Direct Numerical Simulations of Impeller Driven Turbulence and Dynamo Action

The process, in which a magnetic field is amplified by the flow of an electrically conducting fluid, known as dynamo action, is believed to be the origin of many magnetic fields in the universe including the magnetic field of the earth. A successful laboratory experiment investigating the underlying mechanisms is the Von Karman Sodium device, consisting of a cylindrical vessel filled with liquid sodium, stirred by two counter-rotating soft-iron impellers. Despite its success, it leaves important questions unsolved and even raises new ones. The aim of this project are detailed high-resolution direct numerical simulations of the VKS experiment. This type of simulations of a three-dimensional turbulent fluid flow in complex geometries supporting a magnetic field are challenging. We designed a massively parallel pseudo-spectral MHD (magnetohydrodynamics)solver that models the geometry of rotating impellers via a penalisation technique. Benchmarks show a good quantitative agreement with experimental data. The investigation of hydrodynamic properties of the system reveals the generation of conical vortices close to the blades, which may provide a major contribution to dynamo action. We achieve dynamo action in simulations of the full magnetohydrodynamic system. A variation of the impeller material constants (steel!soft iron) towards the experimental value leads to a significant decrease of the dynamo threshold as well as a change of the magnetic field mode as observed in the experiment

Dynamo Enhancement and Mode Selection Triggered by High Magnetic Permeability

International audienceWe 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 jumps of magnetic permeability for the solid impellers, resembling various configurations tested experimentally in the von Kármán 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