340 research outputs found
Multidimensional Modeling of Type I X-ray Bursts. I. Two-Dimensional Convection Prior to the Outburst of a Pure Helium Accretor
We present multidimensional simulations of the early convective phase
preceding ignition in a Type I X-ray burst using the low Mach number
hydrodynamics code, MAESTRO. A low Mach number approach is necessary in order
to perform long-time integration required to study such phenomena. Using
MAESTRO, we are able to capture the expansion of the atmosphere due to
large-scale heating while capturing local compressibility effects such as those
due to reactions and thermal diffusion. We also discuss the preparation of
one-dimensional initial models and the subsequent mapping into our
multidimensional framework. Our method of initial model generation differs from
that used in previous multidimensional studies, which evolved a system through
multiple bursts in one dimension before mapping onto a multidimensional grid.
In our multidimensional simulations, we find that the resolution necessary to
properly resolve the burning layer is an order of magnitude greater than that
used in the earlier studies mentioned above. We characterize the convective
patterns that form and discuss their resulting influence on the state of the
convective region, which is important in modeling the outburst itself.Comment: 47 pages including 18 figures; submitted to ApJ; A version with
higher resolution figures can be found at
http://astro.sunysb.edu/cmalone/research/pure_he4_xrb/ms.pd
Low Mach Number Modeling of Type Ia Supernovae. IV. White Dwarf Convection
We present the first three-dimensional, full-star simulations of convection
in a white dwarf preceding a Type Ia supernova, specifically the last few hours
before ignition. For these long-time calculations we use our low Mach number
hydrodynamics code, MAESTRO, which we have further developed to treat spherical
stars centered in a three-dimensional Cartesian geometry. The main change
required is a procedure to map the one-dimensional radial base state to and
from the Cartesian grid. Our models recover the dipole structure of the flow
seen in previous calculations, but our long-time integration shows that the
orientation of the dipole changes with time. Furthermore, we show the
development of gravity waves in the outer, stable portion of the star. Finally,
we evolve several calculations to the point of ignition and discuss the range
of ignition radii.Comment: 42 pages, some figures degraded to conserve space. Accepted to The
Astrophysical Journal (http://journals.iop.org/
Propagation of the First Flames in Type Ia Supernovae
We consider the competition of the different physical processes that can
affect the evolution of a flame bubble in a Type Ia supernovae -- burning,
turbulence and buoyancy. Even in the vigorously turbulent conditions of a
convecting white dwarf, thermonuclear burning that begins at a point near the
center (within 100 km) of the star is dominated by the spherical laminar
expansion of the flame, until the burning region reaches kilometers in size.
Consequently flames that ignite in the inner ~20 km promptly burn through the
center, and flame bubbles anywhere must grow quite large--indeed, resolvable by
large-scale simulations of the global system--for significant motion or
deformation occur. As a result, any hot-spot that successfully ignites into a
flame can burn a significant amount of white dwarf material. This potentially
increases the stochastic nature of the explosion compared to a scenario where a
simmering progenitor can have small early hot-spots float harmlessly away.
Further, the size where the laminar flame speed dominates other relevant
velocities sets a characteristic scale for fragmentation of larger flame
structures, as nothing--by definition--can easily break the burning region into
smaller volumes. This makes possible the development of semi-analytic
descriptions of the earliest phase of the propagation of burning in a Type Ia
supernovae, which we present here. Our analysis is supported by fully resolved
numerical simulations of flame bubbles.Comment: 33 pages, 14 figures, accepted for publication in Ap
MAESTRO: An Adaptive Low Mach Number Hydrodynamics Algorithm for Stellar Flows
Many astrophysical phenomena are highly subsonic, requiring specialized
numerical methods suitable for long-time integration. In a series of earlier
papers we described the development of MAESTRO, a low Mach number stellar
hydrodynamics code that can be used to simulate long-time, low-speed flows that
would be prohibitively expensive to model using traditional compressible codes.
MAESTRO is based on an equation set derived using low Mach number asymptotics;
this equation set does not explicitly track acoustic waves and thus allows a
significant increase in the time step. MAESTRO is suitable for two- and
three-dimensional local atmospheric flows as well as three-dimensional
full-star flows. Here, we continue the development of MAESTRO by incorporating
adaptive mesh refinement (AMR). The primary difference between MAESTRO and
other structured grid AMR approaches for incompressible and low Mach number
flows is the presence of the time-dependent base state, whose evolution is
coupled to the evolution of the full solution. We also describe how to
incorporate the expansion of the base state for full-star flows, which involves
a novel mapping technique between the one-dimensional base state and the
Cartesian grid, as well as a number of overall improvements to the algorithm.
We examine the efficiency and accuracy of our adaptive code, and demonstrate
that it is suitable for further study of our initial scientific application,
the convective phase of Type Ia supernovae.Comment: Accepted to Astrophysical Journal Suppliment (http://iop.org). 56
pages, 15 figures
The Physics of Flames in Type Ia Supernovae
We extend a low Mach number hydrodynamics method developed for terrestrial
combustion, to the study of thermonuclear flames in Type Ia supernovae. We
discuss the differences between 2-D and 3-D Rayleigh-Taylor unstable flame
simulations, and give detailed diagnostics on the turbulence, showing that the
kinetic energy power spectrum obeys Bolgiano-Obukhov statistics in 2-D, but
Kolmogorov statistics in 3-D. Preliminary results from 3-D reacting bubble
calculations are shown, and their implications for ignition are discussed.Comment: To appear in the Proceedings of the SciDAC 2005 meeting, IOP press
(http://www.iop.org). Some figures degraded in quality to conserve spac
MAESTRO, CASTRO, and SEDONA -- Petascale Codes for Astrophysical Applications
Performing high-resolution, high-fidelity, three-dimensional simulations of
Type Ia supernovae (SNe Ia) requires not only algorithms that accurately
represent the correct physics, but also codes that effectively harness the
resources of the most powerful supercomputers. We are developing a suite of
codes that provide the capability to perform end-to-end simulations of SNe Ia,
from the early convective phase leading up to ignition to the explosion phase
in which deflagration/detonation waves explode the star to the computation of
the light curves resulting from the explosion. In this paper we discuss these
codes with an emphasis on the techniques needed to scale them to petascale
architectures. We also demonstrate our ability to map data from a low Mach
number formulation to a compressible solver.Comment: submitted to the Proceedings of the SciDAC 2010 meetin
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