1,961 research outputs found
The Evolution of Adiabatic Supernova Remnants in a Turbulent, Magnetized Medium
(Abridged) We present the results of three dimensional calculations for the
MHD evolution of an adiabatic supernova remnant in both a uniform and turbulent
interstellar medium using the RIEMANN framework of Balsara. In the uniform
case, which contains an initially uniform magnetic field, the density structure
of the shell remains largely spherical, while the magnetic pressure and
synchrotron emissivity are enhanced along the plane perpendicular to the field
direction. This produces a bilateral or barrel-type morphology in synchrotron
emission for certain viewing angles. We then consider a case with a turbulent
external medium as in Balsara & Pouquet, characterized by .
Several important changes are found. First, despite the presence of a uniform
field, the overall synchrotron emissivity becomes approximately spherically
symmetric, on the whole, but is extremely patchy and time-variable, with
flickering on the order of a few computational time steps. We suggest that the
time and spatial variability of emission in early phase SNR evolution provides
information on the turbulent medium surrounding the remnant. The
shock-turbulence interaction is also shown to be a strong source of
helicity-generation and, therefore, has important consequences for magnetic
field generation. We compare our calculations to the Sedov-phase evolution, and
discuss how the emission characteristics of SNR may provide a diagnostic on the
nature of turbulence in the pre-supernova environment.Comment: ApJ, in press, 5 color figure
Direct Evidence for Two-Fluid Effects in Molecular Clouds
We present a combination of theoretical and simulation-based examinations of
the role of two-fluid ambipolar drift on molecular line widths. The dissipation
provided by ion-neutral interactions can produce a significant difference
between the widths of neutral molecules and the widths of ionic species,
comparable to the sound speed. We demonstrate that Alfven waves and certain
families of magnetosonic waves become strongly damped on scales comparable to
the ambipolar diffusion scale. Using the RIEMANN code, we simulate two-fluid
turbulence with ionization fractions ranging from 10^{-2} to 10^{-6}. We show
that the wave damping causes the power spectrum of the ion velocity to drop
below that of the neutral velocity when measured on a relative basis. Following
a set of motivational observations by Li & Houde (2008), we produce synthetic
line width-size relations that shows a difference between the ion and neutral
line widths, illustrating that two-fluid effects can have an observationally
detectable role in modifying the MHD turbulence in the clouds.Comment: 18 pages, 4 figures, submitted to MNRA
Divergence-Free Adaptive Mesh Refinement for Magnetohydrodynamics
In this paper we present a full-fledged scheme for the second order accurate,
divergence-free evolution of vector fields on an adaptive mesh refinement (AMR)
hierarchy. We focus here on adaptive mesh MHD. The scheme is based on making a
significant advance in the divergence-free reconstruction of vector fields. In
that sense, it complements the earlier work of Balsara and Spicer (1999) where
we discussed the divergence-free time-update of vector fields which satisfy
Stoke's law type evolution equations. Our advance in divergence-free
reconstruction of vector fields is such that it reduces to the total variation
diminishing (TVD) property for one-dimensional evolution and yet goes beyond it
in multiple dimensions. Divergence-free restriction is also discussed. An
electric field correction strategy is presented for use on AMR meshes. The
electric field correction strategy helps preserve the divergence-free evolution
of the magnetic field even when the time steps are sub-cycled on refined
meshes. The above-mentioned innovations have been implemented in Balsara's
RIEMANN framework for parallel, self-adaptive computational astrophysics which
supports both non-relativistic and relativistic MHD. Several rigorous, three
dimensional AMR-MHD test problems with strong discontinuities have been run
with the RIEMANN framework showing that the strategy works very well.Comment: J.C.P., figures of reduced qualit
A Two-Fluid Method for Ambipolar Diffusion
We present a semi-implicit method for isothermal two-fluid ion-neutral
ambipolar drift that is second-order accurate in space and time. The method has
been implemented in the RIEMANN code for astrophysical fluid dynamics. We
present four test problems that show the method works and correctly tracks the
propagation of MHD waves and the structure of two-fluid C-shocks. The accurate
propagation of MHD waves in the two-fluid approximation is shown to be a
stringent test of the algorithm. We demonstrate that highly accurate methods
are required in order to properly capture the MHD wave behaviour in the
presence of ion-neutral friction.Comment: 29 pages, 16 figures, accepted to MNRA
A Two-dimensional HLLC Riemann Solver for Conservation Laws : Application to Euler and MHD Flows
In this paper we present a genuinely two-dimensional HLLC Riemann solver. On
logically rectangular meshes, it accepts four input states that come together
at an edge and outputs the multi-dimensionally upwinded fluxes in both
directions. This work builds on, and improves, our prior work on
two-dimensional HLL Riemann solvers. The HLL Riemann solver presented here
achieves its stabilization by introducing a constant state in the region of
strong interaction, where four one-dimensional Riemann problems interact
vigorously with one another. A robust version of the HLL Riemann solver is
presented here along with a strategy for introducing sub-structure in the
strongly-interacting state. Introducing sub-structure turns the two-dimensional
HLL Riemann solver into a two-dimensional HLLC Riemann solver. The
sub-structure that we introduce represents a contact discontinuity which can be
oriented in any direction relative to the mesh.
