ENGINEERING AND CFD

The process, in which a magnetic field is amplified by the flow of an electrically conducting fluid such as liquid metal or plasma, known as dynamo action, is believed to be the origin of magnetic fields in the universe including the magnetic field of the earth. Laboratory experiments using liquid sodium try to investigate the underlying mechanisms. Especially one setup has been successful in producing a dynamo with a free turbulent flow, the Von Karman Sodium experiment in Cadarache, consisting of a cylindrical vessel filled with liquid sodium, stirred by two counter-rotating soft-iron impellers.

Nevertheless this experiment leaves unresolved questions concerning the interaction of liquid metal and solid impellers. The impact of the specific shape and the magnetic permeability of the impellers on the flow and the magnetic field is not yet clear, since measurements close to the impellers are difficult. In particular, the evolving magnetic field is an axial dipole instead of an equatorial dipole, which contradicts predictions from previous computations using a prescribed flow.

Direct numerical simulations of spatially and temporally resolved fields allow a detailed analysis of flow and magnetic field structures. The simulation of a three-dimensional turbulent fluid flow supporting a magnetic field are performed in a massive parallel pseudo-spectral Navier-Stokes/MHD (magnetohydrodynamic) solver adapted for the use on the BlueGene/Q supercomputer JUQUEEN of the Jülich Supercomputing Centre. The geometry of rotating impellers assembled of several basic geometric objects is modelled via an immersed boundary technique, which is applied to fulfil the boundary conditions for velocity and magnetic field at the fluid-solid-interface. It is desirable to obtain turbulent flows and solids with high permeability in order to approximate experimental conditions for reasons of comparison. Therefore high resolution for sufficient accuracy on small spatial scales is needed, achievable only by using supercomputer power.

Results of this project will give answer to questions considering the mechanism of dynamo action in fluid-solid systems. This includes the type of interaction between solid and fluid, the impact of the magnetic permeability on this interaction, the flow properties in different phases of the dynamo evolution and the development of non-linear states like spontaneous polarity reversals of the magnetic field. Also the method developed in this work is applicable for a large variety of other problems concerning complex moving boundaries.

Researchers involved in this project are Prof. Rainer Grauer and Sebastian Kreuzahler (Ruhr-University Bochum), Dr. Yannick Ponty, and Dr. Holger Homann (Observatoire de la Cote d'Azur, Nice).