The VKS experiment studies dynamo action in the flow generated inside a cylinder filled with liqu... more The VKS experiment studies dynamo action in the flow generated inside a cylinder filled with liquid sodium by the rotation of coaxial impellers (the von Kármán geometry). We report observations related to the self-generation of a stationary dynamo when the flow forcing is symmetric, i.e. when the impellers rotate in opposite directions at equal angular velocities. The bifurcation is found to be supercritical, with a neutral mode whose geometry is predominantly axisymmetric. We then report the different dynamical dynamo regimes observed when the flow forcing is asymmetric, including magnetic field reversals. We finally show that these dynamics display characteristic features of low dimensional dynamical systems despite the high degree of turbulence in the flow. L'expérience VKS : dynamiques d'une dynamo turbulente Résumé L'expérience VKSétudie l'effet dynamo dans l'écoulement de von Kármán engendrée dans un cylindre par la contre-rotation des deux turbines coaxiales. Nous décrivons d'abord l'apparition et l'auto-entretien d'une dynamo statistiquement stationnaire, engendrée quand les turbines sont en contra-rotation exacte. L'in-Preprint submitted to Elsevier Science 9 juillet 2008 stabilité dynamo se développe au travers d'une bifurcation super-critique avec un mode neutre essentiellement axisymétrique. Nous discutons ensuite l'observation de régimes dynamique riches, engendrés lorsque les turbines sont en contra-rotationà des vitesses différentes, c'est-à-dire en présence d'une rotation globale de l'écoulement. Renversements erratiques, oscillations et régimes de bouffées intenses de champ magnétique sont alors observés. Nous montrons que ces comportements, engendrés pour des valeurs voisines des paramètres de contrôle de l'écoulement, ont des caractéristiques très similaires aux systèmes chaotiques de faible dimensionalité en dépit de la turbulence importante de l'écoulement.
46 allée dItalie, F-69364 Lyon (France) PACS 91.25.Cw-Origins and models of the magnetic field; d... more 46 allée dItalie, F-69364 Lyon (France) PACS 91.25.Cw-Origins and models of the magnetic field; dynamo theories PACS 47.65.+a-Magnetohydrodynamics and electrohydrodynamics Abstract.-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.
We report the first experimental observation of a bistable dynamo regime. A turbulent flow of liq... more We report the first experimental observation of a bistable dynamo regime. A turbulent flow of liquid sodium is generated between two disks in the von Kármán geometry (VKS experiment). When one disk is kept at rest, bistability is observed between a stationary and an oscillatory magnetic field. The stationary and oscillatory branches occur in the vicinity of a codimension-two bifurcation that results from the coupling between two modes of magnetic field. We present an experimental study of the two regimes and study in detail the region of bistability that we understand in terms of dynamical system theory. Despite the very turbulent nature of the flow, the bifurcations of the magnetic field are correctly described by a low-dimensional model. In addition, the different regimes are robust; i.e. turbulent fluctuations do not drive any transition between the oscillatory and stationary states in the region of bistability.
Atmospheric motions are governed by turbulent motions associated to nontrivial energy transfers a... more Atmospheric motions are governed by turbulent motions associated to nontrivial energy transfers at small scales (direct cascade) and/or at large scales (inverse cascade). Although it is known that the two cascades coexist, energy fluxes have been previously investigated from the spectral point of view but not on their instantaneous spatial and local structure. Here, we compute local and instantaneous subfilter-scale energy transfers in two sets of reanalyses (NCEP–NCAR and ERA-Interim) in the troposphere and the lower stratosphere for the year 2005. The fluxes are mostly positive (toward subgrid scales) in the troposphere and negative in the stratosphere, reflecting the baroclinic and barotropic nature of the motions, respectively. The most intense positive energy fluxes are found in the troposphere and are associated with baroclinic eddies or tropical cyclones. The computation of such fluxes can be used to characterize the amount of energy lost or missing at the smallest scales in ...
Superfluid turbulence (TSF) project uses liquid helium for the fundamental study of turbulent phe... more Superfluid turbulence (TSF) project uses liquid helium for the fundamental study of turbulent phenomena behind a passive grid and is able to work both in HeI and in HeII. The helium flow is generated by a cold Barber and Nichols circulating pump, whereas helium flow temperature is kept constant by means of a heat exchanger immersed in a saturated bath. This experiment takes profit of the CEA Grenoble refrigerator (nominal capacity of 400 Watt at 1.8 K) to remove the heat due to pressure losses in this high Reynolds number experiment. In order to resolve the Kolmogorov scale associated with high Re flow, local instrumentation (e.g. sub-micrometer anemometer) was developed. The difficulties encountered with this local and fragile instrumentation in a quasi industrial environment are discussed and the adopted solutions are also described. Finally, first results (permanent mass flow rate of a few hundreds g/s) obtained both in the two phases of helium are presented.