The Riemann solver presented here is general and can work with any system of
conservation laws. We also present a second order accurate Godunov scheme that
works in three dimensions and is entirely based on the present multidimensional
HLLC Riemann solver technology. The methods presented are cost-competitive with
traditional higher order Godunov schemes
The accretion and spreading of matter on white dwarfs
For a slowly rotating non-magnetized white dwarf the accretion disk extends
all the way to the star. Here the matter impacts and spreads towards the poles
as new matter continuously piles up behind it. We have solved the 3d
compressible Navier-Stokes equations on an axisymmetric grid to determine the
structure of this boundary layer for different viscosities corresponding to
different accretion rates. The high viscosity cases show a spreading BL which
sets off a gravity wave in the surface matter. The accretion flow moves
supersonically over the cusp making it susceptible to the rapid development of
gravity wave and/or Kelvin-Helmholtz instabilities. This BL is optically thick
and extends more than 30 degrees to either side of the disk plane after 3/4 of
a Keplerian rotation period (t=19s). The low viscosity cases also show a
spreading BL, but here the accretion flow does not set off gravity waves and it
is optically thin.Comment: 6 pages, 5 figures, requires autart.cl
Multidimensional HLLE Riemann solver; Application to Euler and Magnetohydrodynamic Flows
In this work we present a general strategy for constructing multidimensional
Riemann solvers with a single intermediate state, with particular attention
paid to detailing the two-dimensional Riemann solver. This is accomplished by
introducing a constant resolved state between the states being considered,
which introduces sufficient dissipation for systems of conservation laws.
Closed form expressions for the resolved fluxes are also provided to facilitate
numerical implementation. The Riemann solver is proved to be positively
conservative for the density variable; the positivity of the pressure variable
has been demonstrated for Euler flows when the divergence in the fluid
velocities is suitably restricted so as to prevent the formation of cavitation
in the flow.
We also focus on the construction of multidimensionally upwinded electric
fields for divergence-free magnetohydrodynamical flows. A robust and efficient
second order accurate numerical scheme for two and three dimensional Euler and
magnetohydrodynamic flows is presented. The scheme is built on the current
multidimensional Riemann solver. The number of zones updated per second by this
scheme on a modern processor is shown to be cost competitive with schemes that
are based on a one-dimensional Riemann solver. However, the present scheme
permits larger timesteps
A Simple and Accurate Riemann Solver for Isothermal MHD
A new approximate Riemann solver for the equations of magnetohydrodynamics
(MHD) with an isothermal equation of state is presented.
The proposed method of solution draws on the recent work of
Miyoshi and Kusano, in the context of adiabatic MHD, where an approximate
solution to the Riemann problem is sought in terms of an average constant
velocity and total pressure across the Riemann fan.
This allows the formation of four intermediate states enclosed by two
outermost fast discontinuities and separated by two rotational waves and an
entropy mode.
In the present work, a corresponding derivation for the isothermal
MHD equations is presented.
It is found that the absence of the entropy mode leads to a different
formulation which is based on a three-state representation rather than four.
Numerical tests in one and two dimensions demonstrates that the new solver is
robust and comparable in accuracy to the more expensive linearized solver of
Roe, although considerably faster.Comment: 19 pages, 9 figure
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