Dynamo threshold detection in the von Kármán sodium experiment

Sophie Miralles (1) , Nicolas Bonnefoy (1) , Mickaël Bourgoin (2) , Philippe Odier (1) , Jean-François Pinton (1) , Nicolas Plihon (1) , Gautier Verhille (3) , Jean Boisson (4) , François Daviaud (4) , Bérengère Dubrulle (4)

1 Phys-ENS - Laboratoire de Physique de l'ENS Lyon
2 LEGI - Laboratoire des Écoulements Géophysiques et Industriels [Grenoble]
3 IRPHE - Institut de Recherche sur les Phénomènes Hors Equilibre
4 SPEC - UMR3680 - Service de physique de l'état condensé

Predicting dynamo self-generation in liquidmetal experiments has been an ongoing question for many years. In contrast to simple dynamical systems for which reliable techniques have been developed, the ability to predict the dynamo capacity of a flow and the estimate of the corresponding critical value of the magnetic Reynolds number (the control parameter of the instability) has been elusive, partly due to the high level of turbulent fluctuations of flows in such experiments (with kinetic Reynolds numbers in excess of 10^6).We address these issues here, using the von Karman sodium experiment and studying its response to an externally applied magnetic field. We first show that a dynamo threshold can be estimated from analysis related to critical slowing down and susceptibility divergence, in configurations for which dynamo action is indeed observed. These approaches are then applied to flow configurations that have failed to self-generate magnetic fields within operational limits, and we quantify the dynamo capacity of these configurations.

Domaines: Mécanique des fluides [physics.class-ph]; Mécanique des fluides [physics.class-ph]; Mécanique des structures [physics.class-ph]

Sophie Miralles, Nicolas Bonnefoy, Mickaël Bourgoin, Philippe Odier, Jean-François Pinton, et al.. Dynamo threshold detection in the von Kármán sodium experiment. Physical Review E : Statistical, Nonlinear, and Soft Matter Physics, 2013, 88, pp.013002. ⟨10.1103/PhysRevE.88.013002⟩. ⟨hal-00941503⟩