Patrice Le Gal Home Page

Institut de Recherche sur les Phénomènes Hors Equilibre (IRPHE)
UMR 6594, CNRS - Universités d'Aix-Marseille I et II.

Geophysical and Rotating flows research team

Recent Publications

Wakes    Rotating Flows     Turbulence    Vortices   Geophysics  Bioluminescence     Liquid Crystal     Amateur Astronomy
  • Wakes

  • Flow-induced vibrations of high mass ratio flexible filaments freely hanging in a flow
    Lionel schouveiler, Christophe Eloy and Patrice Le Gal
  • Abstract

    The behavior of high mass ratio flexible filaments freely hanging in steady horizontal uniform flows is experimentally and theoretically investigated. When the flow velocity is small, static equilibrium states (image) , where the filaments are inclined to the flow, are observed. Then, above a critical value of the wind velocity, the filaments exhibit periodic oscillations in the vertical plane (image). The problem is theoretically addressed considering the beam theory equations for the filament dynamics where the action of the flowing fluid is modeled using semi-empirical expressions. These equations are first solved for the stationary equilibrium states. Then, the stability of these steady solutions relatively to small perturbations is analyzed. A good agreement between experimental and theoretical results is found.

    Physics of Fluids 17, 047104 (2005)

  • Hysteretic mode exchange in the wake of two circular cylinders in tandem
    Yuji Tasaka, Seiji Kon, Lionel schouveiler and Patrice Le Gal
  • Abstract

    Our experimental study is devoted to the analysis of the °ow past two tandem circular cylinders near the vortex shedding threshold. A recent bi-dimensional numerical analysis of this flow [Mizushima and Suehiro, Phys. Fluids 17, 104107 (2005)] has predicted that the bifurcation diagram should become complex in the vicinity of the instability threshold. Subcritical and saddle node bifurcations that lead to hysteretic exchanges between two different modes of vortex shedding were detected for particular distances of separation of the cylinders. We present here visualizations (image) and velocity measurements of this floow in a water channel that prove the robustness of the complexity
    of the bifurcation diagram in real flows.

    Physics of Fluids 18, 084104 (2006)

  • Rotating flows

  • Turbulence

  • Vortices

  • Geophysics

  • Elliptical instability of the flow in a rotating shell
    Lacaze L., Le Gal P., Le Dizès S.
  • Abstract
    A theoretical and experimental study of the spin-over mode induced by the elliptical instability of a flow contained in a slightly deformed rotating spherical shell is presented. This geometrical configuration mimics the liquid rotating cores of planets when deformed by tides coming from neighboring gravitational bodies. Theoretical estimations for the growth rates and for the non linear amplitude saturations of the unstable mode are obtained and compared to experimental data obtained from Laser Doppler anemometry measurements. Visualizations and descriptions of the various characteristics of the instability are given as functions of the flow parameters.

    Physics of the Earth and planetary interiors 151, pp. 194-205 (2005)

  • Magnetic field induced by elliptical instability in a rotating spheroid
    Lacaze L., Herreman W., Le Bars M., Le Dizès S. and Le Gal P.
  • Abstract
    The tidal or elliptical instability of rotating fluid flows is generated by the resonant interaction of
    inertial waves. In a slightly elliptically deformed rotating sphere, the most unstable linear mode is
    called the spin-over mode and is a solid body rotation versus an axis aligned with the maximum strain
    direction. In the non viscous case, this instability corresponds to the median moment of inertia insta-
    bility of solid rotating bodies. This analogy is furthermore illustrated by an elliptical top experiment,
    (Movie of the elliptical top),
    which shows the expected inviscid heteroclinic behaviour. In geophysics, the elliptical instability may
    appear in the molten liquid cores of rotating planets, which are slightly deformed by tidal gravitational
    effects of close bodies. It may then participate to the general outer core dynamics and possibly to the
    geodynamo process. In this context, Kerswell and Malkus (Kerswell, R.R. and Malkus, W.V.R., Tidal
    instability as the source for Io's magnetic signature, Geophys. Res. Lett., 1998, 25, 603-606) showed
    that the puzzling magnetic field of the Jovian satellite Io may indeed be induced by the elliptically
    unstable motions of its liquid core that deffect Jupiter magnetic field. Our magnetohydrodynamics
    experiment is a toy-experiment
    of this geophysical situation and demonstrates for the first time the
    possibility of an induction of a magnetic field by the flow motions due to the elliptical instability.
    A full analytical calculation of the magnetic dipole induced by the spin-over is presented. Finally,
    exponential growths of this induced magnetic field in a slightly deformed rotating sphere filled with
    Galinstan liquid metal are measured for different rotating rates. Their growth rates compare well with
    theoretical predictions in the limit of a vanishing Lorentz force.

