Andre Giesecke

Role of Soft-Iron Impellers on the Mode Selection in the von Karman-Sodium Dynamo Experiment

A crucial point for the understanding of the von Kármán-sodium (VKS) dynamo experiment is the influence of soft-iron impellers. We present numerical simulations of a VKS-like dynamo with a localized permeability distribution that resembles the shape of the flow driving impellers. It is shown that the presence of soft-iron material essentially determines the dynamo process in the VKS experiment. An axisymmetric magnetic field mode can be explained by the combined action of the soft-iron disk and a rather small alpha effect parametrizing the induction effects of unresolved small scale flow fluctuations.

Electromagnetic induction in non-uniform domains

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.

Synchronized Helicity Oscillations: A Link Between Planetary Tides and the Solar Cycle?

Recent years have seen an increased interest in the question of whether the gravitational action of planets could have an influence on the solar dynamo. Without discussing the observational validity of the claimed correlations, we examine which possible physical mechanism might link the weak planetary forces with solar dynamo action. We focus on the helicity oscillations that were recently found in simulations of the current-driven, kink-type Tayler instability, which is characterized by an m=1

Triadic resonances in nonlinear simulations of a fluid flow in a precessing cylinder

m=1

Generation of axisymmetric modes in cylindrical kinematic mean-field dynamos of VKS type

azimuthal dependence. We show how these helicity oscillations may be resonantly excited by some m=2

Subcritical transition to turbulence of a precessing flow in a cylindrical vessel

m=2

Impact of time-dependent nonaxisymmetric velocity perturbations on dynamo action of von Kármán-like flows

perturbations that reflect a tidal oscillation. Specifically, we speculate that the tidal oscillation of 11.07 years induced by the Venus–Earth–Jupiter system may lead to a 1:1 resonant excitation of the oscillation of the α

Inferring basic parameters of the geodynamo from sequences of polarity reversals

\alpha

Nonlinear Large Scale Flow in a Precessing Cylinder and Its Ability To Drive Dynamo Action

-effect. Finally, we recover a 22.14-year cycle of the solar dynamo in the framework of a reduced zero-dimensional α

On the Synchronizability of Tayler–Spruit and Babcock–Leighton Type Dynamos

\alpha