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Dissertations / Theses on the topic 'Three-dimensional magnetic field'
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1
Lee, Daniel Thomas. "Three-dimensional topology of the magnetic field in the solar corona." Thesis, University of Central Lancashire, 2018. http://clok.uclan.ac.uk/25371/.
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This thesis investigates the topology of the magnetic field in the solar corona, due to a variety of source configurations and types. To fully understand the complex behaviour of the Sun's magnetic field, it is important to have a complete description of the features present in its structure. The magnetic topologies due to network source configurations are investigated using both the point source description and the continuous source description. A series of case studies involving an emerging bipole in a hexagonal arrangement to simulate a supergranular cell are studied. This has a particular focus on the behaviour of coronal nulls located in the topology, and a particular case may form the underpinning of a model for polar plumes. A new topological feature, called a null-like point, is defined by relaxing the definition of a magnetic null point. Separatix-like surfaces, originating from null-like points, allow quasi-separatrix layers to be found in magnetic topologies due to continuously distributed sources. The squashing factor, Q, is mapped across the source configuration, highlighting the locations of the quasi-separatrix layers. Finally, an algorithm is developed which automatically detects and classifies magnetic events local to X-ray bright points (XBPs). Significant peaks are identified in the gradients of flux curves (positive, negative and absolute flux) local to XBP footpoints, allowing instances of flux emergence and cancellation to be identified and linked to the onset and demise of the XBPs studied. The algorithm correctly classifies 90% of all emergence and cancellation events related to the studied XBPs.
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李浩然 and Ho-yin Lie. "A three dimensional approach for determining the surface magnetic field of strange stars." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2000. http://hub.hku.hk/bib/B31224805.
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Yu, Qin. "Calculation of Three Dimensional Magnetic Field Distribution of Large Air-Core Reactor Coils /." The Ohio State University, 1996. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487935573770259.
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Lie, Ho-yin. "A three dimensional approach for determining the surface magnetic field of strange stars." Hong Kong : University of Hong Kong, 2000. http://sunzi.lib.hku.hk/hkuto/record.jsp?B23234532.
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Haider, Shah Ali 1954. "Three-dimensional electromagnetic vector field computation in lossy magnetic media by finite element method." Diss., The University of Arizona, 1996. http://hdl.handle.net/10150/284325.
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A three dimensional finite element method software package has been developed for solving electromagnetic vector fields in conducting, magnetic materials and has been applied in two dimensions to ferromagnetic filaments and in three dimensions to a sphere. The bulk of this dissertation describes the approach to formulating the problem, choosing a solution routine, developing a method of discretization, verifying the accuracy and characterizing the computational efficiency of the package. Spurious vector solutions, which arise in numerical approximations to three dimensional electromagnetic problems, were eliminated by using a node-based formulation, with modified vector wave equation to ensure that divergence free conditions are satisfied. Conjugate gradient, iterative quasi-minimal residual solver (QMR) with a non-zero matrix element storage scheme expedited computation and reduced memory requirements. An automatic mesh generator for hexahedral elements was developed for discretization. The two dimensional study continued earlier analytical and experimental work on induction heating of multi-filament ferromagnetic strands. The present results demonstrate that coupling between filaments does not occur in two dimensions and is, in fact, a three dimensional effect provided the filaments are not in electrical contact. Furthermore, the accuracy of the solution can be established quantitatively by a single parameter, the ratio of one side of the finite element to the electromagnetic skin (or penetration) depth. The three dimensional parametric study investigates the effects on power absorption patterns in the sphere as a function of conductivity and permeability. Primarily, this research demonstrates that these types of problems can be solved accurately. Finally, it is shown that while the discretization must extend completely throughout the sphere for non-magnetic, moderately lossy media (conductivity, σ∼ 1 S/m, relative permeability, μᵣ ∼1), it need only consist of a thin shell of about three skin depths thick for highly conducting magnetic materials. The core of this sphere can be replaced by a field free inner perfectly conducting sphere. While the problem of computing power absorption in ferromagnetic implants for hyperthermia, the motivation for this study, was not solved completely, the foundations have been laid. Dependence of power absorption upon size, shape, permeability and conductivity as well as interactions between filaments of finite length can be addressed with this beginning.
