17 research outputs found
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
Mechanical and thermal characterization of an epoxy foam as thermal layer insulation for a glass fiber reinforced polymer
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Magnetohydrodynamic experiments on cosmic magnetic fields
It is widely known that cosmic magnetic fields, i.e. the fields of planets,
stars, and galaxies, are produced by the hydromagnetic dynamo effect in moving
electrically conducting fluids. It is less well known that cosmic magnetic
fields play also an active role in cosmic structure formation by enabling
outward transport of angular momentum in accretion disks via the
magnetorotational instability (MRI). Considerable theoretical and computational
progress has been made in understanding both processes. In addition to this,
the last ten years have seen tremendous efforts in studying both effects in
liquid metal experiments. In 1999, magnetic field self-excitation was observed
in the large scale liquid sodium facilities in Riga and Karlsruhe. Recently,
self-excitation was also obtained in the French "von Karman sodium" (VKS)
experiment. An MRI-like mode was found on the background of a turbulent
spherical Couette flow at the University of Maryland. Evidence for MRI as the
first instability of an hydrodynamically stable flow was obtained in the
"Potsdam Rossendorf Magnetic Instability Experiment" (PROMISE). In this review,
the history of dynamo and MRI related experiments is delineated, and some
directions of future work are discussed.Comment: 25 pages, 26 figures, to appear in ZAM
Impact of time-dependent non-axisymmetric velocity perturbations on dynamo action of von-K\'arm\'an-like flows
We have performed numerical simulations of the kinematic induction equation
in order to examine the dynamo efficiency of an axisymmetric
von-K\'arm\'an-like flow subject to time-dependent non-axisymmetric velocity
perturbations. The numerical model is based on the setup of the French
Von-K\'arm\'an-Sodium dynamo (VKS) and on the flow measurements from a model
water experiment conducted at the University of Navarra in Pamplona, Spain. Our
simulations show that the interactions of azimuthally drifting flow
perturbations with the fundamental drift of the magnetic eigenmode (caused by
the inevitable equatorial symmetry breaking of the basic flow) essentially
determine the temporal behavior of the dynamo state. We find two distinct
regimes of dynamo action that depend on the (prescribed) drift frequency of an
() vortex-like flow perturbation. For comparatively slowly drifting
vortices we observe a narrow window with enhanced growth-rates and a drift of
the magnetic eigenmode that is synchronized with the perturbation drift. The
resonance-like enhancement of the growth-rates takes place when the vortex
drift frequency roughly equals the drift frequency of the magnetic eigenmode in
the unperturbed system. Outside of this small window, the field generation is
hampered compared to the unperturbed case, and the field amplitude of the
magnetic eigenmode is modulated with approximately twice the vortex drift
frequency. The abrupt transition between the resonant regime and the modulated
regime is identified as an spectral exceptional point where eigenvalues
(growth-rates and frequencies) and eigenfunctions of two previously independent
modes collapse.Comment: 14 pages, 14 Figures. Minor changes to match the published versio
On the magnetic fields generated by experimental dynamos
We review the results obtained by three successful fluid dynamo experiments
and discuss what has been learnt from them about the effect of turbulence on
the dynamo threshold and saturation. We then discuss several questions that are
still open and propose experiments that could be performed to answer some of
them.Comment: 40 pages, 13 figure