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ABSTRACT: We present in this article a novel Lagrangian measurement technique: an
instrumented particle which continuously transmits the force/acceleration
acting on it as it is advected in a flow. We develop signal processing methods
to extract information on the flow from the acceleration signal transmitted by
the particle. Notably, we are able to characterize the force acting on the
particle and to identify the presence of a permanent large-scale vortex
structure. Our technique provides a fast, robust and efficient tool to
characterize flows, and it is particularly suited to obtain Lagrangian
statistics along long trajectories or in cases where optical measurement
techniques are not or hardly applicable.
08/2012;
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ABSTRACT: Accessing and characterizing a flow impose a number of constraints on the
employed measurement techniques; in particular optical methods require
transparent fluids and windows in the vessel. Whereas one can adapt apparatus,
fluid and methods in the lab to these constraints, this is hardly possible for
industrial mixers. We present in this article a novel measurement technique
which is suitable for opaque or granular flows: an instrumented particle, which
continuously transmits the force/acceleration acting on it as it is advected in
a flow. Its density is adjustable for a wide range of fluids and because of its
small size and its wireless data transmission, the system can be used both in
industrial and scientific mixers allowing a better understanding of the flow
within. We demonstrate the capabilities and precision of the particle by
comparing its transmitted acceleration to alternative measurements, in
particular in the case of a turbulent von K\'arm\'an flow. Our technique shows
to be an efficient and fast tool to characterize flows.
06/2012;
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ABSTRACT: Since the turn of the century, experiments have produced laboratory fluid dynamos that enable a study of the effect in controlled
conditions. We review here magnetic induction processes that are believed to underlie dynamo action, and we present results
of these dynamo experiments. In particular, we detail progress that have been made through the study of von Kármán flows,
using gallium or sodium as working fluids.
KeywordsMagnetic fields-Magnetohydrodynamics-Dynamo-Experiments-Instabilities-Turbulence
Space Science Reviews 04/2012; 152(1):543-564. · 3.61 Impact Factor
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Michaël Berhanu,
Gautier Verhille,
Jean Boisson,
Basile Gallet,
Christophe Gissinger,
Stéphan Fauve,
Nicolas Mordant,
François Petrelis,
Mickaël Bourgoin,
Philippe Odier, Jean-François Pinton,
Nicolas Plihon,
Sébastien Aumaitre,
Arnaud Chiffaudel,
François Daviaud,
Bérengère Dubrulle,
Christophe Pirat
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ABSTRACT: The Von Kármán Sodium experiment yields a variety of dynamo regimes, when asymmetry is imparted to the flow by rotating impellers at different speed $F_1$ and $F_2$. We show that as the intensity of forcing, measured as $F_1+F_2$, is increased, the transition to a self-sustained magnetic field is always observed via a supercritical bifurcation to a stationary state. For some values of the asymmetry parameter $\theta=(F_1-F_2)/(F_1+F_2)$, time dependent dynamo regimes develop. They are observed either when the forcing is increased for a given value of asymmetry, or when the amount of asymmetry is varied at sufficiently high forcing. Two qualitatively different transitions between oscillatory and stationary regimes are reported, involving or not a strong divergence of the period of oscillations. These transitions can be {interpreted} using a low dimensional model based on the non linear interactions of two dynamo modes.
Physics of Condensed Matter 04/2012; · 1.53 Impact Factor
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ABSTRACT: The influence of an externally applied magnetic field on flow turbulence is investigated in liquid-gallium von-Kármán (VK) swirling flows. Time-resolved measurements of global variables (such as the flow power consumption) and local recordings of the induced magnetic field are made. From these measurements, an effective Reynolds number is introduced as Rm eff = Rm(1−α √ N), so as to take into account the influence of the interaction parameter N. This effective magnetic Reynolds number leads to unified scalings for both global variables and the locally induced magnetic field. In addition, when the flow rotation axis is perpendicular to the direction of the applied magnetic field, significant flow and induced magnetic field fluctuations are observed at low interaction parameter values, but corresponding to anAlf en speed v A of the order of the fluid velocity fluctuations u rms . This strong increase in the flow fluctuations is attributed to chaotic changes between hydrodynamic and magnetohydrodynamic velocity profiles.