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Rai, Satish C. "Three Dimensional Nanowire Array Piezo-phototronic and Piezo-photo-magnetotronic Sensors." ScholarWorks@UNO, 2015. http://scholarworks.uno.edu/td/2104.
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Piezotronic and piezo-phototronic is a burgeoning field of study which emerges from the coupling of intrinsic materials properties exhibited by non-centrosymmetric semiconductors. In the past decade research efforts were mainly focused on the wurtzite family of 1D nanostructures, with major emphasis on ZnO nanowire nanogenerators, MS piezotronic transistors, LEDs and photodetectors mainly integrated on single nanowires. In view of previously known advantages of charge carrier separation in radial heterojunctions, particularly in type-II core/shell nanowires, it can be anticipated that the performance of photosensing devices can be largely enhanced by piezo-phototronic effect. Moreover, the performance metrics can be further improved in an array of nanowires where geometrical feature enabled multiple reflection can efficiently trap incident illumination. The crux of this dissertation lies in the development of 3D type-II core/shell nanowire array based piezo-phototronic device and also to investigate the effect of magnetic field on ZnO nanowire arrays based piezotronic and piezo-phototronic device for new class of sensors. In this regard, prototype piezo-phototronic broadband photodetectors integrated on two material systems, namely type-II CdSe/ZnTe 3D core/shell nanowire arrays and fully wide band gap type-II ZnO/ZnS 3D core/shell nanowire arrays have been developed where the photodetection performance of each device exhibits high sensitivity, fast response and large responsivity. The application of piezo-phototronic effect further improves the device performance by three to four orders of magnitude change numerically calculated from absolute responsivities at multiple wavelengths. A 3D ZnO nanowire array based new class of piezo-photo-magnetotronic sensor is also developed for detection of pressure, illumination and magnetic field suggesting multiple functionality of a single device where more than one effect can be coupled together to exhibit piezo-magnetotronic or piezo-photo-magnetotronic type of device behavior.
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Feng, Qiang. "Study of the three dimensional random field Ising model : magnetic X-ray and neutron scattering, magnetization, and heat capacity." Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/39766.
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Cloos, Cai Constantin [Verfasser]. "On the long-time behavior of the three-dimensional Dirac-Maxwell equation with zero magnetic field / Cai Constantin Cloos." Bielefeld : Universitätsbibliothek Bielefeld, 2020. http://d-nb.info/1224313135/34.
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Murray, Michelle Joanne. "Solar flux emergence : a three-dimensional numerical study /." St Andrews, 2008. http://hdl.handle.net/10023/441.
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Brown, Daniel Stephen. "Three-dimensional topology of solar coronal magnetic fields." Thesis, University of St Andrews, 1999. http://hdl.handle.net/10023/14036.
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This thesis investigates the topology of the magnetic field in the solar corona. It is important have an understanding of how the highly complex coronal magnetic field behaves in order to study many fundamental coronal phenomena, such as coronal heating events, solar flares and polar plumes. The magnetic fields due to three or four discrete sources are investigated and the corresponding topological states are found. The locations of these states in parameter space is calculated and the bifurcations between states are analysed. A complete analysis has been undertaken for the three-source case and a selective one for the four-source case in order to identify new non-generic behaviour. The thesis goes on to study the topological behaviour of a coronal bright point. Different phases during the lifetime of the bright point are identified and the responsible topological behaviour due to the movement of the magnetic fragments in the photosphere is discussed.
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Al-Salti, Nasser S. "Aspects of three-dimensional MHD : magnetic reconnection and rotating coronae." Thesis, University of St Andrews, 2010. http://hdl.handle.net/10023/947.
