Publications (8)14.85 Total impact

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ABSTRACT: We present direct numerical simulations of the different twodimensional flow regimes generated by a constant spatially periodic forcing balanced by viscous dissipation and large scale drag with a dimensionless damping rate $1/Rh$. The linear response to the forcing is a $6\times6$ square array of counterrotating vortices, which is stable when the Reynolds number $Re$ or $Rh$ are small. After identifying the sequence of bifurcations that lead to a spatially and temporally chaotic regime of the flow when $Re$ and $Rh$ are increased, we study the transitions between the different turbulent regimes observed for large $Re$ by varying $Rh$. A large scale circulation at the box size (the condensate state) is the dominant mode in the limit of vanishing large scale drag ($Rh$ large). When $Rh$ is decreased, the condensate becomes unstable and a regime with random reversals between two large scale circulations of opposite signs is generated. It involves a bimodal probability density function of the large scale velocity that continuously bifurcates to a Gaussian distribution when $Rh$ is decreased further. 
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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 09/2014; 90(31):033015. DOI:10.1103/PhysRevE.90.033015 · 2.33 Impact Factor 
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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Hydrodynamic and magnetic behaviors in a modified experimental setup of the von Kármán sodium flowwhere one disk has been replaced by a propellerare investigated.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 06/2014; 89(61):063023. DOI:10.1103/PhysRevE.89.063023 · 2.33 Impact Factor 
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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. · 2.33 Impact Factor 
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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 03/2014; 89(4). DOI:10.1103/PhysRevE.89.043004 · 2.33 Impact Factor 
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ABSTRACT: Magnetorotational dynamo action in Keplerian shear flow is a threedimensional, nonlinear magnetohydrodynamic process whose study is relevant to the understanding of accretion and magnetic field generation in astrophysics. Transition to this form of dynamo is subcritical and shares many characteristics of transition to turbulence in nonrotating hydrodynamic shear flows. This suggests that these different fluid systems become active through similar generic bifurcation mechanisms, which in both cases have eluded detailed understanding so far. In this paper, we investigate numerically the bifurcation mechanisms at work in the incompressible Keplerian magnetorotational dynamo problem in the shearing box framework. Using numerical techniques imported from dynamical systems research, we show that the onset of chaotic dynamo action at magnetic Prandtl numbers larger than unity is primarily associated with global homoclinic and heteroclinic bifurcations of nonlinear magnetorotational dynamo cycles. These global bifurcations are supplemented by local bifurcations of cycles marking the beginning of perioddoubling cascades. This suggests that nonlinear magnetorotational dynamo cycles provide the pathway to turbulent injection of both kinetic and magnetic energy in incompressible magnetohydrodynamic Keplerian shear flow in the absence of an externally imposed magnetic field. Studying the nonlinear physics and bifurcations of these cycles in different regimes and configurations may subsequently help to better understand the conditions of excitation of magnetohydrodynamic turbulence and instabilitydriven dynamos in various astrophysical systems and laboratory experiments. The detailed characterization of global bifurcations provided for this threedimensional subcritical fluid dynamics problem may also prove useful for the problem of transition to turbulence in hydrodynamic shear flows.Journal of Fluid Mechanics 06/2013; 731. DOI:10.1017/jfm.2013.317 · 2.29 Impact Factor 
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ABSTRACT: The nature of dynamo action in shear flows prone to magnetohydrodynamc instabilities is investigated using the magnetorotational dynamo in Keplerian shear flow as a prototype problem. Using direct numerical simulations and Newton's method, we compute an exact timeperiodic magnetorotational dynamo solution to threedimensional dissipative incompressible magnetohydrodynamic equations with rotation and shear. We discuss the physical mechanism behind the cycle and show that it results from a combination of linear and nonlinear interactions between a largescale axisymmetric toroidal magnetic field and nonaxisymmetric perturbations amplified by the magnetorotational instability. We demonstrate that this largescale dynamo mechanism is overall intrinsically nonlinear and not reducible to the standard meanfield dynamo formalism. Our results therefore provide clear evidence for a generic nonlinear generation mechanism of timedependent coherent largescale magnetic fields in shear flows and call for new theoretical dynamo models. These findings may offer important clues to understanding the transitional and statistical properties of subcritical magnetorotational turbulence.Physical Review E 09/2011; 84(3 Pt 2):036321. DOI:10.1103/PhysRevE.84.036321 · 2.33 Impact Factor 
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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; DOI:10.1080/03091929.2011.648629 · 0.92 Impact Factor
Publication Stats
21  Citations  
14.85  Total Impact Points  
Top Journals
Institutions

2014

Ecole Normale Supérieure de Paris
 Laboratoire de Physique Statistique
Lutetia Parisorum, ÎledeFrance, France 
Paris Diderot University
Lutetia Parisorum, ÎledeFrance, France


2011

French National Centre for Scientific Research
Lutetia Parisorum, ÎledeFrance, France
