The dynamics of the solar corona are dominated by the magnetic field which creates its structure. The magnetic field in most of the corona is ‘frozen’ to the plasma very effectively. The exception is in small localised regions of intense current concentrations where the magnetic field can slip through the plasma and a restructuring of the magnetic field can occur. This process is known as magnetic reconnection and is believed to be responsible for a wide variety of phenomena in the corona, from the rapid energy release of solar flares to the heating of the high-temperature corona. The coronal field itself is three-dimensional (3D), but much of our understanding of reconnection has been developed through two-dimensional (2D) models. This thesis describes several models for fully 3D reconnection, with both kinematic and fully dynamic models presented. The reconnective behaviour is shown to be fundamentally different in many respects from the 2D case. In addition a numerical experiment is described which examines the reconnection process in coronal magnetic flux tubes whose photospheric footpoints are spun, one type of motion observed to occur on the Sun. The large-scale coronal field itself is thought to be generated by a magnetohydrodynamic dynamo operating in the solar interior. Although the dynamo effect itself is not usually associated with reconnection, since the essential element of the problem is to account for the presence of large-scale fields, reconnection is essential for the restructuring of the amplified small-scale flux. Here we examine some simple models of the solar-dynamo process, taking advantage of their simplicity to make a full exploration of their behaviour in a variety of parameter regimes. A wide variety of dynamic behaviour is found in each of the models, including aperiodic modulation of cyclic solutions and intermittency that strongly resembles the historic record of solar magnetic activity.

Rotating convection in the Earth's core produces columnar vortices of radius ~10 km or less near the inner core boundary. Small-scale motions in the core can travel as Alfvén waves in the face of Ohmic diffusion, provided the ratio of the magnetic diffusion time th to the Alfvén wave travel time tA (measured by the Lundquist number S0) is much greater than unity. These motions transfer angular momentum from the core to the mantle, a process that can help explain variations in length of day. Vortices subject to the combined influence of a magnetic field and background rotation give rise to fast and slow Magneto-Coriolis (MC) waves whose damping is not well understood. This thesis investigates the long-time evolution of magneto hydrodynamic (MHD) waves generated by an isolated, small-scale motion in an otherwise quiescent, electrically conducting fluid. The first part of the study focuses on the damping of small-scale Alfvén waves, which is independent of rotation. For a plausible magnetic field strength in the Earth's core, it is shown that flows of lengthscale ~ 5 km or larger can propagate across the core as damped Alfvén waves on sub-decadal timescales. The second part of the study looks at MC waves generated from an isolated blob under rotation and a uniform axial magnetic field. The decay laws for these waves are obtained by considering the decay of fast and slow waves individually. While the fast waves are subject to strongly anisotropic magnetic diffusion, the slow waves diffuse isotopically. New timescales are derived for the onset of damping and the transition from the wave-dominated to the diffusion-dominated (quasi-static) phase of decay. This study shows for the first time that MC waves originating from small-scale vortices of magnetic Reynolds number Rm ~ 1 can be long-lived. The results of this study are extendible to small-scale MHD turbulence under rotation, whose damped wave phase has not been adequately addressed in the literature. Furthermore, it is thought that this study would help place a lower bound on the poloidal magnetic field strength in the Earth’s core.

