196 research outputs found
Transport of magnetic field by a turbulent flow of liquid sodium
We study the effect of a turbulent flow of liquid sodium generated in the von
K\'arm\'an geometry, on the localized field of a magnet placed close to the
frontier of the flow. We observe that the field can be transported by the flow
on distances larger than its integral length scale. In the most turbulent
configurations, the mean value of the field advected at large distance
vanishes. However, the rms value of the fluctuations increases linearly with
the magnetic Reynolds number. The advected field is strongly intermittent.Comment: 4 pages, 6 figure
Generation of magnetic field by dynamo action in a turbulent flow of liquid sodium
We report the observation of dynamo action in the VKS experiment, i.e., the
generation of magnetic field by a strongly turbulent swirling flow of liquid
sodium. Both mean and fluctuating parts of the field are studied. The dynamo
threshold corresponds to a magnetic Reynolds number Rm \sim 30. A mean magnetic
field of order 40 G is observed 30% above threshold at the flow lateral
boundary. The rms fluctuations are larger than the corresponding mean value for
two of the components. The scaling of the mean square magnetic field is
compared to a prediction previously made for high Reynolds number flows.Comment: 4 pages, 5 figure
Magnetic field reversals in an experimental turbulent dynamo
We report the first experimental observation of reversals of a dynamo field
generated in a laboratory experiment based on a turbulent flow of liquid
sodium. The magnetic field randomly switches between two symmetric solutions B
and -B. We observe a hierarchy of time scales similar to the Earth's magnetic
field: the duration of the steady phases is widely distributed, but is always
much longer than the time needed to switch polarity. In addition to reversals
we report excursions. Both coincide with minima of the mechanical power driving
the flow. Small changes in the flow driving parameters also reveal a large
variety of dynamo regimes.Comment: 5 pages, 4 figure
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
Solitary vortex couples in viscoelastic Couette flow
We report experimental observation of a localized structure, which is of a
new type for dissipative systems. It appears as a solitary vortex couple
("diwhirl") in Couette flow with highly elastic polymer solutions. A unique
property of the diwhirls is that they are stationary, in contrast to the usual
localized wave structures in both Hamiltonian and dissipative systems which are
stabilized by wave dispersion. It is also a new object in fluid dynamics - a
couple of vortices that build a single entity somewhat similar to a magnetic
dipole. The diwhirls arise as a result of a purely elastic instability through
a hysteretic transition at negligible Reynolds numbers. It is suggested that
the vortex flow is driven by the same forces that cause the Weissenberg effect.
The diwhirls have a striking asymmetry between the inflow and outflow, which is
also an essential feature of the suggested elastic instability mechanism.Comment: 9 pages (LaTeX), 5 Postscript figures, submitte
Traveling Granular Segregation Patterns in a Long Drum Mixer
Mixtures of granular media often exhibit size segregation along the axis of a
partially-filled, horizontal, rotating cylinder. Previous experiments have
observed axial bands of segregation that grow from concentration fluctuations
and merge in a manner analogous to spinodal decomposition. We have observed
that a new dynamical state precedes this effect in certain mixtures:
bi-directional traveling waves. By preparing initial conditions, we found that
the wave speed decreased with wavelength. Such waves appear to be inconsistent
with simple PDE models which are first order in time.Comment: 11 page
Stochastic Resonance in Noisy Non-Dynamical Systems
We have analyzed the effects of the addition of external noise to
non-dynamical systems displaying intrinsic noise, and established general
conditions under which stochastic resonance appears. The criterion we have
found may be applied to a wide class of non-dynamical systems, covering
situations of different nature. Some particular examples are discussed in
detail.Comment: 4 pages, RevTex, 3 PostScript figures available upon reques
Noise and Periodic Modulations in Neural Excitable Media
We have analyzed the interplay between noise and periodic modulations in a
mean field model of a neural excitable medium. To this purpose, we have
considered two types of modulations; namely, variations of the resistance and
oscillations of the threshold. In both cases, stochastic resonance is present,
irrespective of if the system is monostable or bistable.Comment: 13 pages, RevTex, 5 PostScript figure
Noise suppression by noise
We have analyzed the interplay between an externally added noise and the
intrinsic noise of systems that relax fast towards a stationary state, and
found that increasing the intensity of the external noise can reduce the total
noise of the system. We have established a general criterion for the appearance
of this phenomenon and discussed two examples in detail.Comment: 4 pages, 4 figure
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