Forçage électromagnétique dans les métaux liquides

Abstract : This manuscript describes an experimental study on magnetohydrodynamics, with a particular focus on the electromagnetic driving of liquid metals. Such electromagnetically-driven flows involve transformation of electromagnetic energy into kinetic energy through the Laplace force. The motivation of the present thesis is to examine how an electromagnetic field generates a flow, to study the different ways to ensure such a transfer of energy, or to characterize what bounds the efficiency of this energy conversion. This thesis presents two laboratory experiments studying two different driving : first, the induction of a fluid motion by a traveling magnetic field (similar to an asynchronous motor), then a driving due to the combination of a stationary and uniform magnetic field and a constant electric current (similar to a DC motor). In the first part of the thesis, a predictive scaling law is obtained for the evolution of a fluid subjected to a traveling field in the turbulent regime. It is shown that this driving is strongly limited by turbulence, but also by various mechanisms such as magnetic flux expulsion, or energy transfers to higher harmonics. This limitation results in a bound on the efficiency of this energy conversion, which can never exceed 50%. In a second part, the fluid is subjected to two magnetic fields traveling in opposite directions, thus generating a shear flow. The turbulent fluctuations break the symmetry of the problem and yields a chaotic behavior of the shear layer, revealing a 1/f power spectrum at low frequency. This accumulation of energy at low frequencies is associated with chaotic reversals of large scale coherent structures. The appearance of this 1/f noise is mediated by the symmetry and the turbulence of the flow. Finally, in a last experiment, a thin disc of liquid metal is driven by conduction, leading to the first observation of MHD Keplerian turbulence in the laboratory. It is thus observed that the magnetic field laminarises the flow and that the transition to Keplerian turbulence is continuous. This work shows that it is possible to isolate several mechanisms characterizing electromagnetically-driven flows and to understand the complex dynamics of MHD flows.
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Michaël Pereira. Forçage électromagnétique dans les métaux liquides. Physique [physics]. Sorbonne Université, 2018. Français. ⟨NNT : 2018SORUS069⟩. ⟨tel-02165027⟩

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