25 research outputs found
Extensible Component Based Architecture for FLASH, A Massively Parallel, Multiphysics Simulation Code
FLASH is a publicly available high performance application code which has
evolved into a modular, extensible software system from a collection of
unconnected legacy codes. FLASH has been successful because its capabilities
have been driven by the needs of scientific applications, without compromising
maintainability, performance, and usability. In its newest incarnation, FLASH3
consists of inter-operable modules that can be combined to generate different
applications. The FLASH architecture allows arbitrarily many alternative
implementations of its components to co-exist and interchange with each other,
resulting in greater flexibility. Further, a simple and elegant mechanism
exists for customization of code functionality without the need to modify the
core implementation of the source. A built-in unit test framework providing
verifiability, combined with a rigorous software maintenance process, allow the
code to operate simultaneously in the dual mode of production and development.
In this paper we describe the FLASH3 architecture, with emphasis on solutions
to the more challenging conflicts arising from solver complexity, portable
performance requirements, and legacy codes. We also include results from user
surveys conducted in 2005 and 2007, which highlight the success of the code.Comment: 33 pages, 7 figures; revised paper submitted to Parallel Computin
The excitation of 5-min oscillations in the solar corona
We aim to study excitation of the observed 5-min oscillations in the solar
corona by localized pulses that are launched in the photosphere. We solve the
full set of nonlinear one-dimensional Euler equations numerically for the
velocity pulse propagating in the solar atmosphere that is determined by the
realistic temperature profile. Numerical simulations show that an initial
velocity pulse quickly steepens into a leading shock, while the nonlinear wake
in the chromosphere leads to the formation of consecutive pulses. The time
interval between arrivals of two neighboring pulses to a detection point in the
corona is approximately 5 min. Therefore, the consecutive pulses may result in
the 5-min oscillations that are observed in the solar corona. The 5-min
oscillations observed in the solar corona can be explained in terms of
consecutive shocks that result from impulsive triggers launched within the
solar photosphere by granulation and/or reconnection.Comment: 5 pages, 2 figures, accepted in A&
Supersonic plasma turbulence in the laboratory
The properties of supersonic, compressible plasma turbulence determine the behavior of many terrestrial and astrophysical systems. In the interstellar medium and molecular clouds, compressible turbulence plays a vital role in star formation and the evolution of our galaxy. Observations of the density and velocity power spectra in the Orion B and Perseus molecular clouds show large deviations from those predicted for incompressible turbulence. Hydrodynamic simulations attribute this to the high Mach number in the interstellar medium (ISM), although the exact details of this dependence are not well understood. Here we investigate experimentally the statistical behavior of boundary-free supersonic turbulence created by the collision of two laser-driven high-velocity turbulent plasma jets. The Mach number dependence of the slopes of the density and velocity power spectra agree with astrophysical observations, and supports the notion that the turbulence transitions from being Kolmogorov-like at low Mach number to being more Burgers-like at higher Mach numbers
Electron acceleration by wave turbulence in a magnetized plasma
Astrophysical shocks are commonly revealed by the non-thermal emission of energetic electrons accelerated in situ 1-3 . Strong shocks are expected to accelerate particles to very high energies 4-6 ; however, they require a source of particles with velocities fast enough to permit multiple shock crossings. While the resulting diffusive shock acceleration 4 process can account for observations, the kinetic physics regulating the continuous injection of non-thermal particles is not well understood. Indeed, this injection problem is particularly acute for electrons, which rely on high-frequency plasma fluctuations to raise them above the thermal pool 7,8 . Here we show, using laboratory laser-produced shock experiments, that, in the presence of a strong magnetic field, significant electron pre-heating is achieved. We demonstrate that the key mechanism in producing these energetic electrons is through the generation of lower-hybrid turbulence via shock-reflected ions. Our experimental results are analogous to many astrophysical systems, including the interaction of a comet with the solar wind 9 , a setting where electron acceleration via lower-hybrid waves is possible
Implicit large eddy simulations of anisotropic weakly compressible turbulence with application to core-collapse supernovae
(Abridged) In the implicit large eddy simulation (ILES) paradigm, the
dissipative nature of high-resolution shock-capturing schemes is exploited to
provide an implicit model of turbulence. Recent 3D simulations suggest that
turbulence might play a crucial role in core-collapse supernova explosions,
however the fidelity with which turbulence is simulated in these studies is
unclear. Especially considering that the accuracy of ILES for the regime of
interest in CCSN, weakly compressible and strongly anisotropic, has not been
systematically assessed before. In this paper we assess the accuracy of ILES
using numerical methods most commonly employed in computational astrophysics by
means of a number of local simulations of driven, weakly compressible,
anisotropic turbulence. We report a detailed analysis of the way in which the
turbulent cascade is influenced by the numerics. Our results suggest that
anisotropy and compressibility in CCSN turbulence have little effect on the
turbulent kinetic energy spectrum and a Kolmogorov scaling is
obtained in the inertial range. We find that, on the one hand, the kinetic
energy dissipation rate at large scales is correctly captured even at
relatively low resolutions, suggesting that very high effective Reynolds number
can be achieved at the largest scales of the simulation. On the other hand, the
dynamics at intermediate scales appears to be completely dominated by the
so-called bottleneck effect, \ie the pile up of kinetic energy close to the
dissipation range due to the partial suppression of the energy cascade by
numerical viscosity. An inertial range is not recovered until the point where
relatively high resolution , which would be difficult to realize in
global simulations, is reached. We discuss the consequences for CCSN
simulations.Comment: 17 pages, 9 figures, matches published versio