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SYNS: Numerical Simulation of Hightemperature Plasmas

SYNS 2: Numerical Simulation II

SYNS 2.6: Vortrag

Donnerstag, 2. April 2009, 17:15–17:30, HS Biochemie (groß)

Nonlinear MHD dynamo simulations in spherical geometry — •Klaus Reuter and Frank Jenko — IPP Garching

The MHD dynamo process is commonly believed to cause cosmic magnetic fields, e.g. in planets, stars, and galaxies. In recent years, several experiments which implement turbulent flows of liquid sodium were performed in order to study dynamo action in the laboratory. We present numerical simulations of a mechanically driven, electrically conducting flow in spherical geometry which consists of two counter-rotating flow cells, similar to the flow realized in the Madison Dynamo Experiment. The aim of our studies is to better understand the influence of turbulence on magnetic field amplification which turned out to be highly detrimental in the actual experiment.

At low Reynolds numbers Re, a hydrodynamic instability gives rise to propagating wave features which can either support or hinder dynamo action, depending on their spatio-temporal properties. Turbulent fluctuations which appear at higher Re strongly inhibit the dynamo process. The critical magnetic Reynolds number Rm_c(Re) above which magnetic field amplification sets in is discussed.

Finally, it is shown that the turbulent flow allows for subcritical dynamo action. These subcritical dynamo states can either be reached by suddenly reducing Rm below Rm_c, or by applying external finite amplitude magnetic fields. The latter finding may be useful for the dynamo experiment to reach a state of self-excitation.