"Give me the splendid silent sun with all his beams full-dazzling!"
-- Walt Whitman

Beneath the Surface

Some things are easy to take for granted. Like the Sun showing up every day to cast light and heat in our direction. The ancient Egyptians, who worshipped the Sun as the source of all being, intuitively understood that life on Earth depends absolutely on that daily dose of energy.

In some ways, scientists like University of Minnesota physicists Paul Woodward and David Porter are the modern version of ancient Egyptians. They devote their intellectual energy to unlocking secrets of how the Sun works. To be sure, modern science knows more than Pharoah's priests, yet with the Sun -- as with many things in Nature -- the more we know, the more new puzzles reveal themselves. How long will it be, for instance, till the Sun burns out? Current estimates range between five and six billion years, and the range reflects uncertainty in our knowledge of even this most familiar star.

Woodward and Porter direct their research, part of a National Science Foundation "grand challenge" project in geophysical and astrophysical turbulence, at what happens in a layer of gas -- roughly the outer third of the Sun -- called "the convection zone." In the fall of 1994, using the CRAY T3D at the Pittsburgh Supercomputing Center, they carried out the largest simulation of compressible convection ever done. "We're trying to find out in some detail," says Woodward, "what's going on beneath the surface, where you can't see. This is the first calculation in three dimensions where we had enough resolution to treat the small scale structure of turbulence in this region with a reasonable degree of accuracy."

Visualizations from this computation show features of solar turbulence the researchers didn't expect to see, and they show other features predicted by theory that simulations until now lacked sufficient resolution to test. "This is the first time," says Woodward, "that we've had results we could really sit down and compare with the predictions of simplified models for this kind of convection in stars."

Vorticity in the Convection Zone

This rectangular slab is a volume rendering showing a side view of the solar convection zone, roughly the outer third of the sun. It is idealized, with hard walls at the top and bottom. Energy is added from the bottom, to model radiation from nuclear fusion in the Sun's core. Colored fields represent "vorticity," how strongly the gas is spinning. Black areas are weak; green is slightly stronger, and white the strongest.

The knotted, densely packed vortex tubes, explains David Porter, show vigorously turbulent regions, especially along the top boundary and in the large downflow lane near the right edge. Vertical vortex tubes at the lower center resemble Earth tornados. Here the flow converges in both horizontal directions and expands upward.

Researchers: Paul Woodward & David Porter, University of Minnesota.
Hardware: CRAY T3D
Software: parabolic piecewise method, PPM
Keywords: Astronomy, physics, astrophysics, sun, solar, star, convection zone, compressible convection, turbulence, vorticity, visualization, yellow dwarf, core, hydrogen, helium, roiling, flares, vortex tubes, pressure differentials, gas, fluid dynamics, vortex rings, convection cells, downflow, lanes, plumes.

Related Material on the Web:
More information about this and other research for the Department of Energy.
Projects in Scientific Computing, PSC's annual research report.

References, Acknowledgements & Credits