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Solutions of the magnetohydrodynamic (MHD) equations are very important for modelling laboratory, space and astrophysical plasmas, for example the solar and stellar coronae, as well as for modelling many of the dynamic processes that occur in these different plasma environments such as the fundamental process of magnetic reconnection. Our previous understanding of the behavior of plasmas and their associated dynamic processes has been developed through two-dimensional (2D) models. However, a more realistic model should be three-dimensional (3D), but finding 3D solutions of the MHD equations is, in general, a formidable task. Only very few analytical solutions are known and even calculating solutions with numerical methods is usually far from easy. In this thesis, 3D solutions which model magnetic reconnection and rigidly rotating magnetized coronae are presented. For magnetic reconnection, a 3D stationary MHD model is used. However, the complexity of the problem meant that so far no generic analytic solutions for reconnection in 3D exist and most work consists of numerical simulations. This has so far hampered progress in our understanding of magnetic reconnection. The model used here allows for analytic solutions at least up to a certain order of approximation and therefore gives some better insight in the significant differences between 2D and 3D reconnection. Three-dimensional numerical solutions are also obtained for this model. Rigidly rotating magnetized coronae, on the other hand, are modeled using a set of magnetohydrostatic (MHS) equations. A general theoretical framework for calculating 3D MHS solutions outside massive rigidly rotating central bodies is presented. Under certain assumptions, the MHS equations are reduced to a single linear partial differential equation referred to as the fundamental equation of the theory. As a first step, an illustrative case of a massive rigidly rotating magnetized cylinder is considered, which somehow allows for analytic solutions in a certain domain of validity. In general, the fundamental equation of the theory can only be solved numerically and hence numerical example solutions are presented. The theory is then extended to include a more realistic case of massive rigidly rotating spherical bodies. The resulting fundamental equation of the theory in this case is too complicated to allow for analytic solutions and hence only numerical solutions are obtained using similar numerical methods to the ones used in the cylindrical case.
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Bühler, Leo. "Three-dimensional liquid metal flows in strong magnetic fields." Karlsruhe Forschungszentrum Karlsruhe, 2008. http://d-nb.info/991370600/34.
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Williams, Benjamin Matthew. "The dynamic topology of the solar corona : mapping the Sun's three dimensional magnetic skeleton." Thesis, University of St Andrews, 2018. http://hdl.handle.net/10023/14637.
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Observations of the surface of the Sun reveal multi-scaled, mixed magnetic features that carpet the entire solar surface. Not surprisingly, the global magnetic fields extrapolated from these observations are highly complex. This thesis explores the topology of the Sun's global coronal magnetic fields. The magnetic skeleton of a magnetic field provides us with a way of examining the magnetic field and quantifying its complexity. Using specialised codes to find the magnetic skeletons which were written during the course of this work, we first examine potential field extrapolations of the global solar coronal magnetic field determined from observed synoptic magnetograms from the Heliospheric Magnetic Imager on the Solar Dynamics Observatory. The resolution of the PFSS models is found to be very important for discovering the true nature of the global magnetic skeleton. By increasing the maximum number of harmonics used in the potential field extrapolations and, therefore, the grid resolution, 60 times more null points may be found in the coronal magnetic field. These high resolution fields also have a large global separator network which connects the coronal magnetic field over large distances and involves between 40 % and 60 % of all the null points in the solar atmosphere. This global separator network exists at both solar minimum and solar maximum and has separators that reach high into the solar atmosphere (> 1R☉) even though they connect null points close to the solar surface. These potential field extrapolations are then compared with magnetohydrostatic (MHS) extrapolations of the coronal magnetic field which also provide us with information about the plasma in the corona. With a small component of electric current density in the direction perpendicular to the radial direction, these MHS fields are found to have a plasma beta and pressure typical of the corona. As this small component of electric current density grows, the heliospheric current sheet is warped significantly and the magnetic field, plasma beta and pressure become unphysical. Torsional spine reconnection is also studied local to a single null point. First using a dynamical relaxation of a spiral null point under non-resistive magnetohydrodynamics (MHD) to a MHS equilibrium is form in which a current layer has built up around the spine lines. Then the reconnection under resistive MHD in this current sheet is studied. The current about the spine lines is dissipated and the magnetic energy is mainly converted into heat directly as the field lines untwist about the spine line.
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Murray, Michelle J. "Solar flux emergence : a three-dimensional numerical study." Thesis, University of St Andrews, 2008. http://hdl.handle.net/10023/441.
