Standing inertial waves, energy scaling and dissipation in precession driven flows

A precession dynamo experiment is currently under construction at
Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The experiment is
motivated by the question whether the geodynamo or the ancient lunar
dynamo were powered by a flow driven by precession instead of
or in addition to convection. In the present study we address related
numerical simulations in order to characterize the hydrodynamic flow
field and to make energetic estimations that allow conclusions on the
global power balance.

Simulations of precessing fluids in cylindrical geometry show that
precession is an efficient mechanism to drive substantial flows even
on the lab scale. Using the time-averaged flow field obtained in these
simulations, kinematic dynamo models exhibit dynamo action at
parameters that are well within the range of the planned dynamo
experiment. Our analysis further shows that the standing inertial
wave directly excited by precession is responsible for the magnetic
field excitation when the forcing is sufficiently strong, so that
nonlinear interactions modify the flow and contributions beyond the
resonant Kelvin mode become important. This requires large precession
ratios with the Poincare number (the ratio of precession
frequency to rotation frequency) above Po = 0.10. At this
value we observe an abrupt transition of the flow into a turbulent
behavior with large-scale flow structures becoming less significant.
Our simulations give a detailed characterization of the corresponding
transition. The scaling of global quantities, like flow amplitudes and
energies as well as the laminar and the turbulent dissipation are used
to constrain the energetics of the magnetic field generation and to
predict the most promising parameter regimes suitable for dynamo
action including the specific magnetic field pattern that may emerge
in the planned dynamo experiment.