29,834 research outputs found
The Non-Linear Growth of the Magnetic Rayleigh-Taylor Instability
This work examines the effect of the embedded magnetic field strength on the
non-linear development of the magnetic Rayleigh-Taylor Instability (RTI) (with
a field-aligned interface) in an ideal gas close to the incompressible limit in
three dimensions. Numerical experiments are conducted in a domain sufficiently
large so as to allow the predicted critical modes to develop in a physically
realistic manner. The ratio between gravity, which drives the instability in
this case (as well as in several of the corresponding observations), and
magnetic field strength is taken up to a ratio which accurately reflects that
of observed astrophysical plasma, in order to allow comparison between the
results of the simulations and the observational data which served as
inspiration for this work. This study finds reduced non-linear growth of the
rising bubbles of the RTI for stronger magnetic fields, and that this is
directly due to the change in magnetic field strength, rather than the indirect
effect of altering characteristic length scales with respect to domain size. By
examining the growth of the falling spikes, the growth rate appears to be
enhanced for the strongest magnetic field strengths, suggesting that rather
than affecting the development of the system as a whole, increased magnetic
field strengths in fact introduce an asymmetry to the system. Further
investigation of this effect also revealed that the greater this asymmetry, the
less efficiently the gravitational energy is released. By better understanding
the under-studied regime of such a major phenomenon in astrophysics, deeper
explanations for observations may be sought, and this work illustrates that the
strength of magnetic fields in astrophysical plasmas influences observed RTI in
subtle and complex ways.Comment: Accepted for publication by A&A. 10 pages, 9 figure
A paradigmatic flow for small-scale magnetohydrodynamics: properties of the ideal case and the collision of current sheets
We propose two sets of initial conditions for magnetohydrodynamics (MHD) in
which both the velocity and the magnetic fields have spatial symmetries that
are preserved by the dynamical equations as the system evolves. When
implemented numerically they allow for substantial savings in CPU time and
memory storage requirements for a given resolved scale separation. Basic
properties of these Taylor-Green flows generalized to MHD are given, and the
ideal non-dissipative case is studied up to the equivalent of 2048^3 grid
points for one of these flows. The temporal evolution of the logarithmic
decrements, delta, of the energy spectrum remains exponential at the highest
spatial resolution considered, for which an acceleration is observed briefly
before the grid resolution is reached. Up to the end of the exponential decay
of delta, the behavior is consistent with a regular flow with no appearance of
a singularity. The subsequent short acceleration in the formation of small
magnetic scales can be associated with a near collision of two current sheets
driven together by magnetic pressure. It leads to strong gradients with a fast
rotation of the direction of the magnetic field, a feature also observed in the
solar wind.Comment: 8 pages, 4 figure
The Magnetic Rayleigh-Taylor Instability in Three Dimensions
We study the magnetic Rayleigh-Taylor instability in three dimensions, with
focus on the nonlinear structure and evolution that results from different
initial field configurations. We study strong fields in the sense that the
critical wavelength l_c at which perturbations along the field are stable is a
large fraction of the size of the computational domain. We consider magnetic
fields which are initially parallel to the interface, but have a variety of
configurations, including uniform everywhere, uniform in the light fluid only,
and fields which change direction at the interface. Strong magnetic fields do
not suppress instability, in fact by inhibiting secondary shear instabilities,
they reduce mixing between the heavy and light fluid, and cause the rate of
growth of bubbles and fingers to increase in comparison to hydrodynamics.
Fields parallel to, but whose direction changes at, the interface produce long,
isolated fingers separated by the critical wavelength l_c, which may be
relevant to the morphology of the optical filaments in the Crab nebula.Comment: 14 pages, 9 pages, accepted by Ap
Nonlinear Evolution of the Magnetohydrodynamic Rayleigh-Taylor Instability
We study the nonlinear evolution of the magnetic Rayleigh-Taylor instability
using three-dimensional MHD simulations. We consider the idealized case of two
inviscid, perfectly conducting fluids of constant density separated by a
contact discontinuity perpendicular to the effective gravity g, with a uniform
magnetic field B parallel to the interface. Modes parallel to the field with
wavelengths smaller than l_c = [B B/(d_h - d_l) g] are suppressed (where d_h
and d_l are the densities of the heavy and light fluids respectively), whereas
modes perpendicular to B are unaffected. We study strong fields with l_c
varying between 0.01 and 0.36 of the horizontal extent of the computational
domain. Even a weak field produces tension forces on small scales that are
significant enough to reduce shear (as measured by the distribution of the
amplitude of vorticity), which in turn reduces the mixing between fluids, and
increases the rate at which bubbles and finger are displaced from the interface
compared to the purely hydrodynamic case. For strong fields, the highly
anisotropic nature of unstable modes produces ropes and filaments. However, at
late time flow along field lines produces large scale bubbles. The kinetic and
magnetic energies transverse to gravity remain in rough equipartition and
increase as t^4 at early times. The growth deviates from this form once the
magnetic energy in the vertical field becomes larger than the energy in the
initial field. We comment on the implications of our results to Z-pinch
experiments, and a variety of astrophysical systems.Comment: 25 pages, accepted by Physics of Fluids, online version of journal
has high resolution figure
Influence of turbulence on the dynamo threshold
We use direct and stochastic numerical simulations of the magnetohydrodynamic
equations to explore the influence of turbulence on the dynamo threshold. In
the spirit of the Kraichnan-Kazantsev model, we model the turbulence by a
noise, with given amplitude, injection scale and correlation time. The addition
of a stochastic noise to the mean velocity significantly alters the dynamo
threshold. When the noise is at small (resp. large) scale, the dynamo threshold
is decreased (resp. increased). For a large scale noise, a finite correlation
time reinforces this effect
The Surface Topography of a Magnetic Fluid -- a Quantitative Comparison between Experiment and Numerical Simulation
The normal field instability in magnetic liquids is investigated
experimentally by means of a radioscopic technique which allows a precise
measurement of the surface topography. The dependence of the topography on the
magnetic field is compared to results obtained by numerical simulations via the
finite element method. Quantitative agreement has been found for the critical
field of the instability, the scaling of the pattern amplitude and the detailed
shape of the magnetic spikes. The fundamental Fourier mode approximates the
shape to within 10% accuracy for a range of up to 40% of the bifurcation
parameter of this subcritical bifurcation. The measured control parameter
dependence of the wavenumber differs qualitatively from analytical predictions
obtained by minimization of the free energy.Comment: 21 pages, 16 figures; corrected typos, added reference to Kuznetsov
and Spector(1976), S.J. Fortune(1995) and Harkins&Jordan (1930). Figures
revise
Magnetic properties of colloidal suspensions of interacting magnetic particles
We review equilibrium thermodynamic properties of systems of magnetic
particles like ferrofluids in which dipolar interactions play an important
role. The review is focussed on two subjects: ({\em i}) the magnetization with
the initial magnetic susceptibility as a special case and ({\em ii}) the phase
transition behavior. Here the condensation ("gas/liquid") transition in the
subsystem of the suspended particles is treated as well as the
isotropic/ferromagnetic transition to a state with spontaneously generated
long--range magnetic order.Comment: Review. 62 pages, 4 figure
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