The precession dynamo experiment at HZDR

Abstract:

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. For example, it has long been
discussed whether precession may be a complementary power source for
the geodynamo (Malkus, Science 1968) or for the ancient lunar dynamo
due to the Earth-driven precession of the lunar spin axis (Dwyer, Nature 2011).

We will present the current state of development of the dynamo
experiment together with results from non-linear hydrodynamic
simulations with moderate precessional forcing. Our 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.

More relevant for dynamo action might be free Kelvin modes (the
natural flow eigenmodes in a rotating cylinder) with higher azimuthal
wave number. These modes may become relevant when constituting a
triadic resonance with the fundamental forced mode, i.e., when the
height of the container matches their axial wave lengths. We find
triadic resonances at aspect ratios close to those predicted by the
linear theory except around the primary resonance of the forced
mode. In that regime we still identify free Kelvin modes propagating
in retrograde direction but none of them can be assigned to a triade.

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.