This thesis presents a new computationally efficient method for modelling the response of the solar corona to the release of energy. During impulsive heating events, the coronal temperature increases which leads to a downward heat flux into the transition region (TR). The plasma is unable to radiate this excess conductive heating and so the gas pressure increases locally. The resulting pressure gradient drives an upflow of dense material, creating an increase in the coronal density. This density increase is often called chromospheric evaporation. A process which is highly sensitive to the TR resolution in numerical simulations. If the resolution is not adequate, then the downward heat flux jumps over the TR and deposits the heat in the chromosphere, where it is radiated away. The outcome is that with an under-resolved TR, major errors occur in simulating the coronal density evolution. We address this problem by treating the lower transition region as a discontinuity that responds to changing coronal conditions through the imposition of a jump condition that is derived from an integrated form of energy conservation. In this thesis, it is shown that this method permits fast and accurate numerical solutions in both one-dimensional and multi-dimensional simulations. By modelling the TR with this appropriate jump condition, we remove the influence of poor numerical resolution and obtain the correct evaporative response to coronal heating, even when using resolutions that are compatible with multi-dimensional magnetohydrodynamic simulations.

We present new 3.6 and 4.5 mu m Spitzer phase curves for the highly irradiated hot Jupiter WASP-33b and the unusually dense Saturn-mass planet HD 149026b. As part of this analysis, we develop a new variant of pixel-level decorrelation that is effective at removing intrapixel sensitivity variations for long observations (>10 hr) where the position of the star can vary by a significant fraction of a pixel. Using this algorithm, we measure eclipse depths, phase amplitudes, and phase offsets for both planets at 3.6 and 4.5 mu m. We use a simple toy model to show that WASP-33b's phase offset, albedo, and heat recirculation efficiency are largely similar to those of other hot Jupiters despite its very high irradiation. On the other hand, our fits for HD 149026b prefer a very high albedo. We also compare our results to predictions from general circulation models, and we find that while neither planet matches the models well, the discrepancies for HD 149026b are especially large. We speculate that this may be related to its high bulk metallicity, which could lead to enhanced atmospheric opacities and the formation of reflective cloud layers in localized regions of the atmosphere. We then place these two planets in a broader context by exploring relationships between the temperatures, albedos, heat transport efficiencies, and phase offsets of all planets with published thermal phase curves. We find a striking relationship between phase offset and irradiation temperature: the former drops with increasing temperature until around 3400 K and rises thereafter. Although some aspects of this trend are mirrored in the circulation models, there are notable differences that provide important clues for future modeling efforts.

In a magnetized plasma, naturally occurring pressure anisotropies facilitate instabilities that are expected to modify the transport properties of the system. In this thesis we examine two such instabilities and, where appropriate, their effects on transport. First we consider the collisional (fluid) magnetized magnetorotational instability (MRI) in the presence of the Braginskii viscosity. We conduct a global linear analysis of the instability in a galactic rotation profile for three magnetic field configurations: purely azimuthal, purely vertical and slightly pitched. Our analysis, numerical and asymptotic, shows that the first two represent singular configurations where the Braginskii viscosity's primary role is dissipative and the maximum growth rate is proportional to the Reynolds number when this is small. For a weak pitched field, the Braginskii viscosity is destabilising and when its effects dominate over the Lorentz force, the growth rate of the MRI can be up to 2√2 times faster than the inviscid limit. If the field is strong, an over-stability develops and both the real and imaginary parts of the frequency increase with the coefficient of the viscosity. Second, in the context of the ICM of galaxy clusters, we consider the pressure-anisotropy-driven firehose instability. The linear instability is fast (~ ion cyclotron period) and small-scale (ion Larmor radius ρi) and so fluid theory is inapplicable. We determine its nonlinear evolution in an ab initio kinetic calculation (for parallel gradients only). We use a particular physical asymptotic ordering to derive a closed nonlinear equation for the firehose turbulence, which we solve. We find secular (α t) growth of magnetic fluctuations and a k-||3 spectrum, starting at scales >~ ρi. When a parallel ion heat flux is present, the parallel firehose instability mutates into the new gyrothermal instability. Its nonlinear evolution also involves secular magnetic energy growth, but its spectrum is eventually dominated by modes with a maximal scale ~ρilT/λmfp,(lT is the parallel temperature gradient scale). Throughout we discuss implications for modelling, transport and other areas of magnetized plasma physics.

