No description available.

The objective of this thesis was the design of a regeneratively cooled, high temperature, clean gas plasma generator facility. This facility was desired to extend the high temperature research capabilities of the Department of Mechanical Engineering.

The distinction between the magnetohydrodynamic generator and the conventional wire-wound generator is that the role of the armature in the letter is played by an electrically conducting fluid in the former. This fluid is passed through a transverse electric and magnetic field and between two parallel plate electrodes. The induced electric field in the fluid, which is proportional to the fluid velocity and magnetic flux density, maintains a voltage drop across the electrodes and a current is generated when the electrodes are connected to an external load, closing the circuit. Thus energy is extracted from the fluid and delivered to an external load as electrical power. / Master of Science

Conventional one-point turbulence closures have beenextended with an additional transported scalar for modeling ofmagnetohydrodynamic (MHD) turbulence. The new scalar, α ,captures the length scale anisotropy and tendency towardstwo-dimensionality, which is characteristic feature of MHDturbulence, and allows accurate modeling of the Jouledissipation of turbulence. The concept has been used for both afull Reynolds stress closure, and a three-equationK-ε -αmodel. An exact transport equation forαwas derived from the governing equations. All terms inthe equation require modeling, however. The proposed modeltransport equation for α includes terms for magneticdissipation, nonlinear energy transfer, and effects of meanshear and strain. Modeling of the magnetic and strain-relatedterms was based on rapid distortion analysis of the linearizedequations, while modeling of nonlinear effects isphenomenological in nature. For homogeneous turbulence, themodel was compared with linear theory, direct numericalsimulations and experiments. For turbulence subjected to astrong magnetic field, the model reproduces the energy andlength scale evolution predicted by linear theory. Whennonlinear effects are of importance, it predicts energy decayand length scale evolution in agreement with experiments. Theeddy viscosity and Reynolds stress versions of the modelcoincide with the respective conventional models in the absenceof a magnetic field. The objective of this project has been todevelop efficient MHD turbulence models for engineeringapplications, especially for modeling of continuous steelcasting. The novel MHD turbulence models appear to benumerically robust, and they have been implemented in acommercial flow solver, together with electromagnetic equationsfor the Lorentz forces in the mean momentum equations. <b>Keywords:</b>Turbulence model, magnetohydrodynamics, MHD,magnetohydrodynamic turbulence, computational fluid dynamics,continuous casting, dimensionality, Reynolds stresses, eddyviscosity

<p>Conventional one-point turbulence closures have beenextended with an additional transported scalar for modeling ofmagnetohydrodynamic (MHD) turbulence. The new scalar, α ,captures the length scale anisotropy and tendency towardstwo-dimensionality, which is characteristic feature of MHDturbulence, and allows accurate modeling of the Jouledissipation of turbulence. The concept has been used for both afull Reynolds stress closure, and a three-equation<i>K-ε -α</i>model. An exact transport equation forαwas derived from the governing equations. All terms inthe equation require modeling, however. The proposed modeltransport equation for α includes terms for magneticdissipation, nonlinear energy transfer, and effects of meanshear and strain. Modeling of the magnetic and strain-relatedterms was based on rapid distortion analysis of the linearizedequations, while modeling of nonlinear effects isphenomenological in nature. For homogeneous turbulence, themodel was compared with linear theory, direct numericalsimulations and experiments. For turbulence subjected to astrong magnetic field, the model reproduces the energy andlength scale evolution predicted by linear theory. Whennonlinear effects are of importance, it predicts energy decayand length scale evolution in agreement with experiments. Theeddy viscosity and Reynolds stress versions of the modelcoincide with the respective conventional models in the absenceof a magnetic field. The objective of this project has been todevelop efficient MHD turbulence models for engineeringapplications, especially for modeling of continuous steelcasting. The novel MHD turbulence models appear to benumerically robust, and they have been implemented in acommercial flow solver, together with electromagnetic equationsfor the Lorentz forces in the mean momentum equations.</p><p><b>Keywords:</b>Turbulence model, magnetohydrodynamics, MHD,magnetohydrodynamic turbulence, computational fluid dynamics,continuous casting, dimensionality, Reynolds stresses, eddyviscosity</p>

