Études en magnétohydrodynamique, application à l'effet dynamo

Abstract : The work presented in this thesis investigates magnetohydrodynamic induction and the possibility of dynamo action in unconstrained flows of conducting liquids at high magnetic Reynolds number (Rm). We focus mainly on von Kármán flows of gallium (Rm<5) and sodium (Rm<40). Measurements of the induced magnetic field have been performed for different flow configurations, applied field B0 and Rm's value. We interpretate these experiments by an original perturbative approach of MHD equations. In this approach induction mechanisms result from elementary linear steps where B0 induces a first order B1 field which induces a second order B2 and so forth. We can therefore identify specific mechanisms such as Ω effect, "α" effect and expulsion. In the mechanistic approach adopted here, a dynamo corresponds to a magnetic loop-back mechanism. This condition can be expressed in terms of an eigenvalue problem that we show to be equivalent to the traditional growth rate approach. In von Kármán flows, an "α"Ω loop-back mechanism has been identified experimentally and numerically. However, in the configurations we have considered, expulsion always remains the most efficient mechanism, operating against the dynamo growth. The "α"Ω -expulsion competition is controled by the precise topology of the flow and by the electrical boundary conditions. Because of their very low magnetic Prandtl number, liquid metal flows are highly turbulent and fluctuating. This rise rapid magnetic fluctuations that can be explained in the frame of Kolmogorov phenomenolgy of turbulence. The mean value of induced magnetic field does not appear to be affected by these fluctuations and is well accounted for using the mean flow profile.
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Theses
Astrophysics. Ecole normale supérieure de lyon - ENS LYON, 2003. French


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Contributor : Mickaël Bourgoin <>
Submitted on : Monday, January 31, 2005 - 11:52:46 AM
Last modification on : Tuesday, September 22, 2009 - 3:33:35 PM

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Mickaël Bourgoin. Études en magnétohydrodynamique, application à l'effet dynamo. Astrophysics. Ecole normale supérieure de lyon - ENS LYON, 2003. French. <tel-00008302>

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