Large scales features of a flow driven by precession

J. Léorat

Observatoire de Paris-Meudon, 92195, Meudon, France

Abstract
Conversion of kinetic energy into magnetic energy through induction effects, i.e., dynamo action, is observed in almost all astrophysical bodies. The saturation regime leading to chaotic, periodic or steady dynamos as a result of the nonlinear interactions between kinetic and magnetic fluctuations leaves a large field of open questions, since astrophysical observations as well as numerical simulations both suffer of lack of spatial resolution. As already happened for hydrodynamic turbulence, progress in nonlinear dynamos and, more generally, in MHD turbulence needs specific experimental facilities able to generate flows at magnetic Reynolds numbers (\Rm) beyond one hundred. It is well known that the best fluid for dynamo experiments in view of its large conductivity and rather low working temperature is liquid sodium, although its chemical reactivity introduces a first experimental difficulty. While the scales of astrophysical bodies allow to reach huge \Rm despite moderate speeds, the relatively small scales, which may be involved in laboratory experiments, ask for high velocity flows, and this requirement represents a second main obstacle for dynamo experiments. We will argue below that precession driving is a convenient way to answer these two practical questions, since it allows to sustain a high velocity flow in a closed container using a relatively low mechanical power injection. After the two successful dynamo experiments in Riga and Karlsruhe in 1999, a new generation of MHD experiments is now under study, based on configurations without internal walls, in order to get a less constrained nonlinear saturation regime. Counter-rotating turbines or static blades may drive such flows. While using very different designs, all these experiments share, however, a common property: the achievable \Rm stays below one hundred, i.e., rather close (within 15%, say) to the numerically derived critical value \Rm^∗ of \Rm for kinematic dynamo action. In order to deal with more general properties of the nonlinear saturation regime, which could be of interest for astrophysical dynamos, it is highly desirable that the gap between the critical \Rm^∗ and the effective \Rm is as large as possible. This is a third significant constraint in the design of new fluid dynamo configurations. Refs 18.