Dynamo action in an annular array of helical vortices

We numerically study the induction mechanisms generated from an array of helical motions distributed along a cylinder. Our flow is a very idealized geometry of the columnar structure that has been proposed for the convective motion inside the Earth's core. Using an analytically prescribed flow, we apply a recently introduced iterative numerical scheme [M. Bourgoin, P. Odier, J.-F. Pinton, and Y. Richard, Phys. Fluids 16, 2529 (2004)] to solve the induction equation and analyze the flow response to externally applied fields with simple geometries (e.g., azimuthal, radial). Symmetry properties allow us to build selected induction modes whose interactions lead to dynamo mechanisms. Using an induction operator formalism, we show how dipole and quadrupole dynamos can be envisioned from such motions. The method identifies the main induction mechanisms that generate dynamo action in the selected geometry. Here, it emphasizes the competition between alpha-effect and field expulsion as well as the role of scale separation. (C) 2008 American Institute of Physics.