J. Fluid Mech. 01/2012;
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ABSTRACT: The translation and rotation of a large, neutrally buoyant, particle, freely advected by a turbulent flow is determined experimentally. We observe that, both, the orientation the angular velocity with respect to the trajectory and the translational acceleration conditioned on the spinning velocity provides evidence of a lift force, Flift ∝ ω × νrel, acting on the particle. New results of the dynamics of the coupling between the particle's rotation and its translation are presented.
Journal of Physics Conference Series 12/2011; 318(5):052027.
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ABSTRACT: A new type of velocimeter, capable of local velocity measurements in conducting fluids, is introduced. The principle of the "magnetic-distortion probe" is based on the measurement of the induced magnetic field by the flow of a conducting fluid in the vicinity of a localized magnetic field. The new velocimeter has no moving parts, and can be enclosed in a sealed cap, easing the implementation in harsh environments, such as liquid metals. The proposed method allows one to probe both the continuous part and fluctuations of the velocity, the temporal and spatial resolution being linked to the actual geometric configuration of the probe. A prototype probe has been tested in a gallinstan pipe flow and in a fully turbulent flow of liquid gallium generated by the counter rotation of two coaxial impellers in a cylinder. The signals have been compared to a reference potential probe and show very good agreement both for time-averaged velocities and turbulent fluctuations. The prototype is shown to detect motion from a few cm s(-1) to a few m s(-1). Moreover, the use of the magnetic-distortion probe with large-scale applied magnetic field is discussed.
The Review of scientific instruments 09/2011; 82(9):095112. · 1.52 Impact Factor
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ABSTRACT: The motion of a large, neutrally buoyant, particle freely advected by a turbulent flow is determined experimentally. We demonstrate that both the translational and angular accelerations exhibit very wide probability distributions, a manifestation of intermittency. The orientation of the angular velocity with respect to the trajectory, as well as the translational acceleration conditioned on the spinning velocity, provides evidence of a lift force acting on the particle.
Physical Review Letters 04/2011; 106(15):154501. · 7.37 Impact Factor
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ABSTRACT: We study the six-dimensional dynamics--position and orientation--of a large sphere advected by a turbulent flow. The movement of the sphere is recorded with two high-speed cameras. Its orientation is tracked using a novel, efficient algorithm; it is based on the identification of possible orientation "candidates" at each time step, with the dynamics later obtained from maximization of a likelihood function. Analysis of the resulting linear and angular velocities and accelerations reveal a surprising intermittency for an object whose size lies in the inertial range, close to the integral scale of the underlying turbulent flow.
The Review of scientific instruments 03/2011; 82(3):033906. · 1.52 Impact Factor
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ABSTRACT: We study a simple magnetohydrodynamical approach in which hydrodynamics and MHD turbulence are coupled in a shell model, with given dynamo constraints in the large scales. We consider the case of a low Prandtl number fluid for which the inertial range of the velocity field is much wider than that of the magnetic field. Random reversals of the magnetic field are observed and it shown that the magnetic field has a nontrivial evolution--linked to the nature of the hydrodynamics turbulence.
Physical Review Letters 07/2010; 105(2):024501. · 7.37 Impact Factor
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ABSTRACT: We present an apparatus that generates statistically homogeneous and isotropic turbulence with a mean flow that is less than 10% of the fluctuating velocity in a volume of the size of the integral length scale. The apparatus is shaped as an icosahedron where at each of the 12 vertices the flow is driven by independently controlled propellers. By adjusting the driving of the different propellers the isotropy and homogeneity of the flow can be tuned, while keeping the mean flow weak.
Review of Scientific Instruments 05/2010; 81(5):055112-055112-8. · 1.37 Impact Factor
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ABSTRACT: The dynamics of particles in turbulence when the particle size is larger than the dissipative scale of the carrier flow are studied. Recent experiments have highlighted signatures of particles' finiteness on their statistical properties, namely a decrease of their acceleration variance, an increase of correlation times (at increasing the particles size) and an independence of the probability density function of the acceleration once normalized to their variance. These effects are not captured by point-particle models. By means of a detailed comparison between numerical simulations and experimental data, we show that a more accurate description is obtained once Faxen corrections are included.
