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by Cheng Ting. / "August 2004." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (p. 103-107). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

We solve the equations of radiation hydrodynamics to compute the time evolution toward one-dimensional equilibrium solutions using ageneralized Kramers opacity, κ=κ0 ρa Tb, with adjustable prefactor κ0 and exponents a and b on density ρ and temperature T, respectively. We choose our initial conditions to be isothermal and find that the early time evolution away from the isothermal state is fastest near the height where the optical depth is unity, and is slower both above and below it. In all cases where the quantity n=(3-b)/(1+a) is larger than -1, we find a nearly polytropic solution with ρ<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Cpropto" />Tn in the lower part and a nearly isothermal solution in the upper part with a radiating surface in between, where the optical depth is unity. In the lower part, the radiative diffusivity is found to be approximately constant, while in the upper optically thin part it increases linearly. Interestingly, solutions with different parameter combinations a and b that result in the same value of n are rather similar, but not identical. Increasing the prefactor increases the temperature contrast and lowers the value of the effective temperature. We find that the Péclet number based on sound speed and pressure scale height exceeds numerically manageable values of around 104 when the prefactor κ0 is chosen to be approximately six orders of magnitude below the physically correct value. In the special case where a=-1 and b=3, the value of n is undetermined and the radiative diffusivity is strictly constant everywhere. In that case we find a stratification that is approximately adiabatic. Finally, exploratory two-dimensional calculations are presented where we include turbulent values of viscosity and diffusivity and find that onset of convection occurs when these values are around 3<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Ctimes" />1013 cm2 s-1. The addition of an imposed horizontal magnetic field suppresses small-scale convection, but has not led to instability in the cases investigated so far.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 72-74). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (p. 165-174).

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We present the results of three proposed mechanisms for ejection of gas from a spiral arm into the halo. The mechanisms were modelled using magnetohydrodynamics (MHD) as a theoretical template. Each mechanism was run through simulations using a Fortran code: ZEUS-3D, an MHD equation solver. The first mechanism modelled the gas dynamics with a modified Hartmann flow which describes the fluid flow between two parallel plates. We initialized the problem based on observation of lagging halos; that is, that the rotational velocity falls to a zero at some height above the plane of the disk. When adopting a density profile which takes into account the various warm and cold H$_I$ and H$_{II}$ molecular clouds, the system evolves very strangely and does not reproduce the steady velocity gradient observed in edge-on galaxies. This density profile, adopted from Martos and Cox (1998), was used in the remaining models. However, when treating a system with a uniform density profile, a stable simulation can result. Next we considered supernova (SN) blasts as a possible mechanism for gas ejection. While a single SN was shown to be insufficient to promote vertical gas structures from the disk, multiple SN explosions proved to be enough to promote gas ejection from the disk. In these simulations, gas ejected to a height of 0.5 kpc at a velocity of 130 km s$^{-1}$ from 500 supernovae, extending to an approximate maximum height of 1 kpc at a velocity of $6.7 \times 10^3$ km s$^{-1}$ from 1500 supernovae after 0.15 Myr, the approximate time of propagation of a supernova shock wave. Finally, we simulated gas flowing into the spiral arm at such a speed to promote a jump in the disk gas, termed a hydraulic jump. The height of the jump was found to be slightly less than a kiloparsec with a flow velocity of 41 km s$^{-1}$ into the halo after 167 Myr. The latter models proved to be effective mechanisms through which gas is ejected from the disk whereas the Hartmann flow (or toy model) mechanism remains unclear as the heliocentric velocity profile becomes unstable when run through a time-dependent simulation. Though the cause of this instability is unclear, pressure fluctuations in the system are suspected to play a role. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2012-01-05 17:52:59.532

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