This thesis examines the current view of users of passenger transport in Prague on its customer quality. The motive of this work is to identify the actual company's image with the personal city and suburban transport. For interest Gross, that provide transport and transport services in Prague, implemented rapid changes in Prague public transport at, especially in order to make the whole system more efficient, more user pleasant and friendly and introduce direct connection, where demand requires. The aim of this work is to map changes in public transport on 1st of September 2012, describe their impact on the economic efficiency of the system and try to discover the view of public opinion on quality of PID system. To achieve the first two objectives were used research data that individual interest groups have issued, in order to maintain transparency in the field. The third objective was studied using with the method of questionnaire.

The condition of fluid flow has been proven to have a significant influence on a wide variety of material processes. In electromagnetic levitation (EML) experiments, the internal flow is driven primarily by electromagnetic forces. In 1-g, the positioning forces are very strong and the internal flows are turbulent. To reduce the flows driven by the levitation field, experiments may be performed in reduced gravity and parabolic flights experiments have been adopted as the support in advance. Tracer particles on the surface of levitated droplets in EML experiment performed by SUPOS have been used to investigate the transition from laminar to turbulent flow. A sample of NiAl3 was electromagnetically levitated in parabolic flight and the laminar-turbulent transition observed from the case was studied in this work. For the sample with clearly visible tracer patterns, the fluid flow has been numerical evaluated with magnetohydrodynamic models and the laminar-turbulent transition happened during the acceleration of the flow, instead of steady state. The Reynolds number at transition was estimated approximately as 860 by the experiment record. The predicted time to transition obtained from the results of simulation showed significant difference (~ up to 300 times) compared with the time obtained from the experiment—0.37s. The discrepancy between numerical and experimental results could not be explained by the proposed hypotheses: geometry, boundary conditions or solid core. The simulations predict that the flow would become turbulent almost instantaneously after the droplet was fully molten. There are important physics shown by the simulation which were not captured.

<p> Continuous Casting (CC) is an essential process in the steel industry to transform molten steel into solid product. This process begins with primary cooling (PC) where the molten steel is cooled, and the initial solidification begins. It is important to monitor the process of PC as defects such as thinning of the shell in the mold can lead to breakouts. Key parameters in PC are the mold design, casting condition, and steel composition. In the research conducted, key parameters for PC are investigated to analyze the impact on flow formation and solidification. To optimize mold design, angular taper to the narrow face can be employed to accommodate for any shell shrinkage. Utilizing computational fluid dynamics, a range of mold taper is simulated per the developed solidification model with defined temperature-dependent material properties. When simulated without a taper, significant air gap formation in the corners of the mold is visible due to thermal shrinkage of the shell. This air gap decreases the cooling rate due to the shell’s lack of contact with the cooling mold wall. A parametric study of mold taper ranging from no taper to 3° as well as change in casting conditions, superheat and casting speed, are conducted to analyze the impact of taper with respect to the casting conditions. Per the conditions applied, angular taper between 1° and 2° resulted into reduction of undercooling and overcooling in the corner of the mold which is subjected to cooling from the broad face and narrow face of the cool mold wall. The turbulent flow in the mold region was found to drastically influences the quality of steel produced during continuous casting. The flow itself can lead to surface defects or slag entrainment based on the formation. A high surface wave due to turbulence of the injected melt lead to fluctuations and the instability compromised the quality of the steel produced as well as entrained the slag. To regulate the flow, electromagnetic forces can be applied in the mold, dampening the local turbulent flow. As the electrically conductive molten steel interacts with the induced magnetic field, it reduced the velocity of the steel jet released from the ports of the submerged entry nozzle. Per the simulation-based study conducted increasing the EMBr strength from 2975G to 4350G reduced the peak surface wave height by 59.47% and volume of flux rate of decrease by 4.25%. Additionally, increasing the SEN depth from 110 mm to 350 mm increased the average wave height by 19% and volume of flux rate of decrease by 2.6%. Lastly, increasing the mold width from 1.067 m to 1.50m increased average wave height by 8.71% and volume of flux rate of decrease by 0.9%. </p>

