, we find a natural way to define the concept of central configurations with the effective potentials. We characterize the ordinary central configurations as constrained critical points of the cotangent potential, which helps us to establish the existence of ordi- nary central configurations for any given masses. After these fundamental results, we study central configurations on H2, ordinary central configurations in S3, and special central configurations in S3 in three separate chapters. For central configurations on H2, we generalize the theorem of Moulton on geodesic central configurations, the theorem of Shub on the compactness of central configurations, the theorem of Conley on the index of geodesic central configurations, and the theorem of Palmore on the lower bound for the number of central configurations. We show that all three-body central configurations that form equilateral triangles must have three equal masses. For ordinary central configurations in S3, we construct a class of S3 ordinary central configurations. We study the geodesic central configurations of two and three bodies. Three-body non-geodesic ordinary central configurations that form equilateral trian- gles must have three equal masses. We also put into the evidence some other classes of central configurations. For special central configurations, we show that for any N ≥ 3, there are masses that admit at least one special central configuration. We then consider the Dziobek special central configurations and obtain the central con- figuration equation in terms of mutual distances and volumes formed by the position vectors. We end the thesis with results concerning the stability of relative equilibria associated with 3-body special central configurations. We find that these relative equilibria are Lyapunov stable when confined to S1, and that they are linearly stable on S2 if and only if the angular momentum is bigger than a certain value determined by the configuration. / Graduate

In recent times, high power density trends and temperature constraints in integrated circuits have led to conventional cooling techniques not being sufficient to meet the thermal requirements. The ever-increasing desire to overcome this problem has led to worldwide interest in micro heat sink design of electronic components. It has been found that geometric configurations of micro heat sinks play a vital role in heat transfer performance. Therefore, an effective means of optimally designing these heat sinks is required. Experimentation has extensively been used in the past to understand the behaviour of these heat extraction devices. Computational fluid dynamics (CFD) has more recently provided a more cost-effective and less time-consuming means of achieving the same objective. However, in order to achieve optimal designs of micro heat sinks using CFD, the designer has to be well experienced and carry out a number of trial-and-error simulations. Unfortunately, this will still not always guarantee an accurate optimal design. In this dissertation, a design methodology which combines CFD with a mathematical optimisation algorithm (a leapfrog optimisation program and DYNAMIC-Q algorithm) is proposed. This automated process is applied to three design cases. In the first design case, the peak wall temperature of a microchannel embedded in a highly conductive solid is minimised. The second case involves the optimisation of a double row micropin-fin heat sink. In this case, the objective is to maximise the total rate of heat transfer with the effect of the thermal conductivity also being investigated. The third case extends the micropin-fin optimisation to a heat sink with three rows. In all three cases, fixed volume constraint and manufacturing restraints are enforced to ensure industrial applicability. Lastly, the trends of the three cases are compared. It is concluded that optimal design can be achieved with a combination of CFD and mathematical optimisation. / Dissertation (MEng)--University of Pretoria, 2011. / Mechanical and Aeronautical Engineering / Unrestricted

The traveling salesman problem (TSP) has become a classic in the field of combinatorial optimization. Attracting computer scientists and mathematicians, the problem involves finding a minimum cost Hamiltonian cycle in a weighted graph.

This report is about the implementation of autonomy and control on a water quality measuring unmanned surface vehicle. The project was termed Catfish and involved five teams focusing on different aspects of the initial goal to create an autonomous three-part system; a surface drone, a submerged drone and flying drone. In this iteration of the Catfish project the focus laid on creating the surface drone and submerged drone as the Catfish project will improve over generations of thesis projects. The author of the report was in the Control and Autonomy team and had been tasked with giving the surface drone the autonomy needed to make this project viable. Existing advances made in autonomy was adopted and tested. With the help of estimation algorithms, and sensor fusion, a flight controller navigates the surface drone between a set of GPS waypoints. It is also able to counteract the external forces wind, waves and stream to keep its position. To reach this autonomy four test phases were conducted on a pre-prototype with progressively increased difficult autonomy starting with manual control and ending in advanced autonomy. When the advanced missions were executed the speed and accuracy of two different thruster configurations were examined and the best performing out of the two was implemented on the final prototype the other teams had designed. The project ended with a fully autonomous system that was able to execute all the navigational maneuvers required to operate autonomous water quality measuring missions. / Den här rapporten handlar om implementationen av autonomi och kontroll på en vattenkvalitetsmätande vattenburen drönare. Projektet fick namnet Catfish och blev indelat i fem teams som fokuserade på olika aspekter av ett 3-delsystem; en vattenburen, en undervattens och en flygande drönare. I denna iteration av Catfish projektet fokuserade medlemmarna på att utveckla den vattenburna och undervattens drönaren då projektet kommer fortsätta utvecklas under kommande generationer av Catfish projektrapporter. Författaren av den här rapporten ingick i ett team som hette "Control and Autonomy" och hade i uppgift att installera en autonom intelligens till den vattenburna drönaren för att göra Catfish prototypen användbar. Befintliga framsteg inom forskningsområdet blev granskade och testade. Genom att använda uppskattningsalgoritmer och "sensor fusion" lyckades en "flight controller" navigera drönaren mellan GPS waypoints och även behålla sin position genom att motverka krafterna från vind, vågor och strömmar. För att uppnå denna nivå av autonomi utför en förprototyp fyra test faser av ökad autonomisk svårhetsgrad. Under uppdraget blev hastigheten och precisionen av två olika motoruppsättningar undersöka och den som presterade bäst blev implementerad på den slutgiltig designen som de andra teamen hade utvecklat. Projektet avslutades med att ett fullt autonomt system blev utvecklat som var kapabel till att utföra alla navigationsmanövrar nödvändiga för att genomföra autonoma vattenkvalitetsmätningsuppdrag.

