77 research outputs found
Triadic resonances in non-linear simulations of a fluid flow in a precessing cylinder
We present results from three-dimensional non-linear hydrodynamic simulations
of a precession driven flow in cylindrical geometry. The simulations are
motivated by a dynamo experiment currently under development at
Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in which the possibility of
generating a magnetohydrodynamic dynamo will be investigated in a cylinder
filled with liquid sodium and simultaneously rotating around two axes. In this
study, we focus on the emergence of non-axisymmetric time-dependent flow
structures in terms of inertial waves which - in cylindrical geometry - form
so-called Kelvin modes. For a precession ratio
the amplitude of the forced Kelvin mode
reaches up to one fourth of the rotation velocity of the cylindrical container
confirming that precession provides a rather efficient flow driving mechanism
even at moderate values of . More relevant for dynamo action might
be free Kelvin modes with higher azimuthal wave number. These free Kelvin modes
are triggered by non-linear interactions and may constitute a triadic resonance
with the fundamental forced mode when the height of the container matches their
axial wave lengths. Our simulations reveal triadic resonances at aspect ratios
close to those predicted by the linear theory except around the primary
resonance of the forced mode. In that regime we still identify various free
Kelvin modes, however, all of them exhibit a retrograde drift around the
symmetry axis of the cylinder and none of them can be assigned to a triadic
resonance. The amplitudes of the free Kelvin modes always remain below the
forced mode but may reach up to 6% of the of the container's angular velocity.
The properties of the free Kelvin modes will be used in future simulations of
the magnetic induction equation to investigate their ability to provide for
dynamo action.Comment: 26 pages, 14 figures, submitted to New J. Phy
Towards a precession driven dynamo experiment
The most ambitious project within the DREsden Sodium facility for DYNamo and
thermohydraulic studies (DRESDYN) at Helmholtz-Zentrum Dresden-Rossendorf
(HZDR) is the set-up of a precession-driven dynamo experiment. After discussing
the scientific background and some results of water pre-experiments and
numerical predictions, we focus on the numerous structural and design problems
of the machine. We also outline the progress of the building's construction,
and the status of some other experiments that are planned in the framework of
DRESDYN.Comment: 9 pages, 6 figures, submitted to Magnetohydrodynamic
Mode Conversion and Period Doubling at Plasma- Unity in an Alfv\'en-Wave Experiment with Liquid Rubidium
We report Alfv\'en-wave experiments with liquid rubidium at the Dresden High
Magnetic Field Laboratory (HLD). Reaching up to 63 T, the pulsed magnetic field
exceeds the critical value of 54 T at which the Alfv\'en speed becomes equal to
the sound speed (plasma- unity). At this threshold we observe a period
doubling of an applied 8 kHz CW excitation, a clear footprint for a parametric
resonance between magnetosonic waves and Alfv\'en waves.Comment: 5 pages, 4 figure
Alfv\'en wave experiments with liquid rubidium in a pulsed magnetic field
Magnetic fields are key ingredients for heating the solar corona to
temperatures of several million Kelvin. A particularly important region with
respect to this is the so-called magnetic canopy below the corona, where sound
and Alfv\'en waves have roughly the same speed and can, therefore, easily
transform into each other. We present the results of an Alfv\'en-wave
experiment with liquid rubidium carried out in a pulsed field of up to 63 T. At
the critical point of 54 T, where the speeds of Alfv\'en waves and sound
coincide, a new 4 kHz signal appears in addition to the externally excited 8
kHz torsional wave. This emergence of an Alfv\'en wave with a doubled period is
in agreement with the theoretical predictions of a parametric resonance between
the two wave types. We also present preliminary results from numerical
simulations of Alfv\'en and magneto-sonic waves using a compressible MHD code.Comment: 7 pages, 4 figure
Helical magnetorotational instability in a Taylor-Couette flow with strongly reduced Ekman pumping
The magnetorotational instability (MRI) is thought to play a key role in the
formation of stars and black holes by sustaining the turbulence in
hydrodynamically stable Keplerian accretion discs. In previous experiments the
MRI was observed in a liquid metal Taylor-Couette flow at moderate Reynolds
numbers by applying a helical magnetic field. The observation of this helical
MRI (HMRI) was interfered with a significant Ekman pumping driven by solid
end-caps that confined the instability only to a part of the Taylor-Couette
cell. This paper describes the observation of the HMRI in an improved
Taylor-Couette setup with the Ekman pumping significantly reduced by using
split end-caps. The HMRI, which now spreads over the whole height of the cell,
appears much sharper and in better agreement with numerical predictions. By
analyzing various parameter dependencies we conclude that the observed HMRI
represents a self-sustained global instability rather than a noise-sustained
convective one.Comment: 30 pages, 22 figures, submitted to PR
Magnetic Field Saturation in the Riga Dynamo Experiment
After the dynamo experiment in November 1999 had shown magnetic field
self-excitation in a spiraling liquid metal flow, in a second series of
experiments emphasis was placed on the magnetic field saturation regime as the
next principal step in the dynamo process. The dependence of the strength of
the magnetic field on the rotation rate is studied. Various features of the
saturated magnetic field are outlined and possible saturation mechanisms are
discussed.Comment: 4 pages, 8 figure
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