Instabilities and transition in three-dimensional flows with rotation DYNAMO ACTION IN FINITE CYLINDERS

Using numerical simulations, we investigate two magnetohydrodynamics (MHD) problems in a cylindrical cavity, namely the von Kármán Sodium (VKS) experiment and a precessing cylinder filled with a conducting fluid. We use a parallel code denoted SFEMaNS [1], which is able to integrate nonlinear MHD equations for incompressible fluids in heterogenous domains (with jump distributions of electrical conductivity or magnetic permeability) with axisymmetric interfaces embedded in a vacuum. We numerically demonstrate that using high permeability disks in VKS decreases the dynamo threshold and that precession is able to drive a cylindrical dynamo. 1 Impact of soft iron impellers in the VKS dynamo experiment The VKS experiment [2] leads to dynamo action, at the available power, using soft iron impellers but not using steel impellers. This demonstrates the crucial role played by ferromagnetic material that numerical studies can handle on simplified configurations. Therefore kinematic simulations of the induction equation (with imposed velocity field) are carried out for different setups suitable for this experiment. The impellers are modeled by flat disks of high conductivity or high permeability and drive the flow in a cylinder filled with a conducting fluid. Ohmic decay and kinematic dynamo problems using various configurations and physical parameters are investigated. The material properties of the disks change drastically the field geometry and growth rates while external boundary conditions have nearly no influence. Using a VKS like mean fluid flow and high permeabilit