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Session 74 - The Quiet & Active Sun.
Display session, Friday, January 09
The larger scales of convection in the sun are influenced by rotation, leading to a redistribution of angular momentum which is seen as differential rotation. Previous numerical simulations of such global-scale convection in rotating spherical shells of fluid have been restricted by computational resources to deal with nearly laminar flow regimes for the resolved scales of motion, though provisions were made for diffusive transport by nominally turbulent unresolved scales. The disparities between the differential rotation profiles that those models predict (angular velocity nearly constant on cylinders) and those deduced from the frequency splittings of p modes (nearly constant on radii in the convection zone), coupled with recent advances in high performance computing, encouraged us to begin pursuing numerical simulations of global-scale convection in turbulent parameter regimes. We anticipate that rotationally-constrained turbulence can possess inverse cascades that yield large-scale coherent vorticity structures and strong mean flows which coexist with less persistent smaller-scale turbulence, and yield angular velocity profiles that may be quite different than those obtained with laminar convection.
The dominant challenge to all turbulence simulations is to be able to explicitly describe the dynamics over a broad enough range of length scales. To this end, we have developed a new code for studying rotating anelastic convection in spherical shells, employing expansions in spherical harmonics to resolve horizontal structures and in Chebyshev polynomials to resolve radial structures. Appropriate optimization techniques, including expedient use of inter-processor tranposes and sophisticated load balancing, enable the code to achieve high performance on massively parallel architectures, and currently to reach a speed of around 100Mflop/s per node on the Cray T3E. The new code is enabling us to perform simulations of convection at spatial resolutions significantly higher than those of earlier studies, and thereby to begin to investigate fully turbulent parameter regimes. We present the results from such a simulation, which includes the important effects of penetration into the stable interior, and discuss the corresponding differential-rotation profile obtained.
Program listing for Friday