http://hdl.handle.net/10016/7404 Fri, 24 May 2013 20:19:51 GMT 2013-05-24T20:19:51Z http://hdl.handle.net/10016/8911 Title: Parallelization in time of numerical simulations of fully-developed plasma turbulence using the parareal algorithm Author(s): Samaddar, D.; Newman, David E.; Sánchez, Raúl Abstract: It is shown that numerical simulations of fully-developed plasma turbulence can be successfully parallelized in time using the parareal algorithm. The result is far from trivial, and even unexpected, since the exponential divergence of Lagrangian trajectories as well as the extreme sensitivity to initial conditions characteristic of turbulence set these type of simulations apart from the much simpler systems to which the parareal algorithm has been applied to this day. It is also shown that the parallel gain obtainable with this method is very promising (close to an order of magnitude for the cases and implementations described), even when it scales with the number of processors quite differently to what is typical for spatial parallelization. Description: 16 pages, 12 figures. Tue, 31 Aug 2010 22:00:00 GMT http://hdl.handle.net/10016/8911 2010-08-31T22:00:00Z http://hdl.handle.net/10016/8910 Title: BCYCLIC: A parallel block tridiagonal matrix cyclic solver Author(s): Hirshman, S. P.; Perumalla, K. S.; Lynch, V. E.; Sánchez, Raúl Abstract: A block tridiagonal matrix is factored with minimal fill-in using a cyclic reduction algorithm that is easily parallelized. Storage of the factored blocks allows the application of the inverse to multiple right-hand sides which may not be known at factorization time. Scalability with the number of block rows is achieved with cyclic reduction, while scalability with the block size is achieved using multithreaded routines (OpenMP, GotoBLAS) for block matrix manipulation. This dual scalability is a noteworthy feature of this new solver, as well as its ability to efficiently handle arbitrary (non-powers-of-2) block row and processor numbers. Comparison with a state-of-the art parallel sparse solver is presented. It is expected that this new solver will allow many physical applications to optimally use the parallel resources on current supercomputers. Example usage of the solver in magneto-hydrodynamic (MHD), three-dimensional equilibrium solvers for high-temperature fusion plasmas is cited. Description: 13 pages, 6 figures. Tue, 31 Aug 2010 22:00:00 GMT http://hdl.handle.net/10016/8910 2010-08-31T22:00:00Z http://hdl.handle.net/10016/8906 Title: Persistent dynamic correlations in self-organized critical systems away from their critical point Author(s): Woodard, Ryan; Newman, David E.; Sánchez, Raúl; Carreras, Benjamín A. Abstract: We show that correlated dynamics and long time memory persist in self-organized criticality (SOC) systems even when forced away from the defined critical point that exists at vanishing drive strength. These temporal correlations are found for all levels of external forcing as long as the system is not overdriven. They arise from the same physical mechanism that produces the temporal correlations found at the vanishing drive limit, namely the memory of past events stored in the system profile. The existence of these correlations contradicts the notion that a SOC time series is simply a random superposition of events with sizes distributed as a power law, as has been suggested by previous studies. Description: 16 pages, 12 figures.-- PACS nrs.: 05.65.+b, 05.40.-a, 52.25.Fi, 96.60.Rd.-- ArXiv pre-print available at: http://arxiv.org/abs/cond-mat/0503159 Sun, 31 Dec 2006 23:00:00 GMT http://hdl.handle.net/10016/8906 2006-12-31T23:00:00Z http://hdl.handle.net/10016/8905 Title: The path integral formulation of fractional Brownian motion for the general Hurst exponent Author(s): Calvo, Iván; Sánchez, Raúl Abstract: In 1995, Sebastian (1995 J. Phys. A: Math. Gen. 28 4305) gave a path integral computation of the propagator of subdiffusive fractional Brownian motion (fBm), i.e. fBm with a Hurst or self-similarity exponent H ∈ (0, 1/2). The extension of Sebastian's calculation to superdiffusion, H ∈ (1/2, 1], becomes however quite involved due to the appearance of additional boundary conditions on fractional derivatives of the path. In this communication, we address the construction of the path integral representation in a different fashion, which allows us to treat both subdiffusion and superdiffusion on an equal footing. The derivation of the propagator of fBm for the general Hurst exponent is then performed in a neat and unified way. Description: 5 pages, no figures.-- PACS nrs.: 05.40.-a, 02.50.Ey, 05.10.Gg.-- ArXiv preprint available at: http://arxiv.org/abs/0805.1170 Thu, 17 Jul 2008 22:00:00 GMT http://hdl.handle.net/10016/8905 2008-07-17T22:00:00Z