Numerical simulations for a fluid flow driven dynamo in a precessing cylinder*

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, which

has motivated related studies in the laboratory. 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 dynamo action in order to examine the

interaction of 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 a

transitional regime, within which the flow undergoes a radical change

from a large-scale to a smaller-scale turbulent behavior. In contrast

to the kinematic solution, the character of the dynamo is small-scale

in the MHD models, which in addition exhibit irregular magnetic bursts

with an increase in the magnetic energy by a factor of 3 to 5.

Nevertheless, the magnetic energy remains significantly lower than the

kinetic energy, so that the dynamo process is not particularly

efficient and there is nearly no noticeable feedback of the magnetic

field on the flow.