Journal of Fluid Mechanics 07/2009; · 2.46 Impact Factor
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ABSTRACT: A synthetic fluid dynamo built in the spirit of the Bullard device [E. C. Bullard, Proc. Camb. Phil. Soc., 51, 744 (1955)] is investigated. It is a two-step dynamo in which one process stems from the fluid turbulence, while the other part is an alpha effect achieved by a linear amplification of currents in external coils [M. Bourgoin et al., New J. Phys., 8, 329 (2006)]. Modifications in the forcing are introduced in order to change the dynamics of the flow, and hence the dynamo behavior. Some features, such as on-off intermittency at onset of dynamo action, are very robust. Large scales fluctuations have a significant impact on the resulting dynamo, in particular in the observation of magnetic field reversals.
06/2009;
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Romain Monchaux,
Michael Berhanu,
Sébastien Aumaître,
Arnaud Chiffaudel,
François Daviaud,
Bérengère Dubrulle,
Florent Ravelet,
Stephan Fauve,
Nicolas Mordant,
François Pétrélis,
Mickael Bourgoin,
Philippe Odier, Jean-François Pinton,
Nicolas Plihon,
Romain Volk
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ABSTRACT: Effect of orifice inner lip radius on synthetic jet efficiency Phys. Fluids 24, 115110 (2012) Effect of fluctuations on the onset of density-driven convection in porous media Phys. Fluids 24, 114102 (2012) On the wave breaking phenomena for the generalized periodic two-component Dullin-Gottwald-Holm system J. Math. Phys. 53, 103709 (2012) Bubble nucleation on nano-to micro-size cavities and posts: An experimental validation of classical theory The von Kármán Sodium VKS experiment studies dynamo action in the flow generated inside a cylinder filled with liquid sodium by the rotation of coaxial impellers the von Kármán geometry. We first report observations related to the self-generation of a stationary dynamo when the flow forcing is R -symmetric, i.e., when the impellers rotate in opposite directions at equal angular velocities. The bifurcation is found to be supercritical with a neutral mode whose geometry is predominantly axisymmetric. We then report the different dynamical dynamo regimes observed when the flow forcing is not symmetric, including magnetic field reversals. We finally show that these dynamics display characteristic features of low dimensional dynamical systems despite the high degree of turbulence in the flow. © 2009 American Institute of Physics.
Physics of Fluids 03/2009; 21(24):35108-91108. · 1.93 Impact Factor
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ABSTRACT: Since Larmor at the beginning of the XX^o century, the
magnetic field of the earth is thought to be produced from motions of
the liquid iron core. Part of the kinetic energy of the flow is
converted into magnetic energy. A generic model of the dynamo
instability is based on two induction processes, namely α and
φ. The α- effect is the production of a current density j
parallel to the initial magnetic field b, and the φ-effect is
linked to the velocity v gradients via the term b.∇v in the
induction equation. We developed an experimental semi homogeneous
α-φ dynamo (a model commonly used in astrophysics) in a Von
Kármán flow : motion is imparted to liquid Gallium by the
counter-rotation of two coaxial impellers with blades. The φ effect
is due to the shear in the mid plane of a Von Kármán flow
and fully includes turbulence. The α- effect is simulated by
current flow in two coils. Complex dynamics of the dynamo (On-Off
intermittency, chaotic reversals, excursions) are observed to be linked
with the statistics of the turbulent φ induction process.
10/2008; -1.