This report investigates how prediction of stock markets with Artificial Neural Networks (ANN) is affected by altering aspects of data quantities. A short-term and a long-term perspective considering time delays are examined. Inspired by neurosciences, ANNs have shown great potential in terms of recognising patterns in nonlinear systems. Existing research suggests that ANN is an eminent model to predicting stock markets due to its dynamical characteristics. Closing prices of large-caps within the sectors of IT and Telecommunication represented by the Swedish of OMX30 Stockholm (OMXS30), have been leveraged as data. The ANNs are implemented as multilayer feedforward networks, trained using supervised learning. To identify specific configurations, the models have undergone extensive testing by mean squared errors and statistical analysis. The results obtained suggest that the short-term perspective is optimally predicted for significantly small numbers of time delays, and that optimal configurations do not alter for increasing quantities of data. No significant conclusions could be drawn from the results for the long-term perspective.Key words: ANOVA, Backpropagation, Configurations, Stock Prediction, Artficial Neural Networks

Several heterogeneous, intelligent and distributed devices can be connected to interact with one another over the internet in what is known as the internet of things (IoT). Also, the concept of IoT can be exploited in the industrial environment for increasing the production output of goods and services and for mitigating the risk of disaster occurrences. This application of IoT for enhancing industrial production is known as industrial IoT (IIoT). More so, the benefits of IoT technology can be particularly exploited across the maritime industry in what is termed the internet of maritime things (IoMT) where sensors and actuator devices are implanted on marine equipment in order to foster the communication efficacy of nautical radio networks. Marine explorations may suffer from unwanted situations such as transactional delays, environmental degradation, insecurity, seaport congestions, accidents and collisions etc, which could arise from severe environmental conditions. As a result, there is a need to develop proper communication techniques that will improve the overall quality of service (QoS) and quality of experience (QoE) of marine users. To address these, the merits of contemporaneous technologies such as ubiquitous computing, software-defined networking (SDN) and network functions virtualization (NFV) in addition to salubrious communication techniques including emergent configurations (EC), channel estimation (CE) and communication routing protocols etc, can be utilized for sustaining optimal operation of pelagic networks. Emergent configuration (EC) is a technology that can be adapted into maritime radio networks to support the operation and collaboration of IoT connected devices in order to improve the efficiency of the connected IoT systems for maximum user satisfaction. To meet user goals, the connected devices are required to cooperate with one another in an adaptive, interoperable, and homogeneous manner. In this thesis, a survey on the concept of IoT is presented in addition to a review of IIoT systems. The applications of ubiquitous computing and SDN technology are employed to design a newfangled network architecture which is specifically propounded for enhancing the throughput of oil and gas production in the maritime ecosystem. The components of this architecture work in collaboration with one another by attempting to manage and control the exploration process of deep ocean activities especially during emergencies involving anthropogenic oil and gas spillages. Several heterogeneous, intelligent and distributed devices can be connected to interact with one another over the internet in what is known as the internet of things (IoT). Also, the concept of IoT can be exploited in the industrial environment for increasing the production output of goods and services and for mitigating the risk of disaster occurrences. This application of IoT for enhancing industrial production is known as industrial IoT (IIoT). More so, the benefits of IoT technology can be particularly exploited across the maritime industry in what is termed the internet of maritime things (IoMT) where sensors and actuator devices are implanted on marine equipment in order to foster the communication efficacy of nautical radio networks. Marine explorations may suffer from unwanted situations such as transactional delays, environmental degradation, insecurity, seaport congestions, accidents and collisions etc, which could arise from severe environmental conditions. As a result, there is a need to develop proper communication techniques that will improve the overall quality of service (QoS) and quality of experience (QoE) of marine users. To address these, the merits of contemporaneous technologies such as ubiquitous computing, software-defined networking (SDN) and network functions virtualization (NFV) in addition to salubrious communication techniques including emergent configurations (EC), channel estimation (CE) and communication routing protocols etc, can be utilized for sustaining optimal operation of pelagic networks. Emergent configuration (EC) is a technology that can be adapted into maritime radio networks to support the operation and collaboration of IoT connected devices in order to improve the efficiency of the connected IoT systems for maximum user satisfaction. To meet user goals, the connected devices are required to cooperate with one another in an adaptive, interoperable, and homogeneous manner. In this thesis, a survey on the concept of IoT is presented in addition to a review of IIoT systems. The applications of ubiquitous computing and SDN technology are