294 research outputs found
A Constrained Transport Scheme for MHD on Unstructured Static and Moving Meshes
Magnetic fields play an important role in many astrophysical systems and a
detailed understanding of their impact on the gas dynamics requires robust
numerical simulations. Here we present a new method to evolve the ideal
magnetohydrodynamic (MHD) equations on unstructured static and moving meshes
that preserves the magnetic field divergence-free constraint to machine
precision. The method overcomes the major problems of using a cleaning scheme
on the magnetic fields instead, which is non-conservative, not fully Galilean
invariant, does not eliminate divergence errors completely, and may produce
incorrect jumps across shocks. Our new method is a generalization of the
constrained transport (CT) algorithm used to enforce the condition on fixed Cartesian grids. Preserving at the discretized level is necessary to maintain the
orthogonality between the Lorentz force and . The possibility of
performing CT on a moving mesh provides several advantages over static mesh
methods due to the quasi-Lagrangian nature of the former (i.e., the mesh
generating points move with the flow), such as making the simulation
automatically adaptive and significantly reducing advection errors. Our method
preserves magnetic fields and fluid quantities in pure advection exactly.Comment: 13 pages, 9 figures, accepted to MNRAS. Animations available at
http://www.cfa.harvard.edu/~pmocz/research.htm
The large-scale properties of simulated cosmological magnetic fields
We perform uniformly sampled large-scale cosmological simulations including
magnetic fields with the moving mesh code AREPO. We run two sets of MHD
simulations: one including adiabatic gas physics only; the other featuring the
fiducial feedback model of the Illustris simulation. In the adiabatic case, the
magnetic field amplification follows the scaling derived
from `flux-freezing' arguments, with the seed field strength providing an
overall normalization factor. At high baryon overdensities the amplification is
enhanced by shear flows and turbulence. Feedback physics and the inclusion of
radiative cooling change this picture dramatically. In haloes, gas collapses to
much larger densities and the magnetic field is amplified strongly and to the
same maximum intensity irrespective of the initial seed field of which any
memory is lost. At lower densities a dependence on the seed field strength and
orientation, which in principle can be used to constrain models of cosmic
magnetogenesis, is still present. Inside the most massive haloes magnetic
fields reach values of , in agreement with galaxy
cluster observations. The topology of the field is tangled and gives rise to
rotation measure signals in reasonable agreement with the observations.
However, the rotation measure signal declines too rapidly towards larger radii
as compared to observational data.Comment: 23 pages, 19 figures, 1 table. Accepted for publication in MNRAS.
Edited to match published versio
Reducing noise in moving-grid codes with strongly-centroidal Lloyd mesh regularization
A method for improving the accuracy of hydrodynamical codes that use a moving
Voronoi mesh is described. Our scheme is based on a new regularization scheme
that constrains the mesh to be centroidal to high precision while still
allowing the cells to move approximately with the local fluid velocity, thereby
retaining the quasi-Lagrangian nature of the approach. Our regularization
technique significantly reduces mesh noise that is attributed to changes in
mesh topology and deviations from mesh regularity. We demonstrate the
advantages of our method on various test problems, and note in particular
improvements obtained in handling shear instabilities, mixing, and in angular
momentum conservation. Calculations of adiabatic jets in which shear excites
Kelvin Helmholtz instability show reduction of mesh noise and entropy
generation. In contrast, simulations of the collapse and formation of an
isolated disc galaxy are nearly unaffected, showing that numerical errors due
to the choice of regularization do not impact the outcome in this case.Comment: 9 pages, 14 figures, MNRAS submitte
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