302 research outputs found
Athena: A New Code for Astrophysical MHD
A new code for astrophysical magnetohydrodynamics (MHD) is described. The
code has been designed to be easily extensible for use with static and adaptive
mesh refinement. It combines higher-order Godunov methods with the constrained
transport (CT) technique to enforce the divergence-free constraint on the
magnetic field. Discretization is based on cell-centered volume-averages for
mass, momentum, and energy, and face-centered area-averages for the magnetic
field. Novel features of the algorithm include (1) a consistent framework for
computing the time- and edge-averaged electric fields used by CT to evolve the
magnetic field from the time- and area-averaged Godunov fluxes, (2) the
extension to MHD of spatial reconstruction schemes that involve a
dimensionally-split time advance, and (3) the extension to MHD of two different
dimensionally-unsplit integration methods. Implementation of the algorithm in
both C and Fortran95 is detailed, including strategies for parallelization
using domain decomposition. Results from a test suite which includes problems
in one-, two-, and three-dimensions for both hydrodynamics and MHD are given,
not only to demonstrate the fidelity of the algorithms, but also to enable
comparisons to other methods. The source code is freely available for download
on the web.Comment: 61 pages, 36 figures. accepted by ApJ
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Observation and analysis of in situ carbonaceous matter in Nakhla: part II
Analysis of in situ carbonaceous matter in the Nakhla SNC meteorite has been carried out using a variety of techniques. Laser raman data shows the carbonaceous matter to be highly complex and static mass spectrometry has shown it to have an isotopic composition of '18 to '20' C
An Euler Solver Based on Locally Adaptive Discrete Velocities
A new discrete-velocity model is presented to solve the three-dimensional
Euler equations. The velocities in the model are of an adaptive nature---both
the origin of the discrete-velocity space and the magnitudes of the
discrete-velocities are dependent on the local flow--- and are used in a finite
volume context. The numerical implementation of the model follows the
near-equilibrium flow method of Nadiga and Pullin [1] and results in a scheme
which is second order in space (in the smooth regions and between first and
second order at discontinuities) and second order in time. (The
three-dimensional code is included.) For one choice of the scaling between the
magnitude of the discrete-velocities and the local internal energy of the flow,
the method reduces to a flux-splitting scheme based on characteristics. As a
preliminary exercise, the result of the Sod shock-tube simulation is compared
to the exact solution.Comment: 17 pages including 2 figures and CMFortran code listing. All in one
postscript file (adv.ps) compressed and uuencoded (adv.uu). Name mail file
`adv.uu'. Edit so that `#!/bin/csh -f' is the first line of adv.uu On a unix
machine say `csh adv.uu'. On a non-unix machine: uudecode adv.uu; uncompress
adv.tar.Z; tar -xvf adv.ta
Towards a new generation of multi-dimensional stellar evolution models: development of an implicit hydrodynamic code
This paper describes the first steps of development of a new multidimensional
time implicit code devoted to the study of hydrodynamical processes in stellar
interiors. The code solves the hydrodynamical equations in spherical geometry
and is based on the finite volume method. Radiation transport is taken into
account within the diffusion approximation. Realistic equation of state and
opacities are implemented, allowing the study of a wide range of problems
characteristic of stellar interiors. We describe in details the numerical
method and various standard tests performed to validate the method. We present
preliminary results devoted to the description of stellar convection. We first
perform a local simulation of convection in the surface layers of a A-type star
model. This simulation is used to test the ability of the code to address
stellar conditions and to validate our results, since they can be compared to
similar previous simulations based on explicit codes. We then present a global
simulation of turbulent convective motions in a cold giant envelope, covering
80% in radius of the stellar structure. Although our implicit scheme is
unconditionally stable, we show that in practice there is a limitation on the
time step which prevent the flow to move over several cells during a time step.
Nevertheless, in the cold giant model we reach a hydro CFL number of 100. We
also show that we are able to address flows with a wide range of Mach numbers
(10^-3 < Ms< 0.5), which is impossible with an anelastic approach. Our first
developments are meant to demonstrate that the use of an implicit scheme
applied to a stellar evolution context is perfectly thinkable and to provide
useful guidelines to optimise the development of an implicit multi-D
hydrodynamical code.Comment: 21 pages, 18 figures, accepted for publication in A&
Three-Dimensional Simulations of Jets from Keplerian Disks: Self--Regulatory Stability
We present the extension of previous two-dimensional simulations of the
time-dependent evolution of non-relativistic outflows from the surface of
Keplerian accretion disks, to three dimensions. The accretion disk itself is
taken to provide a set of fixed boundary conditions for the problem. The 3-D
results are consistent with the theory of steady, axisymmetric, centrifugally
driven disk winds up to the Alfv\'en surface of the outflow. Beyond the
Alfv\'en surface however, the jet in 3-D becomes unstable to non-axisymmetric,
Kelvin-Helmholtz instabilities. We show that jets maintain their long-term
stability through a self-limiting process wherein the average Alfv\'enic Mach
number within the jet is maintained to order unity. This is accomplished in at
least two ways. First, poloidal magnetic field is concentrated along the
central axis of the jet forming a ``backbone'' in which the Alfv\'en speed is
sufficiently high to reduce the average jet Alfv\'enic Mach number to unity.
Second, the onset of higher order Kelvin-Helmholtz ``flute'' modes (m \ge 2)
reduce the efficiency with which the jet material is accelerated, and transfer
kinetic energy of the outflow into the stretched, poloidal field lines of the
distorted jet. This too has the effect of increasing the Alfv\'en speed, and
thus reducing the Alfv\'enic Mach number. The jet is able to survive the onset
of the more destructive m=1 mode in this way. Our simulations also show that
jets can acquire corkscrew, or wobbling types of geometries in this relatively
stable end-state, depending on the nature of the perturbations upon them.