In recent years, many alternative models of the substorm process have been proposed to explain different aspects of this magnetospheric phenomenon. Some features in these competing models are compatible while others, such as the nature and location of substorm onset, remain controversial. The objective of this thesis is to assess the viability of the ballooning instability as a mechanism for initiating substorms. A review of the history and development of magnetospheric substorm research as well as a review of substorm models is presented. In these models, the crosstail current disruption responsible for the onset of the expansion phase is usually ascribed to the onset of some microinstability. An alternative triggering mechanism is a macroscopic magnetohydrodynamic instability such as the ballooning instability. To derive a threshold condition for the ballooning instability, a simplified magnetotail geometry with cylindrical symmetry near the equatorial plane is assumed. In such circumstances, the torsion of the magnetic field lines is zero and they can be characterised by their curvature. The hydromagnetic equations with isotropic pressure are linearised to find the dispersion relation. This leads to a threshold condition which depends on the pressure and magnetic field intensity gradients. In order to obtain realistic numerical results for the threshold condition, a quasistatic, self-consistent, two-dimensional numerical model of the magnetotail during conditions typical of substorm growth phase is used. The model involves solving the Grad-Shafranov equation with appropriate boundary conditions. It provides time-dependent magnetospheric magnetic field configurations that are characterised by the development of a minimum in Bz in the equatorial plane. Calculations of the detailed configuration of the magnetotail during onset allow an estimate of the instability criterion. In a model which does not allow an increase of pressure with radius, it is found that the magnetotail is not unstable to ballooning. Part of this work has been presented at a conference, viz.: Dormer, L.A. and A.D.M. Walker, Investigation of local MHD instabilities in the magnetotail using a two-dimensional magnetospheric convection model. Poster presented at the 39th annual South African Institute of Physics conference, University of Bophuthatswana, 1994. / Thesis (M.Sc.)-University of Natal, 1995.

Prepared under D.O.E. Order no. ET-78-X-01-2975. / Sponsored by the U.S. Dept. of Energy.

It has been recognized for some time that the very high fluence of fast (14.1MeV) neutrons produced by deuterium-tritium fusion will represent a major materials challenge for the development of next-generation fusion energy projects such as a fusion component test facility and demonstration fusion power reactor. The best-understood and most promising solutions presently available are a family of low-activation steels originally developed for use in fission reactors, but the ferromagnetic properties of these steels represent a danger to plasma confinement through enhancement of magnetohydrodynamic instabilities and increased susceptibility to error fields. At present, experimental research into the effects of ferromagnetic materials on MHD stability in toroidal geometry has been confined to demonstrating that it is still possible to operate an advanced tokamak in the presence of ferromagnetic components. In order to better quantify the effects of ferromagnetic materials on tokamak plasma stability, a new ferritic wall has been installated in the High Beta Tokamak—Extended Pulse (HBT-EP) device. The development, assembly, installation, and testing of this wall as a modular upgrade is described, and the effect of the wall on machine performance is characterized. Comparative studies of plasma dynamics with the ferritic wall close-fitting against similar plasmas with the ferritic wall retracted demonstrate substantial effects on plasma stability. Resonant magnetic perturbations (RMPs) are applied, demonstrating a 50% increase in n = 1 plasma response amplitude when the ferritic wall is near the plasma. Susceptibility of plasmas to disruption events increases by a factor of 2 or more with the ferritic wall inserted, as disruptions are observed earlier with greater frequency. Growth rates of external kink instabilities are observed to be twice as large in the presence of a close-fitting ferritic wall. Initial studies are made of the influence of mode rotation frequency on the ferritic effect, as well as observations of the effect of the ferritic wall on disruption halo currents.

It is currently believed that angular momentum transport in accretion disks is mediated by magnetohydrodynamic (MHD) turbulence driven by the magnetorotational instability (MRI). More than 15 years after its discovery, an accretion disk model that incorporates the MRI as the mechanism driving the MHD turbulence is still lacking. This dissertation constitutes the first in a series of steps towards establishing the formalism and methodology needed to move beyond the standard accretion disk model and incorporating the MRI as the mechanism enabling the accretion process. I begin by presenting a local linear stability analysis of a compressible, differentially rotating flow and addressing the evolution of the MRI beyond the weak-field limit when magnetic tension forces due to strong toroidal fields are considered. Then, I derive the first formal analytical proof showing that, during the exponential growth of the instability, the mean total stress produced by correlated MHD fluctuations is positive and leads to a net outward flux of angular momentum. I also show that some characteristics of the MHD stresses that are determined during this initial phase are roughly preserved in the turbulent saturated state observed in local numerical simulations. Motivated by these results, I present the first mean-field MHD model for angular momentum transport driven by the MRI that is able to account for a number of correlations among stresses found in local numerical simulations. I point out the relevance of a new type of correlation that couples the dynamical evolution of the Reynolds and Maxwell stresses and plays a key role in developing and sustaining the MHD turbulence. Finally, I address how the turbulent transport of angular momentum depends on the magnitude of the local shear. I show that turbulent MHD stresses in accretion disks cannot be described in terms of shear-viscosity.

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (p. 116-121).