79 research outputs found
A numerical model of the VKS experiment
We present numerical simulations of the magnetic field generated by the flow
of liquid sodium driven by two counter-rotating impellers (VKS experiment).
Using a dynamo kinematic code in cylindrical geometry, it is shown that
different magnetic modes can be generated depending on the flow configuration.
While the time averaged axisymmetric mean flow generates an equatorial dipole,
our simulations show that an axial field of either dipolar or quadrupolar
symmetry can be generated by taking into account non-axisymmetric components of
the flow. Moreover, we show that by breaking a symmetry of the flow, the
magnetic field becomes oscillatory. This leads to reversals of the axial dipole
polarity, involving a competition with the quadrupolar component.Comment: 6 pages, 5 figure
Chaotic magnetic field reversals in turbulent dynamos
We present direct numerical simulations of reversals of the magnetic field
generated by swirling flows in a spherical domain. In agreement with a recent
model, we observe that coupling dipolar and quadrupolar magnetic modes by an
asymmetric forcing of the flow generates field reversals. In addition, we show
that this mechanism strongly depends on the value of the magnetic Prandtl
number.Comment: 4 pages, 5 figure
Bypassing Cowling's theorem in axisymmetric fluid dynamos
We present a numerical study of the magnetic field generated by an
axisymmetrically forced flow in a spherical domain. At small enough Reynolds
number, Re, the flow is axisymmetric and generates an equatorial dipole above a
critical magnetic Reynolds number Rmc . The magnetic field thus breaks
axisymmetry, in agreement with Cowling's theorem. This structure of the
magnetic field is however replaced by a dominant axial dipole when Re is larger
and allows non axisymmetric fluctuations in the flow. We show here that even in
the absence of such fluctuations, an axial dipole can also be generated, at low
Re, through a secondary bifurcation, when Rm is increased above the dynamo
threshold. The system therefore always find a way to bypass the constraint
imposed by Cowling's theorem. We understand the dynamical behaviors that result
from the interaction of equatorial and axial dipolar modes using simple model
equations for their amplitudes derived from symmetry arguments.Comment: 4 pages, 6 figure
Vorticity production through rotation, shear and baroclinicity
In the absence of rotation and shear, and under the assumption of constant
temperature or specific entropy, purely potential forcing by localized
expansion waves is known to produce irrotational flows that have no vorticity.
Here we study the production of vorticity under idealized conditions when there
is rotation, shear, or baroclinicity, to address the problem of vorticity
generation in the interstellar medium in a systematic fashion. We use
three-dimensional periodic box numerical simulations to investigate the various
effects in isolation. We find that for slow rotation, vorticity production in
an isothermal gas is small in the sense that the ratio of the root-mean-square
values of vorticity and velocity is small compared with the wavenumber of the
energy-carrying motions. For Coriolis numbers above a certain level, vorticity
production saturates at a value where the aforementioned ratio becomes
comparable with the wavenumber of the energy-carrying motions. Shear also
raises the vorticity production, but no saturation is found. When the
assumption of isothermality is dropped, there is significant vorticity
production by the baroclinic term once the turbulence becomes supersonic. In
galaxies, shear and rotation are estimated to be insufficient to produce
significant amounts of vorticity, leaving therefore only the baroclinic term as
the most favorable candidate. We also demonstrate vorticity production visually
as a result of colliding shock fronts.Comment: 9 pages, 10 figures, Accepted for publication in A&
Influence of high permeability disks in an axisymmetric model of the Cadarache dynamo experiment
Numerical simulations of the kinematic induction equation are performed on a
model configuration of the Cadarache von-K\'arm\'an-Sodium dynamo experiment.
