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Stokes drift dynamos

Published online by Cambridge University Press:  19 April 2011

W. HERREMAN  and
P. LESAFFRE
W. HERREMAN*
Affiliation:
LIMSI, CNRS, 91403 Orsay CEDEX, France
P. LESAFFRE
Affiliation:
LERMA/LRA, CNRS/UMR8112, Ecole Normale Supérieure, 24, rue Lhomond, 75231 Paris, CEDEX 05, France
*
†Email address for correspondence: wietze.herreman@limsi.fr
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Abstract

Fluid particles can have a mean motion in time, even when the Eulerian mean flow disappears everywhere in space. In the present article, we demonstrate that this phenomenon, known as the Stokes drift, plays an essential role in the problem of magnetic field generation by fluctuation flows (kinematic dynamo) at high Rm. At leading order, the dynamo is generated by the Stokes drift that acts as if it were a mean flow. This result is derived from a mean-field dynamo theory in terms of time averages, which reveals how classical expressions for alpha and beta tensors actually recombine into a single Stokes drift contribution. In a test case, we find fluctuation flows that have a G. O. Roberts flow as Stokes drift and this allows to confront our model to direct integration of the induction equation. We find an excellent quantitative agreement between the prediction of our model and the results of our simulations. We finally apply our Stokes drift model to prove that a broad class of inertial waves in rapidly rotating flows cannot drive a dynamo.

JFM classification

MHD and Electrohydrodynamics: Dynamo theory
Type
Papers
Information
Journal of Fluid Mechanics , Volume 679 , 25 July 2011 , pp. 32 - 57
Copyright
Copyright © Cambridge University Press 2011

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