Resonances of a rotor/stator cavity in the vicinity of the critical point of SF6

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Measurement of particle and bubble accelerations in turbulence

4 pagesWe 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

Induction in a von Karman flow driven by ferromagnetic impellers

We study magnetohydrodynamics in a von K\'arm\'an flow driven by the rotation of impellers made of material with varying electrical conductivity and magnetic permeability. Gallium is the working fluid and magnetic Reynolds numbers of order unity are achieved. We find that specific induction effects arise when the impeller's electric and magnetic characteristics differ from that of the fluid. Implications in regards to the VKS dynamo are discussed.Comment: 14 pages, 7 figure

Transition from hydrodynamic turbulence to magnetohydrodynamic turbulence in von Kármán flows

International audienceThe influence of an externally applied magnetic field on flow turbulence is investigated in liquid-gallium von-Ka ́rma ́n (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 Rmeff = 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 an Alfve'n speed vA of the order of the fluid velocity fluctuations urms. This strong increase in the flow fluctuations is attributed to chaotic changes between hydrodynamic and magnetohydrodynamic velocity profiles

Résonances d'une cavité rotor/stator au voisinage du point critique du SF6

Les très grandes vitesses de rotation des pompes cryogéniques spatiales nécessitent la présence de cavités fluides aux dos des rotors. C'est dans ce contexte, que nous étudions un écoulement de type rotor/stator dans du SF6 car à la fois la viscosité de ce fluide et sa vitesse du son diminuent significativement à proximité de son point critique. Nos résultats obtenus dans une enceinte contenant du SF6 à une pression supérieure à 38 bars et à une température de 45°C, mettent en évidence des résonances de la cavité rotor/stator à des fréquences en accord avec nos modèles théoriques

Large-scale fluctuations and dynamics of the Bullard-von Karman dynamo

International audienceA synthetic fluid dynamo built in the spirit of the Bullard device (Bullard, The stability of a homopolar dynamo, Proc. Camb. Phil. Soc. 1955, 51, 744) is investigated. It is a two-step dynamo in which one process stems from the fluid turbulence, while the other part is achieved by a linear amplification of currents in external coils (Bourgoin et al., A bullard-von Karman dynamo, New J. Phys., 2006, 8, 329). 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-scale fluctuations have a significant impact on the resulting dynamo, in particular in the observation of magnetic field reversals

Vélocimétrie dans les liquides conducteurs : sonde à distorsion magnétique

Nous proposons un nouveau type de sonde pour la mesure de vitesse dans les métaux liquides. Son principe repose sur la mesure du champ magnétique induit par un écoulement de fluide conducteur au voisinage d'un champ magnétique localisé. La méthode utilisée ici permet de mesurer à la fois la moyenne et les fluctuations. Celle-ci a été testé dans un écoulement laminaire et dans un écoulement turbulent de gallium liquide dit de Von Karman. Une excellente corrélation a été observé avec les signaux d'une sonde de potentiel prise comme référence

Helical Ribbons: Simple Chiral Sedimentation

We investigate the sedimentation of chiral particles in viscous fluid flow. We identify helical ribbons as simple particles with strong translation-rotation coupling whose symmetry ensures that the centers of mass, buoyancy, resistance, and mobility coincide. Experimental measurements of both relevant mobility tensors show excellent agreement with simulations of ribbons made of interacting spheres. We observe quasi-periodic angular dynamics causing complex spatial trajectories. In tilt-spin phase space, orbits are closed due to time-reversal and reflection symmetry. Changing the helical ribbon length reveals a bifurcation at which the stable sedimentation orientations switch.Comment: 7 pages, 6 figure

Long-term memory in experiments and numerical simulations of hydrodynamic and magnetohydrodynamic turbulence

International audienceWe analyze time series stemming from experiments and direct numerical simulations of hydrodynamic and magnetohydrodynamic turbulence. Simulations are done in periodic boxes, but with a volumetric forcing chosen to mimic the geometry of the flow in the experiments, the von Kármán swirling flow between two counterrotating impellers. Parameters in the simulations are chosen to (within computational limitations) allow comparisons between the experiments and the numerical results. Conducting fluids are considered in all cases. Two different configurations are considered: a case with a weak externally imposed magnetic field and a case with self-sustained magnetic fields. Evidence of long-term memory and 1/f noise is observed in experiments and simulations, in the case with weak magnetic field associated with the hydrodynamic behavior of the shear layer in the von Kármán flow, and in the dynamo case associated with slow magnetohydrodynamic behavior of the large-scale magnetic field