The effect of a localized axisymmetric distribution of relative permeability
{\mu} that represents soft iron material within the conducting fluid flow is
investigated. The critical magnetic Reynolds number Rm^c for dynamo action of
the first non-axisymmetric mode roughly scales like
Rm^c({\mu})-Rm^c({\mu}->infinity) ~ {\mu}^(-1/2) i.e. the threshold decreases
as {\mu} increases. This scaling law suggests a skin effect mechanism in the
soft iron disks. More important with regard to the Cadarache dynamo experiment,
we observe a purely toroidal axisymmetric mode localized in the high
permeability disks which becomes dominant for large {\mu}. In this limit, the
toroidal mode is close to the onset of dynamo action with a (negative)
growth-rate that is rather independent of the magnetic Reynolds number. We
qualitatively explain this effect by paramagnetic pumping at the fluid/disk
interface and propose a simplified model that quantitatively reproduces
numerical results. The crucial role of the high permeability disks for the mode
selection in the Cadarache dynamo experiment cannot be inferred from
computations using idealized pseudo-vacuum boundary conditions (H x n = 0).Comment: 16 pages, 9 Figures, published in New Journal of Physics 14(2012),
05300
Simulating magnetic fields in the Antennae galaxies
We present self-consistent high-resolution simulations of NGC4038/4039 (the
"Antennae galaxies") including star formation, supernova feedback and magnetic
fields performed with the N-body/SPH code Gadget, in which magnetohydrodynamics
are followed with the SPH method. We vary the initial magnetic field in the
progenitor disks from 1 nG to 100 muG. At the time of the best match with the
central region of the Antennae system the magnetic field has been amplified by
compression and shear flows to an equilibrium field of approximately 10 muG,
independent of the initial seed field. These simulations are a proof of the
principle that galaxy mergers are efficient drivers for the cosmic evolution of
magnetic fields. We present a detailed analysis of the magnetic field structure
in the central overlap region. Simulated radio and polarization maps are in
good morphological and quantitative agreement with the observations. In
particular, the two cores with the highest synchrotron intensity and ridges of
regular magnetic fields between the cores and at the root of the southern tidal
arm develop naturally in our simulations. This indicates that the simulations
are capable of realistically following the evolution of the magnetic fields in
a highly non-linear environment. We also discuss the relevance of the
amplification effect for present day magnetic fields in the context of
hierarchical structure formation.Comment: 18 pages, 14 figures, accepte
A spherical shell numerical dynamo benchmark with pseudo vacuum magnetic boundary conditions
It is frequently considered that many planetary magnetic fields originate as a result of convection within planetary cores. Buoyancy forces responsible for driving the convection generate a fluid flow that is able to induce magnetic fields; numerous sophisticated computer codes are able to simulate the dynamic behaviour of such systems. This paper reports the results of a community activity aimed at comparing numerical results of several different types of computer codes that are capable of solving the equations of momentum transfer, magnetic field generation and heat transfer in the setting of a spherical shell, namely a sphere containing an inner core. The electrically conducting fluid is incompressible and rapidly rotating and the forcing of the flow is thermal convection under the Boussinesq approximation. We follow the original specifications and results reported in Harder & Hansen to construct a specific benchmark in which the boundaries of the fluid are taken to be impenetrable, non-slip and isothermal, with the added boundary condition for the magnetic field <b>B</b> that the field must be entirely radial there; this type of boundary condition for <b>B</b> is frequently referred to as ‘pseudo-vacuum’. This latter condition should be compared with the more frequently used insulating boundary condition. This benchmark is so-defined in order that computer codes based on local methods, such as finite element, finite volume or finite differences, can handle the boundary condition with ease. The defined benchmark, governed by specific choices of the Roberts, magnetic Rossby, Rayleigh and Ekman numbers, possesses a simple solution that is steady in an azimuthally drifting frame of reference, thus allowing easy comparison among results. Results from a variety of types of code are reported, including codes that are fully spectral (based on spherical harmonic expansions in angular coordinates and polynomial expansions in radius), mixed spectral and finite difference, finite volume, finite element and also a mixed Fourier-finite element code. There is good agreement among codes
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
Tayler–Spruit dynamo simulations for the modeling of radiative stellar layers
International audienceContext. Maxwell stresses exerted by dynamo-generated magnetic fields have been proposed as an efficient mechanism to transport angular momentum in radiative stellar layers. Numerical simulations are still needed to understand its trigger conditions and the saturation mechanisms. Aims. The present study follows up on a recent paper where we reported on the first simulations of Tayler-Spruit dynamos. Here we extend the parameter space explored to assess in particular the influence of stratification on the dynamo solutions. We also present numerical verification of theoretical assumptions made previously that were instrumental in deriving the classical prescription for angular momentum transport implemented in stellar evolution codes. Methods. A simplified radiative layer is modeled numerically by considering the dynamics of a stably stratified, differentially rotating, magnetized fluid in a spherical shell. Results. Our simulations display a diversity of magnetic field topologies and amplitudes depending on the flow parameters, including hemispherical solutions. The Tayler-Spruit dynamos reported here are found to satisfy magnetostrophic equilibrium and achieve efficient turbulent transport of angular momentum, following Spruit's heuristic prediction
- …