<p>Active Galactic Nuclei (AGNs) are accreting supermassive black holes at the center of galaxies, known for rich spectral features and multi-time scale variability in their electromagnetic emission. The origin of the variability in AGN light curves can be either intrinsic, meaning related processes that take place inside the AGN system, or extrinsic, i.e., from the propagation of light towards Earth. In this dissertation, I present my work focusing on AGN variability. The first two works focus on the variability of blazars, a subclass of AGN with their relativistic jets beaming towards the observer. The first work combines 3D relativistic magnetohydrodynamics (RMHD) simulations with radiation transfer and shows the kink instability within the blazar jet can cause quasi-periodic radiation signatures within a typical period of time scales from weeks to months. The second work combines 2D Particle-in-Cell (PIC) simulations with radiation transfer and shows that isolated and merging plasmoids due to magnetic reconnection in a blazar environment could produce rich radiation and polarization signatures. The last work explores an extrinsic origin for AGN variability: a scenario in which interstellar medium (ISM) within our galaxy can refract light coming from AGNs. It suggests that plasma structures in ISM with an axisymmetric geometry can account for extreme scattering events (ESEs) in AGN observations. Future research directions include studies of the kink instability in jets that propagate in different environments and simulations of magnetic reconnection in 3D which may reveal additional particle acceleration mechanisms, which may play important role in the resulting radiation and polarization signatures. </p>

Magnetic flux emergence is the subject of how magnetic fields from the solar interior can rise and expand into the atmosphere to produce active regions. It is the link that joins dynamics in the convection zone with dynamics in the atmosphere. In this thesis, we study many aspects of magnetic flux emergence through mathematical modelling and computer simulations. Our primary aim is to understand the key physical processes that lie behind emergence. The first chapter introduces flux emergence and the theoretical framework, magnetohydrodynamics (MHD), that describes it. In the second chapter, we discuss the numerical techniques used to solve the highly non-linear problems that arise from flux emergence. The third chapter summarizes the current literature. In the fourth chapter, we consider how changing the geometry and parameter values of the initial magnetic field can affect the dynamic evolution of the emerging magnetic field. For an initial toroidal magnetic field, it is found that its axis can emerge to the corona if the tube’s initial field strength is large enough. The fifth chapter describes how flux emergence models can produce large-scale solar eruptions. A 2.5D model of the breakout model, using only dynamic flux emergence, fails to produce any large scale eruptions. A 3D model of toroidal emergence with an overlying magnetic field does, however, produce multiple large-scale eruptions and the form of these is related to the breakout model. The sixth chapter is concerned with signatures of flux emergence and how to identify emerging twisted magnetic structures correctly. Here, a flux emergence model produces signatures found in observations. The signatures from the model, however, have different underlying physical mechanisms to the original interpretations of the observations. The thesis concludes with some final thoughts on current trends in theoretical magnetic flux emergence and possible future directions.

La première partie de ce travail se propose de définir une nouvelle version du formalisme d'étude des instabilités MHD de pression dans les structures d'accrétion-éjection magnétisées. Ces processus se produisent dans des plasmas confinés magnétiquement et sont très contraignants dans le domaine de la fusion thermonucléaire mais leur influence est peu étudiée dans des contextes astrophysiques. Dans un cadre d'approximation éliminant les ondes magnétosoniques rapides nous avons développé un système d'équations général permettant de s'intéresser à la fois aux modes instables d'interchange et aux modes de ballooning. L'application de ce système à un jet cylindrique en rotation solide nous montre que le cisaillement magnétique conduit à la déstabilisation des parties internes de ces structures. En outre, tout en clarifiant cette problématique dans une certaine mesure, nous retrouvons que ces flots sont génériquement instables vis-à-vis des modes d'interchange. Par ailleurs, nous avons étudié les méthodes numériques de résolution des équations aux dérivées partielles et plus particulièrement celles de la MHD. A partir d'un algorithme d'intégration élémentaire, nous avons pu évaluer les effets de géométrie, de conditions aux limites et de dissipation artificielle sur le calcul numérique, à travers une série de tests classiques. L'étude de la production de rayons cosmiques de très haute énergie dans les gamma-ray bursts constitue la dernière partie du travail effectué. Dans ces objets, des processus de Fermi accélèrent des particules jusqu'à des énergies de 10 21 eV, lors du croisement de perturbations d'Alfvén relativistes. Une interaction de type faisceau-plasma, entre une coquille de plasma en mouvement relativiste et les baryons qui la traversent, génère ces fronts alfvéniques et un mécanisme de rétrodiffusion redistribue l'énergie disponible entre des perturbations alfvéniques progressive, régressive et des perturbations magnétosoniques.

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.