Numerical simulations for the DRESDYN precession dynamo

A fluid flow of liquid sodium in a cylindrical container, solely driven by precession, is considered as a source for magnetic field generation in the next generation dynamo experiment. The device is currently under development in the framework of the project DRESDYN at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR).

A preparatory small scale water experiment is already in operation in order to provide the hydrodynamic flow properties. First results show that the precessional forcing converts a rather large amount of kinetic energy from the exterior rotation into internal fluid flow. A further striking feature is the abrupt transition from a large scale, quasi-laminar flow to a more disordered turbulent state. In this state the dynamical pressure is reduced while the power required to maintain the rotation of the container is increased.

For slow precession rates, both the amplitude and the shape of the elementary inertial mode obtained from ultrasonic measurements could be well reproduced in hydrodynamic simulations. The simulations also show the presence of the second state at larger precession rates, however, the transition is less abrupt and the degree of turbulence remains much smaller than in the water experiment.

We have used the time-dependent velocity fields obtained from the numerical simulations as input data for kinematic simulations of the magnetic induction equation. So far, the resulting magnetic field growth rates remain below the dynamo threshold for magnetic Reynolds numbers up to Rm=2000 which is in contradiction with results presented by Nore et al (PRE, 84 (1), 016317). Further simulations are necessary in order to restrict the expedient parameter regime and to allow for an optimization of the experimental configuration.

Another possibility for dynamo action may result from the cyclonic vortices aligned parallel to the symmetry axis of the cylinder that have been observed by Mouhali et al (Exp. Fluids 2012, 53 (6), 1693). These vortices provide a efficient source for helicity but, so far, they have not been found in numerical simulations of precession driven flows. Future simulations of magnetic induction caused by precession driven flows will have to be based on analytical models of These vortices and their temporal behavior. This requires more detailed experimental measurements that are expected from the forthcoming upgrade of the water experiment with a 3d-PIV (Particle Image Velocimetry) system.