Because these processes happen beneath the Sun's visible surface -- the photosphere -- they can't be seen from Earth, and it's very difficult to conduct experiments. Computer simulations provide a way to see what's otherwise unseeable. The difficulty has to do with the complexity of the flow. Along with general features that occur over relatively large scales, such as the granulation pattern on the Sun's surface, the researchers need to observe fine details.Vorticity in the Convection Zone
"You want to see the small vortices and how they interact," says Woodward. "Very fast time scales and small length scales are involved in this problem." To approach realistic results, the researchers must allow the gas to be compressible -- its density changes as pressure and temperature change, which increases the computing requirements. The modeling must also replicate large pressure differentials between the bottom and top of the convection zone. With the CRAY T3D, it became possible to more realistically model this depth differential.
In October 1994, PSC made two weeks of dedicated time available. Woodward and Porter are still analyzing the pile of data (250 gigabytes) that resulted, a process that could take a year or longer. Nevertheless, scientific visualization has already revealed phenomena not seen before.
Temperature 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.
This view looking down at the surface of the convection zone shows temperature according to color -- aqua (cold) through blue, green, red to yellow (hot). A cellular network of cool downflow regions surrounds isolated, warm upflows in a pattern that resembles "solar granules" observed on the Sun's surface.
The second slice (right) is from deeper in the convection zone (one pressure scale height below the surface). Here, the cool downflow lanes have merged into fewer, larger cells. Effects of turbulence can be seen in the knotted downflow structure. Some downflow lanes, crushed into isolated downflowing plumes, are seen as blue dots.
In general, the researchers see more turbulence than was apparent in previous simulations. Representative of this are vortex rings, which in the visualizations look like smoke rings, moving up and down. The visualizations also reveal a hierarchy of dynamics. "You have small convection cells at the surface," explains Porter, "and they're embedded in a large convective cell that spans the depth of the layer. So we're seeing the hierarchy of convection theorists had predicted, but only now is numerical experiment capable of resolving and testing these theories."
"This is the first three-dimensional calculation of compressible convection we've done," says Woodward, "where we have real confidence in the results." From here, the researchers plan to add more realism -- a stable layer beneath the convection zone and a free surface on top, like taking the lid off a boiling pot. "It's a scientific philosophy of our grand challenge team that we don't want to take a step towards a more complex formulation of the problem, a more realistic model, until we've understood the simplified one. We're ready now to take that step."
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