employed to design a newfangled network architecture which is specifically propounded for enhancing the throughput of oil and gas production in the maritime ecosystem. The components of this architecture work in collaboration with one another by attempting to manage and control the exploration process of deep ocean activities especially during emergencies involving anthropogenic oil and gas spillages. On the other hand, CE is a utilitarian communication technique that can be exploited during maritime exploration processes which offer additional reinforcement to the capacities of the nautical radio network. This technique enables the receivers of deep-sea networks to efficiently approximate the channel impulse response (CIR) of the wireless communication channel so that the effects of the communication channel on the transmitting aggregated cluster head information can be proficiently understood and predicted for useful decision-making procedures. Two CE schemes named inter-symbol interference/ average noise reduction (ISI/ANR) and reweighted error-reducing (RER) are designed in this study for estimating maritime channels for supporting the communication performances of nautical radio networks in both severe and light-fading environmental conditions. In the proposed RER method, the Manhattan distance of the CIR of an orthodox adaptive estimator is taken, which is subsequently normalised by a stability constant ɛ whose responsibility is for correcting any potential numerical system instability that may arise during the updating stages of the estimation process. To decrease the received signal error, a log-sum penalty function is eventually multiplied by an adjustable leakage (ɛ ) ̈that provides additional stability to the oscillating channel behaviour. The performance of the proposed RER method is further strengthened and made resilient against channel effects by the introduction of a reweighting attractor that further contracts the mean square error of this proposed estimator. In the ISI/ANR technique, the effects of possible ISI that may arise from maritime transmissions is considered and transformed using a low-pass filter that is incorporated for eliminating the effects of channel noise possible effects of multipath propagation. The RER scheme offered superior CE performances in comparison to other customary techniques such as the adaptive recursive least squares and normalised least mean square method in addition to conventional linear approaches such as least squares, linear minimum mean square error and maximum-likelihood estimation method. The proposed ISI/ANR technique offered an improved MSE performance in comparison to all considered linear methods. Finally, from this study, we were able to establish that accurate CE methods can improve the QoS and QoE of nautical radio networks in terms of network data rate and system outage probability. / Thesis (PhD (Computer Engineering))--University of Pretoria, 2021. / University of Pretoria Doctoral research grant, South African National Research Foundation/Research and Innovation Support and Advancement (NRF/RISA) research grant. Center for Connected Intelligence, Advanced Sensor Networks research group, University of Pretoria. / Electrical, Electronic and Computer Engineering / PhD (Computer Engineering) / Unrestricted

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The purpose of this study was to determine if there were differences in standardized test scores, expressed as percentage passing, in math, reading-language arts, science, and social studies by comparing 6th grade students in K-8 schools with those in 6-8 schools. The data were gathered from an analysis of 6th grade student' scores on the 2006-2007 TCAP standardized assessment test in the state of Tennessee. The relationship between grade configuration (6-8 or K-8) and percent of 6th grade students scoring at the below proficient, proficient, or advanced level in each subject area was examined. The analysis was based on 5 research questions. A t-test for independent samples was used to identify the relationships between the independent variables, configuration of the school (K-8 or 6-8), and the dependent variables, the percent of students scoring below proficient, proficient, or advanced. A chi square analysis was used to identify the relationship between the proportion of K-8 schools meeting AYP versus the proportion of 6-8 schools meeting AYP. The study showed no relationship between grade configuration (6-8 or K-8) and percent of 6th grade students scoring at the below proficient level in math, reading-language arts, and social studies. Similarly, there was not a significant difference between grade configuration (6-8 or K-8) and percent of 6th grade students scoring at the proficient level in math and reading-language arts and the advanced level in math, reading-language arts, and science. However, there was a significant relationship between grade configuration (6-8 or K-8) and percent of 6th grade students scoring at the below proficient level and the proficient level in science and the percent of 6th grade students scoring at the proficient level and advanced level in social studies. In science, a lower percentage of 6th grade students in K-8 schools scored below proficient than did 6th grade students in 6-8 schools. In science, a higher percentage of 6th grade students in K-8 schools scored proficient than did 6th grade students in 6-8 schools. In social studies, a higher percentage of 6th grade students in K-8 schools scored proficient than did 6th grade students in 6-8 schools. However, a higher percentage of 6th grade students in 6-8 schools scored advanced than did 6th grade students in 6-8 schools. The study showed a significant difference in the proportion of K-8 schools meeting AYP versus the proportion of 6-8 schools meeting AYP.

