Abstract
More than 100 years ago, Henri Poincare in his pioneering study showed
that the inviscid base flow in a precessing spheroid is described by a
constant vorticity solution, the spin-over mode. Since then there have
been repeated discussions whether the geodynamo is driven (or at least
influenced) by precession. More recently, precession has also been
considered as an important mechanism for the explanation of the ancient
lunar dynamo. Experiments with precessing fluids in cylindrical and in
spherical geometry showed that precession indeed is an efficient
mechanism to drive substantial flows even on the laboratory scale
without making use of propellers or pumps. A precession dynamo
experiment is currently under construction within the project DRESDYN
(DREsden Sodium facility for DYNamo and thermohydraulic studies) at
Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in which a precession driven
flow of liquid sodium will be used to drive dynamo action. In the
present study we address related numerical and experimental examinations
in order to identify parameter regions where the onset of magnetic field
excitation will be possible. Preliminary kinematic dynamo models using a
prescribed flow field from hydrodynamic simulations, exhibit magnetic
field excitation at critical magnetic Reynolds numbers around Rmc ≈ 430,
which is well within the range of the planned liquid sodium experiment.
Our results show that large scale inertial modes excited by precession
are able to excite dynamo action when their structure is sufficient
complex, i.e. the forcing is sufficient strong. More advanced models
that take into account the container’s finite conductivity show that
boundary conditions may play an important role, but the critical
magnetic Reynolds number will still be achievable in the planned
experiment. Finally, we discuss the role of turbulent flow fluctuations
for the occurrence of dynamo action.