Finally, we suggest that jets go into alternating periods of low and high
activity as the disappearance of unstable modes in the sub-Alfv\'enic regime
enables another cycle of acceleration to super-Alfv\'enic speeds.Comment: 57 pages, 22 figures, submitted to Ap
Alfv\'en Reflection and Reverberation in the Solar Atmosphere
Magneto-atmospheres with Alfv\'en speed [a] that increases monotonically with
height are often used to model the solar atmosphere, at least out to several
solar radii. A common example involves uniform vertical or inclined magnetic
field in an isothermal atmosphere, for which the Alfv\'en speed is exponential.
We address the issue of internal reflection in such atmospheres, both for
time-harmonic and for transient waves. It is found that a mathematical boundary
condition may be devised that corresponds to perfect absorption at infinity,
and, using this, that many atmospheres where a(x) is analytic and unbounded
present no internal reflection of harmonic Alfv\'en waves. However, except for
certain special cases, such solutions are accompanied by a wake, which may be
thought of as a kind of reflection. For the initial-value problem where a
harmonic source is suddenly switched on (and optionally off), there is also an
associated transient that normally decays with time as O(t-1) or O(t-1 ln t),
depending on the phase of the driver. Unlike the steady-state harmonic
solutions, the transient does reflect weakly. Alfv\'en waves in the solar
corona driven by a finite-duration train of p-modes are expected to leave such
transients.Comment: Accepted by Solar Physic
Relativistic Hydrodynamics around Black Holes and Horizon Adapted Coordinate Systems
Despite the fact that the Schwarzschild and Kerr solutions for the Einstein
equations, when written in standard Schwarzschild and Boyer-Lindquist
coordinates, present coordinate singularities, all numerical studies of
accretion flows onto collapsed objects have been widely using them over the
years. This approach introduces conceptual and practical complications in
places where a smooth solution should be guaranteed, i.e., at the gravitational
radius. In the present paper, we propose an alternative way of solving the
general relativistic hydrodynamic equations in background (fixed) black hole
spacetimes. We identify classes of coordinates in which the (possibly rotating)
black hole metric is free of coordinate singularities at the horizon,
independent of time, and admits a spacelike decomposition. In the spherically
symmetric, non-rotating case, we re-derive exact solutions for dust and perfect
fluid accretion in Eddington-Finkelstein coordinates, and compare with
numerical hydrodynamic integrations. We perform representative axisymmetric
computations. These demonstrations suggest that the use of those coordinate
systems carries significant improvements over the standard approach, especially
for higher dimensional studies.Comment: 10 pages, 4 postscript figures, accepted for publication in Phys.
Rev.
Three Dimensional Numerical General Relativistic Hydrodynamics I: Formulations, Methods, and Code Tests
This is the first in a series of papers on the construction and validation of
a three-dimensional code for general relativistic hydrodynamics, and its
application to general relativistic astrophysics. This paper studies the
consistency and convergence of our general relativistic hydrodynamic treatment
and its coupling to the spacetime evolutions described by the full set of
Einstein equations with a perfect fluid source. The numerical treatment of the
general relativistic hydrodynamic equations is based on high resolution shock
capturing schemes. These schemes rely on the characteristic information of the
system. A spectral decomposition for general relativistic hydrodynamics
suitable for a general spacetime metric is presented. Evolutions based on three
different approximate Riemann solvers coupled to four different discretizations
of the Einstein equations are studied and compared. The coupling between the
hydrodynamics and the spacetime (the right and left hand side of the Einstein
equations) is carried out in a treatment which is second order accurate in {\it
both} space and time. Convergence tests for all twelve combinations with a
variety of test beds are studied, showing consistency with the differential
equations and correct convergence properties. The test-beds examined include
shocktubes, Friedmann-Robertson-Walker cosmology tests, evolutions of
self-gravitating compact (TOV) stars, and evolutions of relativistically
boosted TOV stars. Special attention is paid to the numerical evolution of
strongly gravitating objects, e.g., neutron stars, in the full theory of
general relativity, including a simple, yet effective treatment for the surface
region of the star (where the rest mass density is abruptly dropping to zero).Comment: 45 pages RevTeX, 34 figure
A comparison of hyperbolic solvers II: ausm-type and Hybrid Lax-Wendroff-Lax-Friedrichs methods for two-phase flows
Matter flows around black holes and gravitational radiation
We develop and calibrate a new method for estimating the gravitational
radiation emitted by complex motions of matter sources in the vicinity of black
holes. We compute numerically the linearized curvature perturbations induced by
matter fields evolving in fixed black hole backgrounds, whose evolution we
obtain using the equations of relativistic hydrodynamics. The current
implementation of the proposal concerns non-rotating holes and axisymmetric
hydrodynamical motions. As first applications we study i) dust shells falling
onto the black hole isotropically from finite distance, ii) initially spherical
layers of material falling onto a moving black hole, and iii) anisotropic
collapse of shells. We focus on the dependence of the total gravitational wave
energy emission on the flow parameters, in particular shell thickness, velocity
and degree of anisotropy. The gradual excitation of the black hole quasi-normal
mode frequency by sufficiently compact shells is demonstrated and discussed. A
new prescription for generating physically reasonable initial data is
discussed, along with a range of technical issues relevant to numerical
relativity.Comment: 27 pages, 12 encapsulated figures, revtex, amsfonts, submitted to
Phys. Rev.
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