  Precession of planets or moons affects internal liquid layers by driving
flows, instabilities and possibly dynamos.The energy dissipated by these
phenomena can influence orbital parameters such as the planet's spin
rate.However, there is no systematic study of these flows in the spherical
shell geometry relevant for planets, and the lack of scaling law prevents
convincing extrapolation to celestial bodies.We have run more than 900
simulations of fluid spherical shells affected by precession, to systematically
study basic flows, instabilities, turbulence, and magnetic field generation.We
observe no significant effects of the inner core on the onset of the
instabilities.We obtain an analytical estimate of the viscous dissipation,
mostly due to boundary layer friction in our simulations.We propose theoretical
onsets for hydrodynamic instabilities, and document the intensity of turbulent
fluctuations.We extend previous precession dynamo studies towards lower
viscosities, at the limits of today's computers.In the low viscosity regime,
precession dynamos rely on the presence of large-scale vortices, and the
surface magnetic fields are dominated by small scales.Interestingly,
intermittent and self-killing dynamos are observed.Our results suggest that
large-scale planetary magnetic fields are unlikely to be produced by a
precession-driven dynamo in a spherical core.But this question remains open as
planetary cores are not exactly spherical, and thus the coupling between the
fluid and the boundary does not vanish in the relevant limit of small
viscosity.Moreover, the fully turbulent dissipation regime has not yet been
reached in simulations.Our results suggest that the melted lunar core has been
in a turbulent state throughout its history.Furthermore, in the view of recent
experimental results, we propose updated formulas predicting the fluid mean
rotation vector and the associated dissipation in both the laminar and the
turbulent regimes.
