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Lorentz force effects in the Bullard–von Kármán dynamo: saturation, energy balance and subcriticality

Published online by Cambridge University Press:  26 June 2015

Sophie Miralles ,
Nicolas Plihon  and
Jean-François Pinton
Sophie Miralles
Affiliation:
Laboratoire de Physique, École Normale Supérieure de Lyon, CNRS & Université de Lyon, 46, Allée d’Italie, 69364 Lyon CEDEX 07, France Institute für Geophysik, ETH Zürich, Sonneggstrasse 5, CH-8092 Zürich, Switzerland
Nicolas Plihon*
Affiliation:
Laboratoire de Physique, École Normale Supérieure de Lyon, CNRS & Université de Lyon, 46, Allée d’Italie, 69364 Lyon CEDEX 07, France
Jean-François Pinton
Affiliation:
Laboratoire de Physique, École Normale Supérieure de Lyon, CNRS & Université de Lyon, 46, Allée d’Italie, 69364 Lyon CEDEX 07, France
*
†Email address for correspondence: nicolas.plihon@ens-lyon.fr
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Abstract

We report an experimental study of a turbulent dynamo in a liquid metal flow. The semi-synthetic dynamo is achieved thanks to an induction process generated by the turbulent shearing motion of liquid gallium and a feedback loop with external amplification, using coils. The external amplification allows the excitation of the dynamo instability at magnetic Reynolds numbers of order-one. This semi-synthetic dynamo is studied here in a regime where saturation is achieved when Lorentz forces modify significantly the bulk flow structure. We describe the supercritical bifurcation, intermittent and saturated regimes, the scalings of the dynamo magnetic field and we detail the power budget. We also report self-killing dynamos for which the dynamo magnetic field cannot be sustained, when the flow is dominated by the action of Lorentz forces, and subcritical regimes in which the flow only sustains a dynamo when it is already dominated by the action of Lorentz forces.

JFM classification

Nonlinear Dynamical Systems: Bifurcation MHD and Electrohydrodynamics: Dynamo theory MHD and Electrohydrodynamics: MHD turbulence
Type
Papers
Information
Journal of Fluid Mechanics , Volume 775 , 25 July 2015 , pp. 501 - 523
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Copyright
© 2015 Cambridge University Press 

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