We present simulations and performance results of nuclear burning fronts in super- novae on the largest domain and at the finest spatial resolution studied to date. These simulations were performed on the Intel ASCI-Red machine at Sandia National Laboratories using FLASH, a code developed at the Center for Astrophysical Thermonuclear Flashes at the University of Chicago. FLASH is a modular, adaptive mesh, parallel simulation code capable of handling compressible, reactive fluid flows in astrophysical environments. FLASH is written primarily in Fortran 90, uses the Message-Passing Interface library for inter-processor communication and portability, and employs the PARAMESH package to manage a block-structured adaptive mesh that places blocks only where resolution is required and tracks rapidly changing flow features, such as detonation fronts, with ease. We describe the key algorithms and their implementation as well as the optimizations required to achieve sustained performance of 238 GFLOPS on 6420 processors of ASCI-Red in 64 bit arithmetic.

Understanding turbulence and mix in compressible flows is of fundamental importance to real-world applications such as chemical combustion and supernova evolution.The ability to run in three dimensions and at very high resolution is required for the simulation to accurately represent the interaction of the various length scales, and consequently, the reactivity of the intermixing species.Toward this end, we have carried out a very high resolution (over 8 billion zones) 3-D simulation of the Richtmyer-Meshkov instability and turbulent mixing on the IBM Sustained Stewardship TeraOp (SST) system, developed under the auspices of the Department of Energy (DOE) Accelerated Strategic Computing Initiative (ASCI) and located at Lawrence Livermore National Laboratory.We have also undertaken an even higher resolution proofof-principle calculation (over 24 billion zones) on 5832 processors of the IBM system, which executed for over an hour at a sustained rate of 1.05 Tflop/s, as well as a short calculation with a modified algorithm that achieved a sustained rate of 1.18 Tflop/s.The full production scientific simulation, using a further modified algorithm, ran for 27,000 timesteps in slightly over a week of wall time using 3840 processors of the IBM system, clocking a sustained throughput of roughly 0.6 teraflop per second (32-bit arithmetic).Nearly 300,000 graphics files comprising over three terabytes of data were produced and post-processed.The capability of running in 3-D at high resolution enabled us to get a more accurate and detailed picture of the fluidflow structure -in particular, to simulate the development of fine scale structures from the interactions of long-and short-wavelength phenomena, to elucidate differences between twodimensional and three-dimensional turbulence, to explore a conjecture regarding the transition from unstable flow to fully developed turbulence with increasing Reynolds number, and to ascertain convergence of the computed solution with respect to mesh resolution.

Significant improvements in real-time eficiency have been obtained for plasntajluid turbulence calculatwns by m"crotasking the nonlinear jlu"d co& KITE in which they are implemented on the CRAY Y-MP C90 at the National Energy Research Supercomputer Center (NERSC).The number of processors accessed concurrently scales linearly with problem size.Close to six concurrent processors have so far been obtained with a three-dimensional nonlinear production calculation at the currently allowed memory size of 80 Mword.With a calculation size corresponding to the maximum allowed memory of 200 Mword in the next system configuration, we expect to be able to access close to ten processors of the C90 concurrently with a commensurate improvement in realtime eficiency.These improvements in pe~ormance are comparable to those expected from a massively parallel implementation of the same calculations on the Intel Paragon.