64 research outputs found
Simulations of Astrophysical Fluid Instabilities
We present direct numerical simulations of mixing at Rayleigh-Taylor unstable
interfaces performed with the FLASH code, developed at the ASCI/Alliances
Center for Astrophysical Thermonuclear Flashes at the University of Chicago. We
present initial results of single-mode studies in two and three dimensions. Our
results indicate that three-dimensional instabilities grow significantly faster
than two-dimensional instabilities and that grid resolution can have a
significant effect on instability growth rates. We also find that unphysical
diffusive mixing occurs at the fluid interface, particularly in poorly resolved
simulations.Comment: 3 pages, 1 figure. To appear in the proceedings of the 20th Texas
Symposium on Relativistic Astrophysic
Large-Scale Simulations of Clusters of Galaxies
We discuss some of the computational challenges encountered in simulating the
evolution of clusters of galaxies. Eulerian adaptive mesh refinement (AMR)
techniques can successfully address these challenges but are currently being
used by only a few groups. We describe our publicly available AMR code, FLASH,
which uses an object-oriented framework to manage its AMR library, physics
modules, and automated verification. We outline the development of the FLASH
framework to include collisionless particles, permitting it to be used for
cluster simulation.Comment: 3 pages, 3 figures, to appear in Proceedings of the VII International
Workshop on Advanced Computing and Analysis Techniques in Physics Research
(ACAT 2000), Fermilab, Oct. 16-20, 200
Onset of Convectionon a Pre-Runaway White Dwarf
Observed novae abundances and explosion energies estimated from observations
indicate that there must be significant mixing of the heavier material of the
white dwarf (C+O) into the lighter accreted material (H+He). Accordingly, nova
models must incorporate a mechanism that will dredge up the heavier white dwarf
material, and fluid motions from an early convection phase is one proposed
mechanism.
We present results from two-dimensional simulations of classical nova
precursor models that demonstrate the beginning of a convective phase during
the `simmering' of a Nova precursor. We use a new hydrostatic equilibrium
hydrodynamics module recently developed for the adaptive-mesh code FLASH. The
two-dimensional models are based on the one-dimensional models of Ami Glasner,
and were evolved with FLASH from a pre-convective state to the onset of
convection.Comment: 5 pages, 4 figures, from the 2002 International Conference on
Classical Novae in Sitges, Spai
Mixing by Non-linear Gravity Wave Breaking on a White Dwarf Surface
We present the results of a simulation of a wind-driven non-linear gravity
wave breaking on the surface of a white dwarf. The ``wind'' consists of H/He
from an accreted envelope, and the simulation demonstrates that this breaking
wave mechanism can produce a well-mixed layer of H/He with C/O from the white
dwarf above the surface. Material from this mixed layer may then be transported
throughout the accreted envelope by convection, which would enrich the C/O
abundance of the envelope as is expected from observations of novae.Comment: 5 pages, 3 figures, to appear in the proceedings of the International
Conference on Classical Nova Explosions, Sitges, Spain, 20-24 May 200
Mixing by Non-linear Gravity Wave Breaking on a White Dwarf Surface
We present the results of a simulation of a wind-driven non-linear gravity
wave breaking on the surface of a white dwarf. The ``wind'' consists of H/He
from an accreted envelope, and the simulation demonstrates that this breaking
wave mechanism can produce a well-mixed layer of H/He with C/O from the white
dwarf above the surface. Material from this mixed layer may then be transported
throughout the accreted envelope by convection, which would enrich the C/O
abundance of the envelope as is expected from observations of novae.Comment: 5 pages, 3 figures, to appear in the proceedings of the International
Conference on Classical Nova Explosions, Sitges, Spain, 20-24 May 200
Mapping Initial Hydrostatic Models in Godunov Codes
We look in detail at the process of mapping an astrophysical initial model
from a stellar evolution code onto the computational grid of an explicit,
Godunov type code while maintaining hydrostatic equilibrium. This mapping
process is common in astrophysical simulations, when it is necessary to follow
short-timescale dynamics after a period of long timescale buildup. We look at
the effects of spatial resolution, boundary conditions, the treatment of the
gravitational source terms in the hydrodynamics solver, and the initialization
process itself. We conclude with a summary detailing the mapping process that
yields the lowest ambient velocities in the mapped model.Comment: 59 pages, 21 figures, accepted to ApJS. Some figures are degraded for
size constraint
On Validating an Astrophysical Simulation Code
We present a case study of validating an astrophysical simulation code. Our
study focuses on validating FLASH, a parallel, adaptive-mesh hydrodynamics code
for studying the compressible, reactive flows found in many astrophysical
environments. We describe the astrophysics problems of interest and the
challenges associated with simulating these problems. We describe methodology
and discuss solutions to difficulties encountered in verification and
validation. We describe verification tests regularly administered to the code,
present the results of new verification tests, and outline a method for testing
general equations of state. We present the results of two validation tests in
which we compared simulations to experimental data. The first is of a
laser-driven shock propagating through a multi-layer target, a configuration
subject to both Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The second
test is a classic Rayleigh-Taylor instability, where a heavy fluid is supported
against the force of gravity by a light fluid. Our simulations of the
multi-layer target experiments showed good agreement with the experimental
results, but our simulations of the Rayleigh-Taylor instability did not agree
well with the experimental results. We discuss our findings and present results
of additional simulations undertaken to further investigate the Rayleigh-Taylor
instability.Comment: 76 pages, 26 figures (3 color), Accepted for publication in the ApJ
The Response of Model and Astrophysical Thermonuclear Flames to Curvature and Stretch
Critically understanding the `standard candle'-like behavior of Type Ia
supernovae requires understanding their explosion mechanism. One family of
models for Type Ia Supernovae begins with a deflagration in a Carbon-Oxygen
white dwarf which greatly accelerates through wrinkling and flame
instabilities. While the planar speed and behavior of astrophysically-relevant
flames is increasingly well understood, more complex behavior, such as the
flame's response to stretch and curvature, has not been extensively explored in
the astrophysical literature; this behavior can greatly enhance or suppress
instabilities and local flame-wrinkling, which in turn can increase or decrease
the bulk burning rate. In this paper, we explore the effects of curvature on
both nuclear flames and simpler model flames to understand the effect of
curvature on the flame structure and speed.Comment: 25 pages; accepted to ApJ; fixed author field
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