Francky Luddens

It is numerically demonstrated by means of a magnetohydrodynamics code that precession can trigger the dynamo effect in a cylindrical container. When the Reynolds number, based on the radius of the cylinder and its angular velocity, increases, the flow, which is initially centrosymmetric, loses its stability and bifurcates to a quasiperiodic motion. This unsteady and asymmetric flow is shown to be capable of sustaining dynamo action in the linear and nonlinear regimes. Themagnetic field thus generated is unsteady and quadrupolar. These numerical evidences of dynamo action in a precessing cylindrical container may be useful for an experiment now planned at the Dresden sodium facility for dynamo and thermohydraulic studies in Germany.

A novel approximation technique using Lagrange finite elements is proposed to solve magnetodynamics problems involving discontinuous magnetic permeability and non-smooth interfaces. The algorithm is validated on benchmark problems and is used for kinematic studies of the Cadarache von Kármán Sodium 2 (VKS2) experimental fluid dynamo.

Kinematic simulations of the induction equation are carried out for different setups suitable for the Von-Kármán-Sodium (VKS) dynamo experiment. The material properties of the flow driving impellers are modeled by means of high-conducting and high-permeability disks in a cylindrical volume filled with a conducting fluid. Two entirely different numerical codes are mutually validated by showing quantitative agreement on Ohmic decay and kinematic dynamo problems using various configurations and physical parameters. Field geometry and growth rates are strongly modified by the material properties of the disks even if the disks are thin. In contrast the influence of external boundary conditions remains small. Utilizing a VKS like mean fluid flow and high-permeability disks yield a reduction of the critical magnetic Reynolds number Rmc for the onset of dynamo action of the simplest non-axisymmetric field mode. However, this threshold reduction is not sufficient to fully explain the VKS experiment. We show that this reduction of Rmc is influenced by small variations in the flow configuration so that the observed reduction may be changed with respect to small modifications of setup and properties of turbulence.