AZIMUTHAL TWIST INFLUENCE ON THE DYNAMO THRESHOLD IN AXISYMMETRICAL FLOWS

J. Burguete, University of Navarra,

joint work with A. de la Torre.


The dynamo action in conducting fluids depends dramatically on the characteristics of the underlying flow. A small modification of one of these properties can destroy the instability, becoming a non-dynamo flow. In this work we present the influence of the twisting of flow elements trajectories in an axisymmetrical flow. Depending on the twist number, the critical magnetic Reynolds number and the topology of the resulting magnetic field change.
Using as starting point an experimentally velocity field (see for instance A. de la Torre et al. Phys. Rev. Lett. 99, 054101 (2007) ) measured in a cylindrical geometry, we have modified the velocity field in order to study the influence of the different geometrical properties. Only when the flow presents closed trajectories with an integer twist number (# turns on the poloidal plane / # turns on the azimuthal direction), the magnetic field growths. The optimum velocity field has a critical Reynolds number of 26. A similar approach has been proposed by A. Shukurov et al. ( Phys. Rev. E 78, 025301(R) (2008) ) as a new experimental approach to perform dynamo experiments in closed tubes as the Perm dynamo experiment.