1,633 research outputs found
A switch to reduce resistivity in smoothed particle magnetohydrodynamics
Artificial resistivity is included in Smoothed Particle Magnetohydrodynamics
simulations to capture shocks and discontinuities in the magnetic field. Here
we present a new method for adapting the strength of the applied resistivity so
that shocks are captured but the dissipation of the magnetic field away from
shocks is minimised. Our scheme utilises the gradient of the magnetic field as
a shock indicator, setting {\alpha}_B = h|gradB|/|B|, such that resistivity is
switched on only where strong discontinuities are present. The advantage to
this approach is that the resistivity parameter does not depend on the absolute
field strength. The new switch is benchmarked on a series of shock tube tests
demonstrating its ability to capture shocks correctly. It is compared against a
previous switch proposed by Price & Monaghan (2005), showing that it leads to
lower dissipation of the field, and in particular, that it succeeds at
capturing shocks in the regime where the Alfv\'en speed is much less than the
sound speed (i.e., when the magnetic field is very weak). It is also simpler.
We also demonstrate that our recent constrained divergence cleaning algorithm
has no difficulty with shock tube tests, in contrast to other implementations.Comment: 8 pages, 7 figures, accepted for publication in MNRA
Is the dust-to-gas ratio constant in molecular clouds?
We perform numerical simulations of dusty, supersonic turbulence in molecular
clouds. We model 0.1, 1 and 10 {\mu}m sized dust grains at an initial
dust-to-gas mass ratio of 1:100, solving the equations of combined gas and dust
dynamics where the dust is coupled to the gas through a drag term. We show
that, for 0.1 and 1 {\mu}m grains, the dust-to-gas ratio deviates by typically
10-20% from the mean, since the stopping time of the dust due to gas drag is
short compared to the dynamical time. Contrary to previous findings, we find no
evidence for orders of magnitude fluctuation in the dust-to-gas ratio for 0.1
{\mu}m grains. Larger, 10 {\mu}m dust grains may have dust-to-gas ratios
increased by up to an order of magnitude locally. Both small (0.1 {\mu}m) and
large ( 1 {\mu}m) grains trace the large-scale morphology of the gas,
however we find evidence for 'size-sorting' of grains, where turbulence
preferentially concentrates larger grains into dense regions. Size-sorting may
help to explain observations of 'coreshine' from dark clouds, and why
extinction laws differ along lines of sight through molecular clouds in the
Milky Way compared to the diffuse interstellar medium.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letters, videos
available at https://www.youtube.com/channel/UC7J6IDzQklFzKV3c6pBqxU
Constrained hyperbolic divergence cleaning in smoothed particle magnetohydrodynamics with variable cleaning speeds
We present an updated constrained hyperbolic/parabolic divergence cleaning
algorithm for smoothed particle magnetohydrodynamics (SPMHD) that remains
conservative with wave cleaning speeds which vary in space and time. This is
accomplished by evolving the quantity instead of . Doing so
allows each particle to carry an individual wave cleaning speed, , that
can evolve in time without needing an explicit prescription for how it should
evolve, preventing circumstances which we demonstrate could lead to runaway
energy growth related to variable wave cleaning speeds. This modification
requires only a minor adjustment to the cleaning equations and is trivial to
adopt in existing codes. Finally, we demonstrate that our constrained
hyperbolic/parabolic divergence cleaning algorithm, run for a large number of
iterations, can reduce the divergence of the field to an arbitrarily small
value, achieving to machine precision.Comment: 23 pages, 16 figures, accepted for publication in Journal of
Computational Physic
Simulating Astrophysical Magnetic Fields with Smoothed Particle Magnetohydrodynamics
Numerical methods to improve the treatment of magnetic fields in smoothed
field magnetohydrodynamics (SPMHD) are developed and tested. Chapter 2 is a
review of SPMHD. In Chapter 3, a mixed hyperbolic/parabolic scheme is developed
which cleans divergence error from the magnetic field. Average divergence error
is an order of magnitude lower for all test cases considered, and allows for
the stable simulation of the gravitational collapse of magnetised molecular
cloud cores. The effectiveness of the cleaning may be improved by explicitly
increasing the hyperbolic wave speed or by cycling the cleaning equations
between timesteps. In the latter, it is possible to achieve DivB=0. Chapter 4
develops a switch to reduce dissipation of the magnetic field from artificial
resistivity. Compared to the existing switch in the literature, this leads to
sharper shock profiles in shocktube tests, lower overall dissipation of
magnetic energy, and importantly, is able to capture magnetic shocks in the
highly super-Alfvenic regime. Chapter 5 compares these numerical methods
against grid-based MHD methods (using the Flash code) in simulations of the
small-scale dynamo amplification of a magnetic field in driven, isothermal,
supersonic turbulence. Both codes exponentially amplify the magnetic energy at
a constant rate, though SPMHD shows a resolution dependence that arises from
the scaling of the numerical dissipation terms. The time-averaged saturated
magnetic spectra have similar shape, and both codes have PDFs of magnetic field
strength that are log-normal, which become lopsided as the magnetic field
saturates. We conclude that SPMHD is able to reliably simulate the small-scale
dynamo amplification of magnetic fields. Chapter 6 concludes the thesis and
presents some preliminary work demonstrating that SPMHD can activate the
magneto-rotational instability in 2D shearing box tests.Comment: PhD thesis, Monash University, 2015. Chapter 2 is a review of SPMHD.
Chapters 3, 4 and 5 are adapted from published or submitted works, though
Chapters 3 and 6 also contain some unpublished work. 170 pages, 60 figure
Investigating prescriptions for artificial resistivity in smoothed particle magnetohydrodynamics
In numerical simulations, artificial terms are applied to the evolution
equations for stability. To prove their validity, these terms are thoroughly
tested in test problems where the results are well known. However, they are
seldom tested in production-quality simulations at high resolution where they
interact with a plethora of physical and numerical algorithms. We test three
artificial resistivities in both the Orszag-Tang vortex and in a star formation
simulation. From the Orszag-Tang vortex, the Price et. al. (2017) artificial
resistivity is the least dissipative thus captures the density and magnetic
features; in the star formation algorithm, each artificial resistivity
algorithm interacts differently with the sink particle to produce various
results, including gas bubbles, dense discs, and migrating sink particles. The
star formation simulations suggest that it is important to rely upon physical
resistivity rather than artificial resistivity for convergence.Comment: 8 pages, 7 figures. Proceedings of the "12th international SPHERIC
workshop", Ourense, Spain, 13-15 June 201
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