70 research outputs found
Ambivalent effects of added layers on steady kinematic dynamos in cylindrical geometry: application to the VKS experiment
The intention of the ''von Karman sodium'' (VKS) experiment is to study the
hydromagnetic dynamo effect in a highly turbulent and unconstrained flow. Much
effort has been devoted to the optimization of the mean flow and the lateral
boundary conditions in order to minimize the critical magnetic Reynolds number
and hence the necessary motor power. The main focus of this paper lies on the
role of ''lid layers'', i.e. layers of liquid sodium between the impellers and
the end walls of the cylinder. First, we study an analytical test flow to show
that lid layers can have an ambivalent effect on the efficiency of the dynamo.
The critical magnetic Reynolds number shows a flat minimum for a small lid
layer thickness, but increases for thicker layers. For the actual VKS geometry
it is shown that static lid layers yield a moderate increase of the critical
magnetic Reynolds number by approximately 12 per cent. A more dramatic increase
by 100 until 150 per cent can occur when some rotational flow is taken into
account in those layers. Possible solutions of this problem are discussed for
the real dynamo facility.Comment: 24 pages, 11 figures, minor changes, to appear in European Journal of
Mechanics B/Fluid
On the properties of steady states in turbulent axisymmetric flows
We experimentally study the properties of mean and most probable velocity
fields in a turbulent von K\'arm\'an flow. These fields are found to be
described by two families of functions, as predicted by a recent statistical
mechanics study of 3D axisymmetric flows. We show that these functions depend
on the viscosity and on the forcing. Furthermore, when the Reynolds number is
increased, we exhibit a tendency for Beltramization of the flow, i.e. a
velocity-vorticity alignment. This result provides a first experimental
evidence of nonlinearity depletion in non-homogeneous non-isotropic turbulent
flow.Comment: latex prl-stationary-051215arxiv.tex, 9 files, 6 figures, 4 pages
(http://www-drecam.cea.fr/spec/articles/S06/008/
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
Towards an experimental von Karman dynamo: numerical studies for an optimized design
Numerical studies of a kinematic dynamo based on von Karman type flows
between two counterrotating disks in a finite cylinder are reported. The flow
has been optimized using a water model experiment, varying the driving
impellers configuration. A solution leading to dynamo action for the mean flow
has been found. This solution may be achieved in VKS2, the new sodium
experiment to be performed in Cadarache, France. The optimization process is
described and discussed, then the effects of adding a stationary conducting
layer around the flow on the threshold, on the shape of the neutral mode and on
the magnetic energy balance are studied. Finally, the possible processes
involved into kinematic dynamo action in a von Karman flow are reviewed and
discussed. Among the possible processes we highlight the joint effect of the
boundary-layer radial velocity shear and of the Ohmic dissipation localized at
the flow/outer-shell boundary
Design and Experimental Validation of a Ducted Counter-rotating Axial-flow Fans System
An experimental study on the design of counter-rotating axial-flow fans was
carried out. The fans were designed using an inverse method. In particular, the
system is designed to have a pure axial discharge flow. The counter-rotating
fans operate in a ducted-flow configuration and the overall performances are
measured in a normalized test bench. The rotation rate of each fan is
independently controlled. The relative axial spacing between fans can vary from
17% to 310%. The results show that the efficiency is strongly increased
compared to a conventional rotor or to a rotor-stator stage. The effects of
varying the rotation rates ratio on the overall performances are studied and
show that the system has a very flexible use, with a large patch of high
efficient operating points in the parameter space. The increase of axial
spacing causes only a small decrease of the efficiencyComment: accepted in Journal of Fluid Engineerin
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
Design and Experimental Validation of a Ducted Counter-Rotating Axial-Flow Fans System
An experimental study on the design of counter-rotating axial-flow fans was carried out. The fans were designed using an inverse method. In particular, the system is designed to have a pure axial discharge flow. The counter-rotating fans operate in a ducted-flow configuration and the overall performances are measured in a normalized test bench. The rotation rate of each fan is independently controlled. The relative axial spacing between fans can vary from 17% to 310%. The results show that the efficiency is strongly increased compared to a conventional rotor or to a rotor-stator stage. The effects of varying the rotation rates ratio on the overall performances are studied and show that the system has a very flexible use, with a large patch of high efficient operating points in the parameter space. The increase of axial spacing causes only a small decrease of the efficiency
The integral equation approach to kinematic dynamo theory and its application to dynamo experiments in cylindrical geometry
The conventional magnetic induction equation that governs hydromagnetic
dynamo action is transformed into an equivalent integral equation system. An
advantage of this approach is that the computational domain is restricted to
the region occupied by the electrically conducting fluid and to its boundary.
This integral equation approach is first employed to simulate kinematic dynamos
excited by Beltrami-like flows in a finite cylinder. The impact of externally
added layers around the cylinder on the onset of dynamo actions is
investigated. Then it is applied to simulate dynamo experiments within
cylindrical geometry including the von Karman sodium (VKS) experiment and the
Riga dynamo experiment. A modified version of this approach is utilized to
investigate magnetic induction effects under the influence of externally
applied magnetic fields which is also important to measure the proximity of a
given dynamo facility to the self-excitation threshold.Comment: 22 pages, 14 figure
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