Numerical studies of a kinematic dynamo based on von Kármán type flows between two counterrotatin... more Numerical studies of a kinematic dynamo based on von Kármán 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 briefly 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 Kármán 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.
-Une oscillation collective périodique de la position des rouleaux est observée au cours d'expéri... more -Une oscillation collective périodique de la position des rouleaux est observée au cours d'expériences de convection de Rayleigh-Bénard en géométrie quasi-unidimensionnelle-c.a.d. quand l'une des dimensions horizontales est inférieure à l'épaisseur de la couche de fluide. Ces oscillations sont liées à la présence de très petites longueurs d'onde et conduisent directement à des comportements d'intermittences turbulentes spatio temporelles lorsque la géométrie est annulaire. Abstract-Collective periodic oscillations of the rolls' position are observed in experiments of Rayleigh-Benard convection, when one of the horizontal extensions is smaller than the depth of the fluid layer (quasi one dimensional geometry). The oscillations are related to the presence of very short wavelengths and can lead directly to turbulent spatio temporal intermittencies, when the convection is achieved in an annular geometry.
Rayleigh-Bénard and Bénard-Marangoni instabilities have been studied for roughly a century and ha... more Rayleigh-Bénard and Bénard-Marangoni instabilities have been studied for roughly a century and have served as prototypes for the transition to temporal chaos as well as spatiotemporal chaos of an initially stationary pattern. Using the Marangoni effect [1,2] with an horizontal temperature gradient to drive the system out of equilibrium, one can observe propagating waves instabilities: hydrothermal waves [3]. This paper presents different instability regimes of thermocapillary flows in extended geometry, focusing on propagating waves. We first introduce thermocapillary flows, and give some indications about physical effects involved. We then review experimental results in cylindrical geometry and illustrate how rich those systems are.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2003
The microdisplacements generated by a small localized overload at the free surface are visualized... more The microdisplacements generated by a small localized overload at the free surface are visualized experimentally inside a packing of steel beads. For a triangular packing, beads rearrangements remain confined in two inverted triangles on both sides of the applied overload. This pattern disappears for stronger disorder. A simple model allows us to account for these observations and to relate them to the stress function response measured via photoelastic visualizations. This provides a different tool to probe the mechanical Green's function in weakly confined packings of rigid grains the description of which is the most challenged.
Above a critical flow rate, a single jet impinging from below on a water-air interface oscillates... more Above a critical flow rate, a single jet impinging from below on a water-air interface oscillates while arrays of jets exhibit collective behaviors. The aim of the paper is to explain the physics governing such an array. First, a series of experiments shows that the instability mechanism leading to the oscillation of a single jet is due to a feedback effect between surface distortion and the unstable jet as in the ''jet-edge'' system and that several jets are coupled through surface waves. Then a modelization is proposed in which jets are considered as oscillators close to their limit cycle coupled by delayed action to take into account the wave propagation. A good agreement is obtained between the predictions of the model and experiments on sets of two or four jets. The long range of the surface waves and the large number of oscillators render this jet configuration very similar to idealized phase coupled nonlinear oscillators investigated in theoretical models but with a not instantaneous coupling ensured by traveling waves. This allows in particular the existence of various synchronization modes, which appear as geometrical patterns. ͓S1063-651X͑98͒07407-8͔
This paper provides a prescription for the turbulent viscosity in rotating shear flows for use e.... more This paper provides a prescription for the turbulent viscosity in rotating shear flows for use e.g. in geophysical and astrophysical contexts. This prescription is the result of the detailed analysis of the experimental data obtained in several studies of the transition to turbulence and turbulent transport in Taylor-Couette flow. We first introduce a new set of control parameters, based on dynamical rather than geometrical considerations, so that the analysis applies more naturally to rotating shear flows in general and not only to Taylor-Couette flow. We then investigate the transition thresholds in the supercritical and the subcritical regime in order to extract their general dependencies on the control parameters. The inspection of the mean profiles provides us with some general hints on the mean to laminar shear ratio. Then the examination of the torque data allows us to propose a decomposition of the torque dependence on the control parameters in two terms, one completely given by measurements in the case where the outer cylinder is at rest, the other one being a universal function provided here from experimental fits. As a result, we obtain a general expression for the turbulent viscosity and compare it to existing prescription in the literature. Finally, throughout all the paper we discuss the influence of additional effects such as stratification or magnetic fields.
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Papers by F. Daviaud