Greifswald 2009 – wissenschaftliches Programm
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P: Fachverband Plasmaphysik
P 22: Theorie/Modellierung II
P 22.1: Fachvortrag
Donnerstag, 2. April 2009, 12:10–12:35, HS Biochemie (groß)
Nonlinear MHD dynamo simulations in spherical geometry — •Klaus Reuter and Frank Jenko — Max-Planck-Institut für Plasmaphysik
The MHD dynamo process is commonly believed to cause e.g. planetary and stellar magnetic fields. In recent years, several experiments which use turbulent flows of liquid sodium were performed 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 aims of these studies are to better understand the underlying physics and to possibly optimize the experimental setup.
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 resulting critical magnetic Reynolds number Rmc(Re) above which magnetic field amplification sets in is presented.
Finally, it is shown that the turbulent flow allows for subcritical dynamo action. These subcritical dynamo states can either be reached by suddenly reducing the magnetic Reynolds number of the fluid, or by applying external finite amplitude magnetic fields. The latter finding may be useful for the dynamo experiment to reach self-excitation.