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Flux is continually emerging on the Sun, making its way from the solar interior up into the atmosphere. Emergence occurs on small-scales in the quiet Sun where magnetic fragments emerge, interact and cancel and on large-scales in active regions where magnetic fields emerge and concentrate to form sunspots. This thesis has been concerned with the large-scale emergence process and in particular the results from previous solar flux emergence modelling endeavours. Modelling uses numerical methods to evolve a domain representing simplified layers of the Sun’s atmosphere, within which the subsurface layer contains magnetic flux. The flux is initialised such that it will rises towards the surface at the start of the simulation. Once the flux reaches the solar surface, it can only emerge into the atmosphere if a magnetic buoyancy instability occurs, after which it expands rapidly both vertically and horizontally. The aim of this thesis is to test the robustness of these general findings from simulations to date upon the seed magnetic field. More explicitly, we have used three-dimensional numerical simulations to investigate how variations in the subsurface magnetic field modify the emergence process and the resulting atmospheric field. We initially consider a simple constant twist flux tube for the seed field and vary the tube’s magnetic field strength and degree of twist. Additionally, we have examined the effects of using non-constant twist flux tubes as the seed field by choosing two different profiles for the twist that are functions of the tube’s radius. Finally, we have investigated the effects of increasing the complexity of the seed field by positioning two flux tubes below the solar surface and testing two different configurations for the tubes. In both cases, the magnetic fields of the two tubes are such that, once the tubes come into contact with each other, reconnection occurs and a combined flux system is formed. From our investigations, we conclude that the general emergence results given by previous simulations are robust. However, for constant twist tubes with low field strength and twist, the buoyancy instability fails to be launched when the tubes reach the photosphere and they remain trapped in the low atmosphere. Similarly, when the non-constant twist profile results in a low tension force throughout the tube, we find that the buoyancy instability is not initialised.
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Bühler, Leo [Verfasser]. "Three-dimensional liquid metal flows in strong magnetic fields / L. Bühler." Karlsruhe : Forschungszentrum Karlsruhe, 2008. http://d-nb.info/991370600/34.
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Fonseca, Carissa Grace. "Assessment of left ventricular diastolic function with three dimensional cardiac magnetic resonance imaging." Thesis, University of Auckland, 2004. http://hdl.handle.net/2292/5715.
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Measurement of diastolic left ventricular (LV) function is vitally important in the assessment of cardiac disease. However, only limited information on tissue function can be obtained with current clinical techniques. This Thesis developed and investigated novel parameters of both global and regional myocardial function, using cardiac magnetic resonance imaging (MRI) with three-dimensional tissue tagging. Multidirectional peak myocardial shortening strains and strain rates, as well as the peak systolic displacement and velocity of the mitral valve annulus plane (MVP), were considered as parameters of LV systolic function. The corresponding peak diastolic strain relaxation rates and peak diastolic MVP velocity were used to assess diastolic function. The effects of normal ageing were studied in people with no evidence of cardiac disease, and compared with the effects of disease in patients with type 2 diabetes mellitus (DM). In normal healthy subjects, systolic strain parameters were preserved, while diastolic parameters were impaired, with age. DM patients showed impaired diastolic function on correction for age, and systolic functional parameters were also impaired, even though LV ejection fraction was normal. MVP systolic and diastolic motion were reduced both with age and in DM patients. Systolic LV torsion was increased with age and in DM, with no corresponding increase in torsional relaxation. Both systolic and diastolic function parameters were regionally heterogeneous. With normal ageing, diastolic function was impaired in a regionally non-uniform manner. Thus, a complete assessment of LV function requires measurement of LV tissue mechanics as well as chamber haemodynamics. MRI provides valuable information regarding myocardial tissue behaviour, contributing to systolic and/or diastolic dysfunction, which cannot be obtained otherwise. Systolic tissue dysfunction may develop concomitantly in patients with diastolic dysfunction, even when global ejection fraction is preserved. Regional analyses provide important information on how local changes contribute to global function. The influence of age must be taken into account in studies of disease.
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Dugas, Alain R. J. "A three-dimensional FEM method to predict current densities induced in conducting objects exposed to ELF magnetic fields." Thesis, University of Ottawa (Canada), 1993. http://hdl.handle.net/10393/6721.
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This thesis studies a numerical method to predict the current densities induced in conducting objects exposed to Extremely Low Frequency (ELF) magnetic fields. The Three Dimensional Finite Element Method (3-D FEM) was chosen to accomplish this task. The 3-D FEM numerical formulation is derived and presented as is the resulting algorithm written in FORTRAN code. The results are then presented and compared to other verified solutions to verify the validity of the formulation and the code. These results show that the 3-D FEM formulation works well for very simple objects. But, due to the fact that the developed algorithm requires much computer memory and that the governing coupled equations to solve produce matrices which are not always well conditioned, complex objects cannot yet be analyzed using this method.
