259 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
Observation of gravity-capillary wave turbulence
We report the observation of the cross-over between gravity and capillary
wave turbulence on the surface of mercury. The probability density functions of
the turbulent wave height are found to be asymmetric and thus non Gaussian. The
surface wave height displays power-law spectra in both regimes. In the
capillary region, the exponent is in fair agreement with weak turbulence
theory. In the gravity region, it depends on the forcing parameters. This can
be related to the finite size of the container. In addition, the scaling of
those spectra with the mean energy flux is found in disagreement with weak
turbulence theory for both regimes
Existence and stability of singular patterns in a Ginzburg–Landau equation coupled with a mean field
We study singular patterns in a particular system of parabolic partial differential equations which consist of a Ginzburg–Landau equation and a mean field equation. We prove the existence of the three simplest concentrated periodic stationary patterns (single spikes, double spikes, double transition layers) by composing them of more elementary patterns and solving the corresponding consistency conditions. In the case of spike patterns we prove stability for sufficiently large spatial periods by first showing that the eigenvalues do not tend to zero as the period goes to infinity and then passing in the limit to a nonlocal eigenvalue problem which can be studied explicitly. For the two other patterns we show instability by using the variational characterization of eigenvalues
Mechanism for Surface Waves in Vibrated Granular Material
We use molecular dynamics simulations to study the formation of surface waves
in vertically vibrated granular material. We find that horizontal movements of
particles, which are essential for the formation of the waves, consist of two
distinct processes. First, the movements sharply increase while the particles
are colliding with a bottom plate, where the duration of the collisions is very
short compared to the period of the vibration. Next, the movements gradually
decrease between the collisions, during which the particles move through the
material. We also find that the horizontal velocity field after the collisions
is strongly correlated to the surface profile before the collisions.Comment: 6 pages, 3 figures (included
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
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
Observation of intermittency in wave turbulence
We report the observation of intermittency in gravity-capillary wave
turbulence on the surface of mercury. We measure the temporal fluctuations of
surface wave amplitude at a given location. We show that the shape of the
probability density function of the local slope increments of the surface waves
strongly changes across the time scales. The related structure functions and
the flatness are found to be power laws of the time scale on more than one
decade. The exponents of these power laws increase nonlinearly with the order
of the structure function. All these observations show the intermittent nature
of the increments of the local slope in wave turbulence. We discuss the
possible origin of this intermittency.Comment: new version to Phys. Rev. Let
A quasi-elastic regime for vibrated granular gases
Using simple scaling arguments and two-dimensional numerical simulations of a
granular gas excited by vibrating one of the container boundaries, we study a
double limit of small and large , where is the restitution
coefficient and the size of the container. We show that if the particle
density and where is the particle diameter, are
kept constant and small enough, the granular temperature, i.e. the mean value
of the kinetic energy per particle, , tends to a constant whereas the
mean dissipated power per particle, , decreases like when
increases, provided that . The relative fluctuations
of , and the power injected by the moving boundary, , have simple
properties in that regime. In addition, the granular temperature can be
determined from the fluctuations of the power injected by the moving
boundary.
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