In this dissertation, we are concerned with the numerical simulation for flows of electrically conducting fluids exposed to an external magnetic field (also known as magnetohydrodynamics or in short MHD). The aim of the present dissertation is twofold. First, the in-house CFD hydrodynamic solver SFELES is extended to MHD problems. Second, MHD turbulence is studied in the simple configuration of a MHD pipe flow within an external transverse magnetic field. Chapter 2 of this dissertation aims at reminding the physical equations that govern incompressible MHD problems. Two equivalent formulations are put forward in the particular case of quasi-static MHD. Chapter 3 is devoted to the detailed development of the hybrid spectral - stabilized finite element methods for quasi-static MHD problems. The extension of SFELES is made for both Cartesian and axisymmetric systems of coordinates. The short chapter 4 follows to provide the performances of SFELES executed by several processes in a parallel environment. The addition of a parallel direct solver is studied in regards with the memory and time requirements. The extension of SFELES is then validated in chapter 5 with test cases of increasing complexity. For this purpose, laminar flows with an existing analytical-asymptotic solution are considered. The subject of chapter 6 is the MHD turbulent pipe flow within an external transverse and uniform magnetic field. The results are partially compared with the corresponding hydrodynamic flow and with a few data available in the literature. / Le thème de cette thèse de doctorat est la simulation numérique d'écoulements de fluides conducteurs d'électricité qui sont exposés à un champ magnétique extérieur (également connu sous le nom de magnétohydrodynamique ou encore MHD). L'objectif de ce travail est double. Premièrement, le code CFD maison SFELES est étendu aux problèmes MHD. Deuxièmement, la turbulence MHD est étudiée dans la configuration de l'écoulement en conduite cylindrique à l'intérieur d'un champ magnétique transverse. Le chapitre 2 de cette thèse a pour but de rappeler les équations qui gouvernent les problèmes de MHD incompressible. Deux formulations équivalente sont mises en évidence dans le cas particulier de la MHD quasi-statique. Le chapitre 3 est dévoué au développement détaillé des méthodes spectrale - éléments finis pour la MHD quasi-statique. L'extension de SFELES est réalisée dans les systèmes de coordonnées cartésiennes et axisymétriques. Le court chapitre 4 suit pour fournir les performances de SFELES exécuté sur plusieurs processeurs dans un environnement parallèle. L'ajout d'un solveur parallèle direct est étudié en ce qui concerne les demandes en temps et mémoire. L'extension de SFELES est alors validée dans le chapitre 5 avec des cas d'étude de complexité croissante. Dans ce but, des écoulements laminaires avec solution théorique-asymptotique sont envisagés. Le sujet du chapitre 6 est l'écoulement MHD turbulent en conduite cylindrique à l'intérieur d'un champ magnétique transverse et uniforme. Les résultats sont partiellement comparés avec l'écoulement hydrodynamique correspondant et avec des données disponibles dans la littérature. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Dans cette dissertation, nous considérons l’écoulement des liquides conducteurs d’électricité dans un champ magnétique externe. De tels écoulements sont décrits par les équations de la magnétohydrodynamique (MHD) quasi-statique, et sont fréquemment rencontrés dans des applications pratiques. Il suit qu’il y a un intérêt fort pour des outils numérques qui peuvent simuler ces écoulements dans des géometries complexes.<p>La première partie de cette thèse (chapitres 2 et 3) est dédiée à la présentation de la machinerie numérique qui a été utilisée et implémentée afin de résoudre les équations de la MHD quasi-statistique (incompressible). Plus précisément, nous avons contribué au développement d’un solveur volumes finis non-structuré parallèle. La discussion sur ces méthodes est accompagnée d’une analyse numérique qui est aussi valable pour des mailles non-structurées. Dans le chapitre 3, nous vérifions notre implémentation par la simulation d’un certain nombre de cas tests avec un accent sur des écoulements dans un champ magnétique intense.<p>Dans la deuxième partie de cette thèse (chapitres 4-6), nous avons utilsé ce solveur pour étudier des écoulements MHD de proche paroi .La première géometrie considérée (chapitre 4) est celle d’une conduite circulaire infini d’axe à haut nombre de Hartmann. Nous avons investitgué la sensitivité des résultats numériques au schéma de discrétisation et à la topologie de la maille. Nos résultats permettent de caractériser in extenso l’écoulement MHD dans une conduite avec des bords bien conducteurs par moyen des lois d’échelle.<p>Le sujet du cinquième chapitre est l’écoulement dans une conduite toroïdale à section carée. Une étude du régime laminaire confirme une analyse asymptotique pour ce qui concerne les couches de cisaillement. Nous avons aussi effectué des simulations des écoulements turbulents afin d’évaluer l’effet d’un champ magnétique externe sur l’état des couches limites limites.<p>Finalement, dans le chapitre 6, nous investiguons l’écoulement MHD et dans un U-bend et dans un coude arrière. Nous expliquons comment générer une maille qui permet de toutes les couches de cisaillement à un coût computationelle acceptable. Nous comparons nos résultats aux solutions asymptotiques. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