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Nencib, Nadim. "Conception et validation d'un dispositif de caractérisation magnétique sous excitation bidimensionnelle : comportement des toles FeSi en champ tournant." Grenoble INPG, 1994. http://www.theses.fr/1994INPG0109.
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On s'intéresse à caractériser et comprendre le comportement des tôles magnétiques fonctionnant sous un champ d'excitation bidimensionnel. Un dispositif de mesure performant a été développé. Il permet de déterminer les propriétés magnétiques d'échantillons de 300 x 300 mm2 de dimension en régime d'enduction tournante, d'une part, en régime unidirectionnel d'orientation quelconque dans le plan de la tôle, d'autre part. Il a été optimisé suite à des simulations bidimensionnelle et tridimensionnelle utilisant les logiciels FLUX2D et FLUX3D dans lesquelles l'influence de nombreux paramètres physiques et géométriques ont été analysées. Des tôles FeSi GO et Fe Si NO ont été caractérisées en champ tournant et en champ uniaxial. Les pertes et les courbes B(H) obtenues sont analysées en termes d'anisotropie. Enfin, une représentation géométrique des pertes en champ tournant équivalente au cycle d'hystérésis en champ uniaxial est proposée
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Le, Dorze Fabrice. "Modélisation des effets de boucles d'immunisation dans les navires." Phd thesis, Grenoble INPG, 1997. http://www.theses.fr/1997INPG0183.
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"Ce mémoire présente le calcul des effets de boucles d'immunisation dans les navires par la méthode des éléments finis. Le système 'd'immunisation est constitué de circuits électriques intérieurs au navire et destinés à compenser l'anomalie magnétique générée par le bâtiment sous l'effet du champ terrestre. La difficulté rencontrée réside dans la proximité entre les circuits électriques et les tôles ferromagnétiques, n en résulte une aimantation très localisée de ces tôles, qui ne peut être correctement calculée qu'en utilisant un maillage extrêmement fin. Sans outil numérique adapté, cette modélisation se révèle donc coûteuse, voire irréalisable dans un cas réel. Une nouvelle technique, la méthode de saut ou de coupure de potentiel, est proposée pour résoudre le problème. Physiquement, l'effet u an circuit sur une tôle se traduit par une quasi-discontinuité du potentiel scalaire j réduit, inconnue principale du système et représentant la réaction du matériau ferromagnétique. Cet effet local, compte tenu de la ( disproportion entre les dimensions des tôles et les distances tôles - circuits, peut être ramené à un problème bidimensionnel et calculé par une méthode analytique ou numérique simple. Le saut de potentiel réduit ainsi calculé est imposé dans la modélisation tridimensionnelle globale comme contraint sur les noeuds des éléments finis de la tôle proches de la boucle. Les différentes validations "Amériques effectuée:: révèlent des résultats très satisfaisants sur des géométries simplifiées et prometteurs sur une géométrie complexe proche de celle d'un navire réel. Le gain en temps de calcul et en nombre d'inconnues du système est très important par rapport à une résolution s'appuyant sur un maillage très fin. Les validations expérimentales mettent en évidence la cohérence des modèles mathématiques utilisés et le comportement linéaire des matériaux sous ('effets des boucles, rendant inutile la prise en compte de l'hystérésis dans leur modélisation en général et dans le calcul des sauts de potentiel en particulier"<br>This thesis présent the computatior. Of degaussing coils effects in ships by die finite cléments method. The degaussing System is cornposed of electrical circuit the liull to compensate the magnelic anomaly generated by me vessel in the eanh's magnetic field. The difficuiiy is the shori distance between the coil and die ferromagnetic sheets. The magnetization of thesi sheets is sa localized that it can be correctly caleulated only by using an extrême fine mesh. Without spécial numerical tool, this modelling is expensive, and may be unrealisable in a real case. A new technique, called the reduced scalar potential jump method is proposed to solve die problem. Physically. Me coil effecton a sheet générâtes a quasi disconlinuity of die reduced scalar potential which is the mean unknown of the System and represents die reaction of die ferromagnetic material. Taking into lunt die disproportion between die sheet dimensions and die distances coil-sheet, this local effect can