30,683 research outputs found
Convergence and Divergence of the Solutions of a Neutral Difference Equation
We investigate the asymptotic behavior of the solutions of a neutral type difference equation of the form Δ[()+(())]+()(())=0, where () is a general retarded argument, () is a general deviated argument (retarded or advanced), ∈ℝ, (())≥0 is a sequence of positive real numbers such that ()≥, ∈ℝ+, and Δ denotes the forward difference operator Δ()=(+1)−(). Also, we examine the asymptotic behavior of the solutions in case they are
continuous and differentiable with respect to
An Explicit Scheme for Incorporating Ambipolar Diffusion in a Magnetohydrodynamics Code
We describe a method for incorporating ambipolar diffusion in the strong
coupling approximation into a multidimensional magnetohydrodynamics code based
on the total variation diminishing scheme. Contributions from ambipolar
diffusion terms are included by explicit finite difference operators in a fully
unsplit way, maintaining second order accuracy. The divergence-free condition
of magnetic fields is exactly ensured at all times by a flux-interpolated
constrained transport scheme. The super time stepping method is used to
accelerate the timestep in high resolution calculations and/or in strong
ambipolar diffusion. We perform two test problems, the steady-state oblique
C-type shocks and the decay of Alfv\'en waves, confirming the accuracy and
robustness of our numerical approach. Results from the simulations of the
compressible MHD turbulence with ambipolar diffusion show the flexibility of
our method as well as its ability to follow complex MHD flows in the presence
of ambipolar diffusion. These simulations show that the dissipation rate of MHD
turbulence is strongly affected by the strength of ambipolar diffusion.Comment: 25 pages, 5 figures, ApJS accepte
A Simflowny-based high-performance 3D code for the generalized induction equation
In the interior of neutron stars, the induction equation regulates the
long-term evolution of the magnetic fields by means of resistivity, Hall
dynamics and ambipolar diffusion. Despite the apparent simplicity and
compactness of the equation, the dynamics it describes is not trivial and its
understanding relies on accurate numerical simulations. While a few works in 2D
have reached a mature stage and a consensus on the general dynamics at least
for some simple initial data, only few attempts have been performed in 3D, due
to the computational costs and the need for a proper numerical treatment of the
intrinsic non-linearity of the equation. Here, we carefully analyze the general
induction equation, studying its characteristic structure, and we present a new
Cartesian 3D code, generated by the user-friendly, publicly available {\em
Simflowny} platform. The code uses high-order numerical schemes for the time
and spatial discretization, and relies on the highly-scalable {\em SAMRAI}
architecture for the adaptive mesh refinement. We present the application of
the code to several benchmark tests, showing the high order of convergence and
accuracy achieved and the capabilities in terms of magnetic shock resolution
and three-dimensionality. This paper paves the way for the applications to a
realistic, 3D long-term evolution of neutron stars interior and, possibly, of
other astrophysical sources.Comment: 23 pages, 13 figures. In pres
A three-dimensional numerical method for modelling weakly ionized plasmas
Astrophysical fluids under the influence of magnetic fields are often
subjected to single-fluid or two-fluid approximations. In the case of weakly
ionized plasmas however, this can be inappropriate due to distinct responses
from the multiple constituent species to both collisional and non-collisional
forces. As a result, in dense molecular clouds and proto-stellar accretion
discs for instance, the conductivity of the plasma may be highly anisotropic
leading to phenomena such as Hall and ambipolar diffusion strongly influencing
the dynamics.
Diffusive processes are known to restrict the stability of conventional
numerical schemes which are not implicit in nature. Furthermore, recent work
establishes that a large Hall term can impose an additional severe stability
limit on standard explicit schemes. Following a previous paper which presented
the one-dimensional case, we describe a fully three-dimensional method which
relaxes the normal restrictions on explicit schemes for multifluid processes.
This is achieved by applying the little known Super TimeStepping technique to
the symmetric (ambipolar) component of the evolution operator for the magnetic
field in the local plasma rest-frame, and the new Hall Diffusion Scheme to the
skew-symmetric (Hall) component.Comment: 13 pages, 9 figures, accepted for publication in MNRA
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