Numerical Simulations for the planned precession dynamo experiment at HZDR

In a next generation dynamo experiment currently under development at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) a fluid flow of liquid sodium, solely driven by precession, will be considered as a possible source for magnetic field generation. The experiment is mainly motivated by alternative concepts for astrophysical dynamos that are based on mechanical flow driving like the geodynamo model by Malkus (Science 1968, 160, 3825, 259-264) or the model for the ancient lunar dynamo from Dwyer (Nature 2011, 479, 7372, 212-214).

I will present results from non-linear hydrodynamic simulations with moderate precessional forcing dedicated to the planned experiment.
The simulations reveal a non-axisymmetric forced mode with an amplitude of up to one fourth of the rotation velocity of the cylindrical container confirming that precession provides a rather efficient flow driving mechanism even at moderate precession rates.

Promising candidates for exciting a dynamo may be triadic resonances in terms of three distinct inertial modes (so called Kelvin modes, the natural flow eigenmodes in a rotating cylinder) which may emerge from first order non-linear interactions when the height of the container matches their axial wave lengths.
The involved free Kelvin modes are characterized by a high azimuthal wave number and an axial structure comparable to the structure of the columnar convection cells that are responsible for dynamo action in geodynamo simulations.
We find triadic resonances at aspect ratios close to those predicted by the quasi-linear theory except around the primary resonance of the forced mode. In that regime the fundamental forced mode becomes unstable at low Ekman number thus inhibiting the triadic resonance.

Our results will enter into the development of flow models that will be used in kinematic simulations of the electromagnetic induction equation in order to determine whether a precession driven flow will be capable to drive a dynamo at all and to limit the parameter space within which the occurrence of dynamo action is most promising.