be considered as a bidimcnsicnal problem and calculated by a simple analytical or numerical method. The potential jump so computed is imposed in die whole diree-dimensional rnodelling as a constraint on the nodes of die sheet near die coil. The différent numerical validations reveal very satisfying results on simple geometries, and promising resulls on a complex geometry, nearer to a real ship. The gain in terms of computation time and nodes number is significant, compared to the modelling using a very fine mesh. The expérimental results reveal die reliability of die used mathematica! models and the linear behaviour of die ferromagnetic materials under me coils effect So it is useless to take die hystérésis into account in the gênerai rnodelling of degaussing coils, and especially in the calculus of die potential jumps
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Frerichs, Heinke Gerd [Verfasser]. "Three-dimensional plasma transport in open chaotic magnetic fields : a computational assessment for tokamak edge layers / vorgelegt von Heinke Gerd Frerichs. [Forschungszentrum Jülich, Energieforschung (IEF), Plasma Physics (IEF-4)]." Jülich : Forschungszentrum, Zentralbibliothek, 2010. http://d-nb.info/1009786954/34.
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Kessler, Philippe. "Simulation des grandes échelles d’écoulements compressibles en géométrie complexe." Grenoble INPG, 1996. http://www.theses.fr/1996INPG0201.
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La transition a la turbulence d'ecoulements en geometrie complexe est etudiee par simulation numerique compressible bidimensionnelle et tridimensionnelle. On presente une simulation des grandes echelles d'un jet rond d'air, comparee a une experience en laboratoire et a une autre simulation numerique incompressible. Cette comparaison nous a permis de valider notre outil numerique. Nous montrons un bon accord entre les resultats experimentaux et notre simulation numerique en zone transitionnelle. On applique notre outil numerique a une geometrie complexe et industrielle: l'entree d'air en incidence. Cette etude est effectuee avec un code a gestion multi-domaine des transferts d'informations par methode aux caracteristiques. Les resultats sont compares a ceux d'une experience faite au ceat de poitiers. Nous avons mis en evidence les tourbillons de kelvin-helmholtz, ainsi que les tourbillons longitudinaux, responsables des fluctuations de pression au niveau du premier etage du compresseur. Enfin, on a reproduit une experience magnetohydrodynamique faite au laboratoire madylam, en geometrie bidimensionnelle ronde. L'influence du champ magnetique sur un ecoulement de mercure soumis aux forces de laplace cree un cisaillement au niveau de l'electrode. On presente une evolution temporelle de la vorticite et de la pression, ainsi que les statistiques obtenues en regime permanent. Nous montrons des resultats qualitatifs satisfaisants, mais par contre sur les statistiques, nous surestimons les valeurs moyennes et fluctuantes. Nous montrons que les caracteristiques de l'ecoulement ne sont pas bidimensionnelles, mais doivent etre tridimensionnelles
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Chen, Bo-sian, and 陳柏先. "Three Dimensional Magnetostatic Field Analysis of Magnetic Spur Gear Pairs." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/68987232849273080568.
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碩士<br>雲林科技大學<br>機械工程系碩士班<br>98<br>Non-contact magnetic gears have gained significant attention due to no mechanical fatigue, no lubrication, reduced maintenance, improved reliability, overload protection, physical isolation between input and output shafts, and potential for very high efficiency. The purpose of this work is to study the three-dimensional magnetostatic field analysis of magnetic spur gear pairs. First, a detailed literature review regarding several kinds of magnetic gears for the last three decades is presented. Next, the three-dimensional magnetostatic field analysis of a magnetic spur gear pair is proposed with the aid of the current aheet model. The drag torques of an external magnetic gear pair and an internal magnetic gear pair are further calculated. Finally, the validity of the air gap flux density and the drag torque are verified by the finite-element method.
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Su, Luong Van, and 梁文史. "Giant Magnetoresistance Spin Valve Films for Three-Dimensional Magnetic Field Sensors." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/6c6p69.
