Impact of Impellers on the Axisymmetric Magnetic Mode in the VKS2 Dynamo Experiment

Abstract

In the von Kármán Sodium 2 (VKS2) successful dynamo experiment of September 2006, the observed magnetic field showed a strong axisymmetric component, implying that nonaxisymmetric components of the flow field were acting. By modeling the induction effect of the spiraling flow between the blades of the impellers in a kinematic dynamo code, we find that the axisymmetric magnetic mode is excited. The control parameters are the magnetic Reynolds number of the mean flow, the coefficient measuring the induction effect alpha, and the type of boundary conditions. We show that using realistic values of alpha, the observed critical magnetic Reynolds number, Rm;{c} approximately 32, can be reached easily with ferromagnetic boundary conditions. We conjecture that the dynamo action achieved in this experiment may not be related to the turbulence in the bulk of the flow, but rather to the alpha effect induced by the impellers.

Effects of discontinuous magnetic permeability on magnetodynamic problems

“…One possible scenario to explain this behavior is that small scale helical turbulence may have induction effects via the so called a-effect. A source for the a-effect could be the helical flow induced by the shear between outwardly expelled fluid trapped between the impeller blades and the slower moving fluid in the bulk of the container [19,27,12].…”

Laboratory Dynamo Experiments

“…The conversion by the α-effect of a toroidal field into a poloidal one is often thought to rely on helical flow motions, but the source of the α term in VKS is yet unclear. Several mechanisms have been proposed (Laguerre et al 2008;Monchaux et al 2009) and further measurements are underway. A point which deserves clarification concerns the role of the ferromagnetic impellers which are used to drive the flow.…”

The VKS experiment: turbulent dynamical dynamos

“…It was found to be an equatorial dipole [26,27,16], thus leading to a strong radial component in the mid-plane between the impellers. A simple model of an α − ω dynamo has been proposed to understand the experimentally observed axial dipole [28,29]: it takes into account the helical nature of the velocity corresponding to the flow ejected by the centrifugal force close to each impeller between the blades. This gives an α-effect localized close to the disks that transforms an azimuthal field into a poloidal one, whereas differential rotation converts also very efficiently the poloidal component into toroidal by ω effect.…”