Purpose - The aim of the study is to further bridge the gap regarding adoption of Circular Business Models (CBM) more specifically in this case, the barriers to the adoption of Green Steel CBMs, and how Business Models (BM) can be configured for the adoption of Green Steel CBMs. Method - The research questions in the study were answered through a thematic analysis based on a three-phase data collection consisting of two informal meetings with industry specialists and 25 semi-structured interviews, followed by eleven additional interviews from a Green Steel conference to validate the findings of the semi-structured interviews. Findings - The analysis of the interviews resulted in eight identified barriers towards Green Steel CBM adoption, and eight enabling factors for the configuration of BM components specified for Green Steel CBM adoption. Furthermore, the findings present a framework showcasing how to utilize the enabling factors to overcome the barriers for the facilitation of CBMs in the steel industry.  Theoretical contribution – The study contributes to existing literature by providing a steel industry perspective on the barriers for the adoption of CBMs and presenting a framework for the configuration of BMs to overcome the barriers. The study further contributes with managerial insights to the literature towards the Green Steel transition, in contrast to the existing technical perspectives that previously has been conducted. Practical contribution - The study offers practical guidance for decision-makers in the steel industry and other industries undergoing similar transitions. The framework work as guidance for the configuration and adoption of Green Steel CBMs, highlighting the role of collaborations between stakeholders to enable new revenue streams and economic opportunities. Limitation and future research – The study was conducted with a restricted number of interviews and geographical reach, making the generalizability of the study limited. Future research could broaden the scope to include perspectives from additional regions, industry segments and stakeholders. / Syfte - Syftet med studien är att överbrygga kunskapsgapet gällande adopteringen av cirkulära affärsmodeller, mer specifikt i detta fall, barriärer för adopteringen av cirkulära affärsmodeller för Grönt Stål och hur affärsmodeller kan konfigureras för adopteringen av affärsmodeller för Grönt Stål. Metod - Forskningsfrågorna i studien besvarades genom en tematisk analys baserad på en datainsamling i tre faser bestående av två informella möten med branschspecialister och 25 semistrukturerade intervjuer, följt av elva intervjuer från en konferens om Grönt Stål för att validera resultaten av de semistrukturerade intervjuerna. Resultat - Analysen av intervjuerna resulterade i åtta identifierade barriärer för adopteringen av cirkulära affärsmodeller för Grönt Stål, samt åtta möjliggörande faktorer för konfigurationen av affärsmodells komponenter som specificerats för adopteringen av cirkulära affärsmodeller för Grönt Stål. Ytterligare presenteras resultaten i ett ramverk som visar hur man kan använda de möjliggörande faktorerna för att övervinna barriärerna och främja cirkulära affärsmodeller i stålindustrin. Teoretiskt bidrag – Studien bidrar till befintlig litteratur genom att tillhandahålla ett stålindustriperspektiv på barriärerna för adopteringen av cirkulära affärsmodeller och presentera ett ramverk för konfigurationen av affärsmodeller för att övervinna barriärerna.  Praktiskt bidrag - Studien ger praktisk vägledning för beslutsfattare inom stålindustrin och andra industrier som genomgår liknande omställningar. Ramverket fungerar som vägledning för konfigurationen och adopteringen av cirkulära affärsmodeller för Grönt Stål, och belyser rollen av samarbeten mellan intressenter för att möjliggöra nya affärsmöjligheter.  Avgränsningar och framtida studier – Studien genomfördes med ett begränsat antal intervjuer och geografisk räckvidd, vilket gjorde studiens generaliserbarhet begränsad. Framtida forskning skulle kunna bredda omfattningen till att inkludera perspektiv från ytterligare regioner, branschsegment och intressenter.