Yannick Ponty - Université Côte d'Azur, CNRS, Observatoire de la Côte d'Azur - Keywords: Dynamo, turbulence, magnetohydrodynamic

Yannick Ponty - Université Côte d'Azur, CNRS, Observatoire de la Côte d'Azur - Keywords: Dynamo, turbulence, magnetohydrodynamic

Contribution title: Turbulence fluid-structure Dynamo

The experimental observation of the dynamo instability has been a long quest
requiring careful ow optimization, which has only be achieved in the Riga, Karlsruhe and von Karman sodium (VKS) experiments. While the behavior of the two former experiments could be explained from computations using simplified flows, this is not the case for the VKS experiment - in which a strongly turbulent liquid sodium flow is driven by the counter-rotation of impellers
fitted with blades, in a cylindrical vessel. Two major puzzles in the understanding of the dynamo mechanism are still unanswered: (i) the dynamo instability was only observed
in the presence of impellers having high magnetic permeability and (ii) the time-averaged dynamo magnetic field in the saturated regime has an axial dipolar structure , while an equatorial dynamo dipole is expected from computations.

We present results from consistent dynamo simulations, where the electrically conducting and incompressible flow inside a cylinder vessel is forced by moving impellers numerically implemented by a penalization method. The numerical scheme models also the jumps of magnetic permeability for the solid impellers, resembling various configurations tested experimentally in the von-Karman Sodium experiment.

The most striking experimental observations are reproduced in our set of simulations. In particular, we report on the existence of a time averaged axisymmetric dynamo mode, self-consistently generated when the magnetic permeability of the impellers exceeds a threshold. We describe a possible scenario involving both the turbulent flow in the vicinity of the impellers and the high magnetic permeability of the impellers [1]. Non linear behaviors, including oscillation and reversals will be also presented.

[1] Dynamo enhancement and mode selection triggered by high magnetic permeability
S. Kreuzahler, Y. Ponty, N. Plihon, H. Homann, R. Grauer ( Accepté Phys. Rev. Lett. ) https://arxiv.org/abs/1706.00260