A precession-driven flow in a cylindrical geometry

A magnetohydrodynamic dynamo process is supposed to take place in the
interior of the Sun or stars as well as in planets and smaller
celestial bodies like the ancient moon or the asteroid Vesta. The
ubiquity and diversity of astrophysical dynamo action, and the
importance of the magnetic fields for formation and evolution of the
objects generating them, has motivated related studies in the
laboratory. A new dynamo experiment is currently under construction
at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), in which a fluid flow
of liquid sodium will be forced by a precessing cylindrical container.
In the 1970s, a similar but much smaller experiment was conducted by
R. Gans, who found a threefold amplification of an external magnetic
field, indicating that dynamo action can be expected in the vicinity
of the transition from a laminar flow state to vigorous turbulence if
the system is sufficiently large. In the new dynamo precession
experiment the knowledge of the velocity field is necessary in order
to achieve the optimal structure of the flow and to understand the
interaction of fluid flow and magnetic field and the associated
transfer of kinetic energy into magnetic energy.

Our present study focuses on the experimental and numerical estimation
of the structure of a precession driven flow field, its scaling with
rotation and precession, and its dynamical evolution. We determine the
amplitude of characteristic flow contributions in terms of inertial
modes, which allows a robust and accurate detection of the transition
between laminar and turbulent state. It further turns out that most of
the kinetic energy can be related to very few large scale inertial
modes. Finally, the determination of the orientation of the rotation
axis of the fluid motion shows that the qualitative behavior of the
precession driven flow in a cylinder is roughly similar to the
predictions given by Busse's theory for a planetary-like setup in a
spheroid.