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Simulation improves understanding of turbulence
Using Livermore’s BlueGene/L supercomputer, Laboratory scientists have discovered a turbulence Reynolds number effect that could accelerate flames in type Ia supernovae at rates exceeding those predicted by standard numerical simulations. The effect derives from the turbulence energy budget (the conversion of potential energy to kinetic energy) associated with unstable Rayleigh–Taylor flame fronts. Spontaneous mixing of fluids at unstable interfaces occurs in a variety of atmospheric, oceanic, geophysical, and astrophysical flows. Rayleigh–Taylor instability is a process that occurs when a light fluid pushes on a heavy fluid and the fluids seek to reduce their combined potential energy. This process plays a key role in all known types of fusion.
The simulation carried out by Andrew Cook and Bill Cabot of Livermore’s Defense and Nuclear Technologies Directorate is a computation of evolving heavy fluid on top of a lighter fluid, subject to a gravitational field. This Rayleigh–Taylor simulation achieved a Reynolds number of 32,000 and is the first to reach past the mixing transition while resolving all hydrodynamic scales of motion. The findings, which appeared in the August 2006 issue of Nature Physics, have important implications for the study of turbulent combustion in supernovae, specifically, and for the understanding of turbulent convection in general.
Contact: Andrew Cook (925) 423-2856 (cook33@llnl.gov) or Bill Cabot (925) 423-9272 (cabot1@llnl.gov).

Research links warming sea surfaces to human influence
Scientists from the Laboratory’s Program for Climate Model Diagnosis and Intercomparison, along with collaborators from 10 other research centers, have shown that rising sea surface temperatures (SSTs) in the hurricane breeding grounds of the Atlantic and Pacific oceans are likely to be directly linked to human activities. The team, whose findings appeared in the September 19, 2006, issue of the Proceedings of the National Academy of Sciences, used 22 climate models to estimate the magnitude of SST changes arising from natural processes and external forcing (human influence) from 1906 to 2005.
The researchers found an 84 percent likelihood that external forcing explains at least 67 percent of observed SST increases in the Atlantic and Pacific cyclogenic regions during the period studied. Previous research has associated the warming of SSTs with an increase in hurricane intensity. The research team used nearly all the world’s climate models to estimate how the climate of an “undisturbed Earth” may have evolved. Comparing this undisturbed scenario with current global conditions, researchers posit that natural processes alone cannot explain the observed SST increases in the hurricane formation regions. They also note that although rising SSTs are not the sole cause of increased hurricane intensity, these increases are likely to be one of the most important influences on hurricane strength.
Contact: Benjamin Santer (925) 422-2486 (santer1@llnl.gov).

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UCRL-52000-06-11 | November 8, 2006