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Sébastien Aumaître,
Michael Berhanu,
Mickael Bourgoin,
Arnaud Chiffaudel,
François Daviaud,
Bérengère Dubrulle,
Stephan Fauve,
Louis Marié,
Romain Monchaux,
Nicolas Mordant,
Philippe Odier,
François Pétrélis, Jean-François Pinton,
Nicolas Plihon,
Florent Ravelet,
Romain Volk
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ABSTRACT: The VKS experiment studies dynamo action in the flow generated inside a cylinder filled with liquid sodium by the rotation of coaxial impellers (the von Karman geometry). We report observations related to the self-generation of a stationary dynamo when the flow forcing is symmetric, i.e. when the impellers rotate in opposite directions at equal angular velocities. The bifurcation is found to be supercritical, with a neutral mode whose geometry is predominantly axisymmetric. We then report the different dynamical dynamo regimes observed when the flow forcing is asymmetric, including magnetic field reversals. We finally show that these dynamics display characteristic features of low dimensional dynamical systems despite the high degree of turbulence in the flow.
Comptes Rendus Physique 09/2008; 9:689-701. · 1.36 Impact Factor
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ABSTRACT: The Taylor–Green flow is a model flow sharing many properties with the von Kármán flow, in which experimental turbulent dynamo action has recently been achieved. We present here recent numerical results on the Taylor–Green dynamo instability, both in the linear and non-linear regime. Various properties are considered, such as the influence of turbulence, the energy transfer between different scales, the spatial structure of the neutral mode, the nature of the bifurcation and the saturation mechanisms. We also discuss the role of the velocity fluctuations on the dynamo onset. To cite this article: Y. Résumé Étude linéaire et non linéaire d'une dynamo produite par un forçage de Taylor–Green. Un écoulement turbulent forçé par un tourbillon de type Taylor–Green, partage de nombreuses propriétés avec l'écoulement de von Kármán dans lequel une dynamo turbulente a été récemment mise en évidence expérimentalement. Nous présentons des résultats récents de dynamos numériques engendrées par des tourbillons de Taylor–Green dans les régimes linéaire et non linéaire. Nous discutons certaines de ses propriétés comme l'influence de la turbulence, le transfert d'énergie entre différentes échelles, la structure du mode neutre, la nature de la bifurcation et les mécanismes de saturation. Nous discutons également le rôle joué par les fluctuations de vitesse sur le seuil de la dynamo. Pour citer cet article : Y.
01/2008; 907007(9):749-756.
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ABSTRACT: We make time resolved velocity measurements of steel spheres in free fall through liquid using a continuous ultrasound technique. We explore two different ways to induce large changes in drag on the spheres: 1) a small quantity of viscoelastic polymer added to water and 2) altering the surface of the sphere. Low concentration polymer solutions and/or a pattern of grooves in the sphere surface induce an early drag crisis, which may reduce drag by more than 50 percent compared to smooth spheres in pure water. On the other hand, random surface roughness and/or high concentration polymer solutions reduce drag progressively and suppress the drag crisis. We also present a qualititative argument which ties the drag reduction observed in low concentration polymer solutions to the Weissenberg number and normal stress difference. Comment: 8 pages
08/2007;
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ABSTRACT: Magnetic fields are ubiquitous in the cosmos and play an important dynamical role, as in the solar wind, in stars or in the interstellar medium. In planets and stars, magnetic fields are believed to be generated by a dynamo instability, in which the stretching of magnetic field lines by vigorous motions of electrically conducting fluids exceeds the Joule dissipation. Such magnetohydrodynamic (MHD) flows have large Reynolds numbers and thus nonlinear mode coupling leads to multi-scale interactions and to the formation of complex flows with, in the small scales, the presence of strong intermittent structures. Moreover, tearing mode instabilities develop and reconnection takes place, as in the magnetopause, or in the heating of solar and stellar corona. At what rate does dissipation occur, as the Reynolds number increases? What is the origin of these structures, and how fast are they formed? What is the origin of such magnetic fields (the dynamo problem)? How does the dynamo work when the magnetic Prandtl number PM—the ratio of viscosity to magnetic diffusivity—differs substantially