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LI, I. FEI, and 李逸飛. "Three-Dimensional Simulation of Vertical Bridgman Crystal Growth in Uniform Magnetic Fields." Thesis, 2002. http://ndltd.ncl.edu.tw/handle/58035785462231068719.
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碩士<br>國立臺灣大學<br>化學工程學研究所<br>90<br>Three-dimensional (3D) simulation is carried out for the vertical Bridgman growth of gallium-doped germanium in a graphite ampoule under arbitrary gravitational and magnetic orientations. Both pseudo-steady state and fully time dependent calculations are carried out. For both fields in the axial direction, calculated results are in good agreement with those obtained by using a 2D model based on the stream function/vorticity formation. The scaling law for velocity damping by the magnetic field agrees well with the boundary layer approximation being |max|Ha-2, where max is the maximum melt velocity and Ha the Hartmann number. The flow damping by the horizontal magnetic field shows the same trend, but much more efficient. However, for 3D calculations at a large magnetic field strength, due to the poorer numerical resolution of the grid in the thin Hartmann layer near the wall, the flow damping fails to follow the scaling law. For stronger flows at normal gravity or tilted gravity, using the axial magnetic field is hard to damp the flow sufficiently unless having very large field strength.
Once the flow is suppressed, the radial segregation tends to increase first due to poorer dopant mixing. With a high enough strength, the radial segregation could be then reduced until the diffusion limit is reached, where the segregation is controlled by the interface concavity. Similarly, the effective segregation coefficient Keff is approaching to the diffusion limit with the increasing magnetic fields. Again, for both the horizontal magnetic field is far more efficient than the axial one. Interestingly, there is one exception. When the gravity is aligned with the growth axis, i.e., the perfect vertical growth configuration, the flow cell is stretched along the axial direction by the axial magnetic field. As a result, the back mixing increases and thus Keff decreases with the magnetic field strength at the beginning. Once the magnetic field is increased further, back mixing is then reduced until the diffusion limit is reached. Axial dopant segregation is also investigated through the fully transient calculations. Beside the more detailed information at different stages of the growth can be obtained, the trend in the flow damping and segregation is similar to that using the pseudo steady state calculations.
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Beck, Christian [Verfasser]. "The three-dimensional topology of magnetic fields in and around sunspots / Christian Beck." 2006. http://d-nb.info/978646843/34.
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St, Aubin Joel. "Three dimensional simulation and magnetic decoupling of the linac in a linac-MR system." Phd thesis, 2010. http://hdl.handle.net/10048/1405.
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Real time image guided radiotherapy has been proposed by integrating an in-line 6 MV linear accelerator (linac) to a magnetic resonance (MR) imager in either a parallel or transverse configuration. In either configuration, magnetic interference in the linac is caused by its immersion in the magnetic fringe fields of the MR imager. Thus in order to minimize the effect of the magnetic interference, investigations on linac performance in external magnetic fields was completed through various simulations.
Finite difference and finite element methods as well as particle simulations were performed in order to design an electron gun and an in-line 6 MV linac waveguide. Monte Carlo simulations provided calculations of dose distributions in a water tank from the derived electron phase space at the linac target. The entire simulation was validated against measurements taken from a commercial medical in-line 6 MV linac, other simulation programs, and theory.
The validated linac simulation was used to investigate linac performance in external magnetic fields. The results of this investigation showed that the linac had a much lower tolerance to transverse magnetic fields compared to longitudinal fields. While transverse magnetic fields caused a global deflection of the electron beam away from the central axis of the waveguide, longitudinal fields changed the optics of the electron gun in a suboptimal way. Both transverse and longitudinal magnetic fields caused excessive beam loss if the field strength was large enough. Heating caused by excessive beam loss in external magnetic fields was shown to have little effect on the resonant frequency of the waveguide, and any change in dosimetry, if it existed, was shown to be easily corrected using the jaws or multileaf collimators (MLCs). It was determined that the low-field parallel configuration linac-MR system investigated did not require any magnetic shielding, so the focus was on shielding the transverse configuration. Using beam loss, MLC motor tolerance to magnetic fields, and MR imager homogeneity as constraints, passive and active magnetic shielding was designed and optimized. Thus through the parallel configuration, or using magnetic shielding, magnetic interference has been reduced to within the linac operational tolerance.<br>Medical Physics