The VKS experiment: turbulent dynamical dynamos

Abstract

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. 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.

Energy transfers in dynamos with small magnetic Prandtl numbers

“…The divergence from equipartition is possibly due to the multiple attractors present in the chaotic regime [24,29,30]. It is conjectured that the equipartition is probably a robust property of the attractor of the fully-developed turbulence.…”

Magnetic induction maps in a magnetized spherical Couette flow experiment

“…In several of these experiments, the mean flow was well characterized, and was expected to yield a dynamo above a critical magnetic Reynolds number that was achievable. It therefore came as a surprise that none of these experiments produced a dynamo, although a rich variety of dynamo behaviours have been discovered in the VKS experiment in Cadarache, France (Berhanu et al, 2007;Monchaux et al, 2007;Aumaitre et al, 2008) when ferromagnetic disks stir the fluid.…”

Laboratory Dynamo Experiments

“…The hundred-fold increase is also one order of magnitude larger than the induction effects and field amplification previously recorded in the VKS experiment with externally applied magnetic field, either homogeneously over the flow volume (Bourgoin et al 2002) or localized at the flow boundary (Volk et al 2006a). The most salient features of the dynamo observed with a symmetric forcing are the following (Monchaux et al , 2009Aumaître et al 2008):…”