One of the most persistent problem in solar physics is the identification of the mechanism that heats the solar corona and accelerates the fast solar wind. Magneto-hydrodynamic (MHD)waves play a crucial role in heating of the solar corona and acceleration of the solar wind. Different types of oscillations have been now observed by various instruments. These are interpreted as due to ubiquitous presence of MHD waves. The magnetic field plays a fundamental role in the propagation and properties of these MHD waves. The topology (structure)of the magnetic fields are different in different regions of the solar atmosphere viz., active regions (high-lying closed magnetic fields), quiet Sun (low-lying closed magnetic fields) and coronal holes (open magnetic fields). The purpose of this dissertation is to study the nature of these propagating MHD waves in different regions of the solar atmosphere. It is believed that polar coronal holes which connects the inner corona and the solar wind, are the source regions of the fast solar wind. The on-disk part of a polar coronal hole can be divided into network and internetwork regions. Long time series(sit-and-stare)data have been obtained from the SUMER/SoHO spectrometer in N iv 765Å and Ne viii 770Å spectral lines to search for the presence of waves in these two different regions from a statistical approach. The network bright regions indicate the presence of compressional waves with a dominant period of ≈ 25 min in both the lines. Moreover, we found that there is a difference in the nature of the wave propagation in the bright (‘network’), as opposed to the dark (‘internetwork’) regions, with the latter sometimes showing evidence of downwardly propagating waves that are not seen in the former. This is consistent with the magnetic topology, as open field lines are rooted in network regions whereas internetwork region has low lying closed field lines. From a measurement of propagation speeds, we found all waves are subsonic, indicating that the majority of them are slow magneto-acoustic in nature. The off-limb part of coronal holes can be divided into plume and inter-plume regions. The simultaneous observations were performed with EIS/Hinode and SUMER/SoHO spectrometer in Fe xii 195Å and Ne viii 770Å spectral lines respectively. We detected the presence of accelerating waves in a polar inter-plume region with a period of 15 min to 20 min in both the spectral lines and a propagation speed increasing from 130 ± 14 km s−1 just above the limb, to 330 ± 140 kms s−1 around 160” above the limb. These waves can be traced to originate from a bright region of the on-disk part of the coronal hole which can be visualized as the base of the coronal funnels. The adjacent plume region also shows the presence of propagating disturbance with the same range of periodicity but with propagation speeds in the range of 135 ± 18 kms s−1 to 165 ± 43 kms s−1 only. We found that the waves within the plumes are not observable (may be getting dissipated) far off-limb whereas this is not the case in the inter-plume region. We suggested that the waves are likely either Alfv´enic or fast magneto-acoustic in the inter-plume regions and slow magneto-acoustic in the plume regions. These results support the view that the inter-plume regions area preferred channel for the acceleration of the fast solar wind. The quiet Sun can be further divided into bright magnetic (network), bright non-magnetic and dark non-magnetic (internetwork) regions. Simultaneous observations were performed in Ca ii filtergram from SOT/Hinode, TRACE 1550Åpassband and with SUMER/SoHO spectrometer in N iv 765ÅandNe viii 770Åspectral lines to study the oscillations in these different regions. We detected the presence of long period oscillations with periods between 15 min to 30 min in bright magnetic regions. The oscillations were detected from chromospheric height to low coronal heights. Power maps showed that low period powers are mainly concentrated in dark regions whereas long period powers are concentrated in bright magnetic regions. We proposed that these 15 min and above periods can propagate up to the coronal heights through ‘magneto¬acoustic portals’. However in this case only with the spectral imaging data, it was not possible to identify the mode of wave propagation. To detect the presence of waves in active regions, we have analysed the imaging and spec¬troscopic data acquired during the total solar eclipse of 2006 and 2009 respectively. We found the oscillations of periods 27 s and 20 s in imaging data obtained in green (Fe xiv 5303Å) and red (Fe x 6374Å) coronal emission lines respectively. Significant oscillations with high proba¬bility estimates were detected at boundary of active region and in the neighbourhood, rather than within the loops itself. We also reported the detection of oscillations in intensity, velocity and line width having periods in the range of 25 s to 50 s with spectroscopic data again obtained in green and red coronal emission lines. These high frequency oscillations were interpreted in terms of presence of fast magneto-acoustic waves or torsional Alfv´en waves. These detected propagating MHD waves may carry sufficient energy to heat the corona and provide enough momenta to accelerate the fast solar wind. In addition, these waves may also provide input for the measurement of coronal magnetic field using the technique of ‘coronal seismology’.