from unity? For example, in the interstellar medium, it can be as large as 1014, whereas in stars such as the Sun and for planets such as Earth, it can be very low (lower than 10−5, the value for the Earth fluid core); similarly, in liquid breeder reactors and in laboratory experiments with liquid metals, PM 1. Huge scale separation is needed, and several dynamical regimes have been identified, depending on turbulence, rotation, flow helicity, wave interactions, etc. The progress reported in this invited focus issue of New Journal of Physics concerns observations, experiments, modeling, theory and direct numerical simulations. It is the hope of the editors of this collection that a general overview of where this dynamic scientific field is presently, and where it is going in the near future (with several large-scale projects, from detailed satellite observations to experiments and petascale computing), will be valuable to our readers. Focus on Magnetohydrodynamics and the Dynamo Problem Contents Toward coupling flow driven and magnetically driven dynamos Eric G Blackman Bifurcations and dynamo action in a Taylor–Green flow B Dubrulle, P Blaineau, O Mafra Lopes, F Daviaud, J-P Laval and R Dolganov 2D enslaving of MHD turbulence Sergey Nazarenko Numerical insights into magnetic dynamo action in a turbulent regime Sasa Kenjeres and Kemal Hanjalic Magnetic helicity effects in astrophysical and laboratory dynamos A Brandenburg and P J Käpylä Adaptive mesh refinement with spectral accuracy for magnetohydrodynamics in two space dimensions D Rosenberg, A Pouquet and P D Mininni Hydrodynamic and magnetohydrodynamic computations inside a rotating sphere P D Mininni, D C Montgomery and L Turner Structure and stability of the magnetic solar tachocline G Rüdiger and L L Kitchatinov Interactive desktop analysis of high resolution simulations: application to turbulent plume dynamics and current sheet formation John Clyne, Pablo Mininni, Alan Norton and Mark Rast Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbers A A Schekochihin, A B Iskakov, S C Cowley, J C McWilliams, M R E Proctor and T A Yousef Theory of the Lorentz force flowmeter André Thess, Evgeny Votyakov, Bernard Knaepen and Oleg Zikanov Turbulent cascades, transfer, and scale interactions in magnetohydrodynamics A Alexakis, P D Mininni and A Pouquet Global solar dynamo models: simulations and predictions of cyclic photospheric fields and long-term non-reversing interior fields M Dikpati and P A Gilman Dynamo action at low magnetic Prandtl numbers: mean flow versus fully turbulent motions Y Ponty, P D Mininni, J-F Pinton, H Politano and A Pouquet Experiments on the magnetorotational instability in helical magnetic fields Frank Stefani, Thomas Gundrum, Gunter Gerbeth, Günther Rüdiger, Jacek Szklarski and Rainer Hollerbach A non-local shell model of hydrodynamic and magnetohydrodynamic turbulence F Plunian and R Stepanov Shell models for Hall effect induced magnetic turbulence Peter Frick, Rodion Stepanov and Matthias Rheinhardt Lagrangian analysis of alignment dynamics for isentropic compressible magnetohydrodynamics J D Gibbon and D D Holm Magnetic field generation by coherent turbulence structures D Kivotides, A J Mee and C F Barenghi Kinematic dynamos in multiple scale flows A Tilgner Steve Cowley, Imperial College, London, UK and University of California, Los Angeles, USA Jean-François Pinton, Ecole Normale Supérieure de Lyon, France Annick Pouquet, National Center for Atmospheric Research, Boulder, Colorado, USA
New Journal of Physics 08/2007; 9(8). · 4.18 Impact Factor
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ABSTRACT: We use an extended laser Doppler technique to track optically the velocity of individual particles in a high Reynolds number turbulent flow. The particle sizes are of the order of the Kolmogorov scale and the time resolution, 30 microseconds, resolves the fastest scales of the fluid motion. Particles are tracked for mean durations of the order of 10 Kolmogorov time scales. The fastest scales of the particle motion are resolved and the particle acceleration is measured. For neutrally buoyant particles, our measurement matches the performance of the silicon strip detector technique introduced at Cornell University \cite{Voth,MordantCornell}. This reference dynamics is then compared to that of slightly heavier solid particles (density 1.4) and to air bubbles. We observe that the acceleration variance strongly depends on the particle density: bubbles experience higher accelerations than fluid particles, while heavier particles have lower accelerations. We find that the probability distribution functions of accelerations normalized to the variance are very close although the air bubbles have a much faster dynamics. Comment: 4 pages
08/2007;
