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ABSTRACT: We consider a fluid dynamo model generated by the flow on both sides of a moving layer. The magnetic permeability of the layer is larger than that of the flow. We show that there exists an optimum value of magnetic permeability for which the critical magnetic Reynolds number for dynamo onset is smaller than for a nonmagnetic material and also smaller than for a layer of infinite magnetic permeability. We present a mechanism that provides an explanation for recent experimental results. A similar effect occurs when the electrical conductivity of the layer is large. Physical review. E, Statistical, nonlinear, and soft matter physics. 09/2014; 90(31):033015.

Sophie Miralles, Johann Herault,
Stephan Fauve,
Christophe Gissinger,
François Pétrélis,
François Daviaud,
Bérengère Dubrulle,
Jean Boisson,
Mickaël Bourgoin,
Gautier Verhille,
Philippe Odier,
JeanFrançois Pinton,
Nicolas Plihon [Show abstract] [Hide abstract]
ABSTRACT: Hydrodynamic and magnetic behaviors in a modified experimental setup of the von Kármán sodium flowwhere one disk has been replaced by a propellerare investigated. When the rotation frequencies of the disk and the propeller are different, we show that the fully turbulent hydrodynamic flow undergoes a global bifurcation between two configurations. The bistability of these flow configurations is associated with the dynamics of the central shear layer. The bistable flows are shown to have different dynamo efficiencies; thus for a given rotation rate of the softiron disk, two distinct magnetic behaviors are observed depending on the flow configuration. The hydrodynamic transition controls the magnetic field behavior, and bifurcations between high and low magnetic field branches are investigated. Physical review. E, Statistical, nonlinear, and soft matter physics. 06/2014; 89(61):063023.

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ABSTRACT: We measure the decay rates of magnetic field modes in a turbulent flow of liquid sodium below the dynamo threshold. We observe that turbulent fluctuations induce energy transfers between modes with different symmetries (dipolar and quadrupolar). Using symmetry properties, we show how to measure the decay rate of each mode without being restricted to the one with the smallest damping rate. We observe that the respective values of the decay rates of these modes depend on the shape of the propellers driving the flow. Dynamical regimes, including field reversals, are observed only when the modes are both nearly marginal. This is in line with a recently proposed model. Physical Review E 04/2014; 89(41):043004. · 2.31 Impact Factor

[Show abstract] [Hide abstract]
ABSTRACT: We measure the decay rates of magnetic field modes in a turbulent flow of liquid sodium below the dynamo threshold. We observe that turbulent fluctuations induce energy transfers between modes with different symmetries (dipolar and quadrupolar). Using symmetry properties, we show how to measure the decay rate of each mode without being restricted to the one with the smallest damping rate. We observe that the respective values of the decay rates of these modes depend on the shape of the propellers driving the flow. Dynamical regimes, including field reversals, are observed only when the modes are both nearly marginal. This is in line with a recently proposed model. 03/2014; 89(4).

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ABSTRACT: We present a study of several systems in which a large scale field is
generated over a turbulent background. These large scale fields usually break a
symmetry of the forcing by selecting a direction. Under certain conditions, the
large scale field displays reversals so that the symmetry of the forcing is
recovered statistically. We present examples of such dynamics in the context of
the dynamo instability, of two dimensional turbulent Kolmogorov flows and of
turbulent RayleighB\'enard convection. In these systems reversals occur
respectively for the dynamo magnetic field, for the large scale circulation
generated by a periodic forcing in space and for the large scale roll generated
by turbulent thermal convection. We compare the mechanisms involved and show
that their properties depend on some symmetries of the system and on the way
they are broken. Geophysical & Astrophysical Fluid Dynamics 02/2011; · 1.19 Impact Factor