Les travaux de cette thèse portent sur le refroidissement descomposants électroniques de puissance par métal liquide. Les efforts se sontconcentrés plus particulièrement autour de deux fonctions : la pompeélectromagnétique servant à mettre le fluide en mouvement et le refroidisseur àminicanaux situé sous la source de dissipation.Le mémoire de thèse se structure en quatre chapitres équivalents. Dans lepremier, l’apport des métaux liquides pour le refroidissement des composantsactifs de puissance est démontré. Dans un deuxième temps, l’étude théorique etexpérimentale d’une pompe électromagnétique à conduction est effectuée. Lesystème de refroidissement est plus particulièrement abordé dans le troisièmechapitre. Enfin, des réflexions sur la mise en oeuvre des refroidisseurs à métauxliquides en électronique de puissance sont discutées dans la dernière partie.Grâce à elles, nous voyons que le champ d’application de ces travaux favorisel'émergence de solutions innovantes pour la gestion thermique des composantsélectronique de puissance. / The work presented in this Phd manuscript deals with cooling powerelectronics devices using an electrical conductive fluid. Two important functionshave been considered: the study and the realization of the electromagnetic pumpwhich circulated the fluid in the cooling loop. The second function was study andrealization of the cooler which evacuated the heat from the electronics device.This document has four chapters: introduction to power electronics coolingsystem with liquid metal, electromagnetic pump study, cooler study, and at lastreflections on realizing liquid metal cooler for power electronics devices. Theresults of this work concern a wide range of applications, especially towards newthermal management solutions of power electronics devices.

This paper contains an introduction that describes the current situation of the problem and its energy supply improvement. It discusses everything from power substations to trolleys which are fed through sections. There is a reference to the network operating deficiencies and possible solutions. The second part describes the development of these networks and their modernization in a growing city. It is verified by calculation if were our considerations good and proposed network will be normally reliable and capable. The final part deals with the economic balance of the project and its various alternatives. Development of energy networks and facilities cost many finance and these investments do not show immediately. Future will be paid off because as soon the changes come less problems may occur.

The main reason of doing this master thesis is reconstruction crossroad Pisarecka – Veslarska in Brno with growing traffic intensity up every year. Obviously had to be observed the best safety draw around the walking and bicycle area. This crossroad is using like transfer place. The result of this work is to elaborate of the situation including charasteristic cut sections and longitudinal profile of main road. Part of this work is an estimate of the financial cost to realize this building.

The diploma thesis deals with the reconstruction of the intersection of streets Merhautova and Provazníkova in Brno in means of to establish a comfortable transit hub between all passing lines. The reconstruction of the intersection includes a proposal for new tram stops and adjustment of pedestrian routes allowing easy transfer between cruise lines, trolley buses and trams.

MSc (Mathematics) / Department of Mathematcs and Applied Mathematics / Magnetohydrodynamics ows have gained signi cant attention due to their importance in engineering applications. In this study, we numerically analysed the Dufour and Soret e ects on an unsteady MHD mixed convection ow past an in nite vertical plate with viscous dissipation. The governing non-linear partial di erential equations (PDEs) are transformed into a system of ordinary di erential equations (ODEs) by the suitable similarity transformations. The resulting equations consist of the momentum, energy and mass di usion equations. These resulting equations are solved using the Spectral Local Linearization Method (SLLM). Results obtained by the SLLM are in good agreement with the bvp4c technique. The e ects of di erent physical parameters entering into the problem are displayed graphically. The values of the Skin-friction (f0(0)), Nusselt number (􀀀 0(0)) and Sherwood number (􀀀 0(0)) are shown in tabular form for di erent values of the parameters. From the results, it is noted that the Soret number (Sr) and the Dufour number (Du) have negligible e ects on temperature pro le, whereas the decrease in the Soret number (Sr) leads to a decrease in both velocity and concentration of the uid, and the increase in Dufour number (Du) reduces the velocity and also has negligilbe e ect on the concentration pro le. / NRF