John Hawley, a UVa astronomer, is studying the gas accretion disks thought to surround black holes. His simulations of these disks include fluid dynamics, gravity, and magnetic fields. The magnetic field is the critical element that provides the viscosity thought necessary to produce the luminous outflows of gas observed in nature. The magnetic viscosity leads to turbulent flow and infall, as was predicted by theory and confirmed by simulation.
This movie represents the results of the simulation. The image to the right is an equatorial view of the accretion disk, which is shaped like a squashed doughnut. The black hole is at the center of the circle. The image on the left is a slice through the accretion disk from the center to the edge. In this view, the black hole is on the left.
Initially, the disk is rotating around the black hole, with a slight seed perturbation in the gas velocities. This perturbation grows, amplified by the interaction between the gas and the small initial magnetic field. As time passes, the resulting turbulence causes the gas to lose angular momentum and flow toward the black hole. Most of the gas falls into the black hole, but some is heated to a very high temperature and flows away from the black hole through the surfaces of the disk.
This problem is usually run on a Cray T3E supercomputer, where it achieves a performance of roughly 2.25 gigaflops on 64 300mhz processors. On Centurion I's 64 533mhz 21164 Alpha processors, this code achieves performance of 4.5 gigaflops, which is twice as fast as the Cray. This particular case uses 128 cubed grid zones to model a rather small initial disk. Simulations of larger, more realistic disks would require many times this number of grid zones.
These results are quoted in "Global Magnetohydrodynamical Simulations of Accretion Tori" available online at www.astro.virginia.edu/~jh8h/torus3d/torus3d.html.
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