310 research outputs found
Achieving Extreme Resolution in Numerical Cosmology Using Adaptive Mesh Refinement: Resolving Primordial Star Formation
As an entry for the 2001 Gordon Bell Award in the "special" category, we
describe our 3-d, hybrid, adaptive mesh refinement (AMR) code, Enzo, designed
for high-resolution, multiphysics, cosmological structure formation
simulations. Our parallel implementation places no limit on the depth or
complexity of the adaptive grid hierarchy, allowing us to achieve unprecedented
spatial and temporal dynamic range. We report on a simulation of primordial
star formation which develops over 8000 subgrids at 34 levels of refinement to
achieve a local refinement of a factor of 10^12 in space and time. This allows
us to resolve the properties of the first stars which form in the universe
assuming standard physics and a standard cosmological model. Achieving extreme
resolution requires the use of 128-bit extended precision arithmetic (EPA) to
accurately specify the subgrid positions. We describe our EPA AMR
implementation on the IBM SP2 Blue Horizon system at the San Diego
Supercomputer Center.Comment: 23 pages, 5 figures. Peer reviewed technical paper accepted to the
proceedings of Supercomputing 2001. This entry was a Gordon Bell Prize
finalist. For more information visit http://www.TomAbel.com/GB
Simulating Galaxy Formation
A review on numerical simulations of galaxy formation is given. Different
numerical methods to solve collisionless and gas dynamical systems are outlined
and one particular simulation technique, Smoothed Particle Hydrodynamics, is
discussed in some detail. After a short discussion of the most relevant
physical processes which affect the dynamics of the gas, the success and
shortcomings of state of the art simulations are discussed via the example of
the formation of disk galaxies.Comment: 24 pages, uuencoded postscript file, 5 figures, 2 figures included
Proc. ``International School of Physics Enrico Fermi'', Course CXXXII: Dark
Matter in the Universe, Varenna 1995, eds.: S. Bonometto, J. Primack, A.
Provenzale, IOP, to appear; complete version available at
http://www.mpa-garching.mpg.de/Galaxien/prep.htm
ColDICE: a parallel Vlasov-Poisson solver using moving adaptive simplicial tessellation
Resolving numerically Vlasov-Poisson equations for initially cold systems can
be reduced to following the evolution of a three-dimensional sheet evolving in
six-dimensional phase-space. We describe a public parallel numerical algorithm
consisting in representing the phase-space sheet with a conforming,
self-adaptive simplicial tessellation of which the vertices follow the
Lagrangian equations of motion. The algorithm is implemented both in six- and
four-dimensional phase-space. Refinement of the tessellation mesh is performed
using the bisection method and a local representation of the phase-space sheet
at second order relying on additional tracers created when needed at runtime.
In order to preserve in the best way the Hamiltonian nature of the system,
refinement is anisotropic and constrained by measurements of local Poincar\'e
invariants. Resolution of Poisson equation is performed using the fast Fourier
method on a regular rectangular grid, similarly to particle in cells codes. To
compute the density projected onto this grid, the intersection of the
tessellation and the grid is calculated using the method of Franklin and
Kankanhalli (1993) generalised to linear order. As preliminary tests of the
code, we study in four dimensional phase-space the evolution of an initially
small patch in a chaotic potential and the cosmological collapse of a
fluctuation composed of two sinusoidal waves. We also perform a "warm" dark
matter simulation in six-dimensional phase-space that we use to check the
parallel scaling of the code.Comment: Code and illustration movies available at:
http://www.vlasix.org/index.php?n=Main.ColDICE - Article submitted to Journal
of Computational Physic
Chaos and Turbulent Nucleosynthesis Prior to a Supernova Explosion
Three-dimensional (3D), time dependent numerical simulations, of flow of
matter in stars, now have sufficient resolution to be fully turbulent. The late
stages of the evolution of massive stars, leading up to core collapse to a
neutron star (or black hole), and often to supernova explosion and
nucleosynthesis, are strongly convective because of vigorous neutrino cooling
and nuclear heating. Unlike models based on current stellar evolutionary
practice, these simulations show a chaotic dynamics characteristic of highly
turbulent flow. Theoretical analysis of this flow, both in the
Reynolds-averaged Navier-Stokes (RANS) framework and by simple dynamic models,
show an encouraging consistency with the numerical results. It may now be
possible to develop physically realistic and robust procedures for convection
and mixing which (unlike 3D numerical simulation) may be applied throughout the
long life times of stars. In addition, a new picture of the presupernova stages
is emerging which is more dynamic and interesting (i.e., predictive of new and
newly observed phenomena) than our previous one.Comment: 11 pages, 2 figures, Submitted to AIP Advances: Stardust, added
figures and modest rewritin
GenASiS: General Astrophysical Simulation System. I. Refinable Mesh and Nonrelativistic Hydrodynamics
GenASiS (General Astrophysical Simulation System) is a new code being
developed initially and primarily, though by no means exclusively, for the
simulation of core-collapse supernovae on the world's leading capability
supercomputers. This paper---the first in a series---demonstrates a centrally
refined coordinate patch suitable for gravitational collapse and documents
methods for compressible nonrelativistic hydrodynamics. We benchmark the
hydrodynamics capabilities of GenASiS against many standard test problems; the
results illustrate the basic competence of our implementation, demonstrate the
strengths and limitations of the HLLC relative to the HLL Riemann solver in a
number of interesting cases, and provide preliminary indications of the code's
ability to scale and to function with cell-by-cell fixed-mesh refinement.Comment: Belated update to version accepted ApJ
Gravitational Collapse in Turbulent Molecular Clouds. I. Gasdynamical Turbulence
Observed molecular clouds often appear to have very low star formation
efficiencies and lifetimes an order of magnitude longer than their free-fall
times. Their support is attributed to the random supersonic motions observed in
them. We study the support of molecular clouds against gravitational collapse
by supersonic, gas dynamical turbulence using direct numerical simulation.
Computations with two different algorithms are compared: a particle-based,
Lagrangian method (SPH), and a grid-based, Eulerian, second-order method
(ZEUS). The effects of both algorithm and resolution can be studied with this
method. We find that, under typical molecular cloud conditions, global collapse
can indeed be prevented, but density enhancements caused by strong shocks
nevertheless become gravitationally unstable and collapse into dense cores and,
presumably, stars. The occurance and efficiency of local collapse decreases as
the driving wave length decreases and the driving strength increases. It
appears that local collapse can only be prevented entirely with unrealistically
short wave length driving, but observed core formation rates can be reproduced
with more realistic driving. At high collapse rates, cores are formed on short
time scales in coherent structures with high efficiency, while at low collapse
rates they are scattered randomly throughout the region and exhibit
considerable age spread. We suggest that this naturally explains the observed
distinction between isolated and clustered star formation.Comment: Minor revisions in response to referee, thirteen figures, accepted to
Astrophys.
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