Speaker
Description
A magnetohydrodynamic dynamo process is believed to occur 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,
motivating similar studies in laboratory settings. Currently, a new
dynamo experiment is under construction at Helmholtz-Zentrum
Dresden-Rossendorf (HZDR), in which liquid sodium will be forced by a
precessing cylindrical container.
In the present study, we conduct related numerical simulations of
precession-driven dynamo action in order to examine the interaction
between flow and field and the associated transfer of kinetic energy
into magnetic energy. We compare self-consistent simulations of the
complete set of magnetohydrodynamic equations with a simplified
kinematic approach solely based on the magnetic induction equation
with a prescribed velocity field. In both cases, we observe an
optimal parameter range for the onset of dynamo action in the
transitional regime in which the flow undergoes a radical change from
a large-scale to a smaller-scale turbulent behavior. The behavior is
in agreement with previous results by R. Gans from the 1970s, obtained
at a similar but much smaller experiment, which yields a threefold
amplification of an external magnetic field at the transition from a
laminar flow state to vigorous turbulence.
The various kinematic models reveal a strong influence of the
electromagnetic properties of outer layers resembling container walls
and/or the outer laboratory area and go along with an existence of two
different branches of dynamo action. In contrast to the kinematic
solution, the character of the dynamo is small-scale in the MHD
models, which exhibit irregular magnetic bursts with an increase in
the magnetic energy by a factor of 3 to 5 but still remaining
significantly lower than the kinetic energy. As the magnetic field is
small-scale and weak, the nonlinear feedback on the flow through the
Lorentz force remains small and arises essentially in terms of a
slight damping of the fast time-scales.