    Geophysical and Astrophysical Fluid Dynamics (100), pp. 299-317 (2006)

  • Coriolis effects on the elliptical instability in cylindrical and spherical rotating containers
      Le Bars M., Le Dizès S. and Le Gal P.
  • Abstract
    The effects of the Coriolis force on the elliptical instability are studied experimentally in cylindrical and spherical rotating containers placed on a table rotating at a fixed rate $\tilde{\Omega}^G$. For a given set-up, changing the ratio ΩG of global rotation $\tilde{\Omega}^G$ to flow rotation $\tilde{\Omega}^F$ leads to the selection of various unstable modes due to the presence of resonance bands , in close agreement with the normal-mode theory. No instability occurs when ΩG varies between −3/2 and −1/2 typically. On decreasing ΩG toward −1/2, resonance bands are first discretized for ΩG<0 and progressively overlap for −1/2 xs226A ΩG < 0. Simultaneously, the growth rates and wavenumbers of the prevalent stationary unstable mode significantly increase, in quantitative agreement with the viscous short-wavelength analysis. New complex resonances have been observed for the first time for the sphere, in addition to the standard spin-over. We argue that these results have significant implications in geo- and astrophysical contexts.

    Journal of Fluid Mechanics (585), pp. 323-342 (2007)

  •   Experimental analysis of the Strato-rotational Instability in a cylindrical Couette flow
  • Le Bars M. and Le Gal P.

    This study is devoted to the experimental analysis of the stratorotational instability (SRI). This
    instability affects the classical cylindrical Couette flow when the fluid is stably stratified in the axial
    direction. In agreement with recent theoretical and numerical analyses, we describe for the first time in
    detail the destabilization of the stratified flow below the Rayleigh line (i.e., the stability threshold without
    stratification). We confirm that the unstable modes of the SRI are nonaxisymmetric, oscillatory, and take
    place as soon as the azimuthal linear velocity decreases along the radial direction. This new instability is
    relevant for accretion disks.
  • Law of spreading of the crest of a breaking wave
  • Pomeau Y., Jamin T., Le Bars M., Le Gal P and Audoly B.
    In a wide range of conditions ocean waves break. This can be seen as the manifestation of a singularity in the dynamics of the fluid surface, moving under the effect of the fluid motion of the underlying fluid. We show that, in the real world,
    at the onset of breaking, the wave crest expands in the span-wise direction as the square root of time. This is first derived from a theoretical analysis and then compared with experimental findings. The agreement is excellent.
  • Bioluminescence

  • Dinoflagellate bioluminescence in response to mechanical stimuli in water flows
    Cussatlegras Anne-Sophie and Patrice Le Gal
  • Abstract
    Bioluminescence of plankton organisms induced by water movements has long been observed and is still under
    investigations because of its great complexity. In particular, the exact mechanism occurring at the level of the cell has
    not been yet fully understood. This work is devoted to the study of the bioluminescence of the dinoflagellates plankton
    species Pyrocystis noctiluca in response to mechanical stimuli generated by water flows. Several experiments were performed
    with different types of flows in a Couette shearing apparatus. All of them converge to the conclusion that stationary
    homogeneous laminar shear does not trigger massive bioluminescence, but that acceleration and shear are both
    necessary to stimulate together an intense bioluminescence response. The distribution of the experimental bioluminescence
    thresholds is finally calculated from the light emission response for the Pyrocystis noctiluca species.

    Non linear processes in Geophysics,
    11, pp. 1-7 (2004)

  • Variability in the bioluminescence response of the dinoflagellate Pyrocystis lunula
    Cussatlegras Anne-Sophie and Patrice Le Gal
  • Abstract
    Light emission in dinoflagellates is induced by water motions. But although it is known that mechanical stimulations of these
    organisms trigger the bioluminescent response, the exact mechanism that involves some cell membrane excitations by fluid
    motions is not yet fully understood and is still controversial. We show in this experimental study that the accelerated shear flow,
    created by abrupt rotations of one or both co-axial cylinders of a Couette shearing chamber excites the light emission from cultured
    dinoflagellates Pyrocystis lunula. Following our first results published earlier that state that pure laminar shear does not excite the
    main bioluminescent response in dinoflagellates, our present experiments show that both shear and acceleration in the flow are
    needed to trigger the bioluminescent response. Besides, the probability to stimulate this bioluminescent response under acceleration
    and shear is deduced from the response curves. This response follows a Gaussian distribution that traduces a heterogeneity in
    individual cell thresholds for the stimulation of bioluminescence in a dinoflagellate population. All these results will have a
    repercussion in the possible applications of dinoflagellate bioluminescence in flow visualizations and measurements. Moreover,
    this study opens a new way in studying mechanically-induced stimulus thresholds at the cell level.

    Journal of Experimental Marine Biology and Ecology,
    343, pp. 74-81 (2007)

  • Liquid Crystal

  • Patrice Le Gal, Institut de Recherche sur les Phénomènes Hors Equilibre, Technopôle de Chateau-Gombert , 49 Av. F. Joliot-Curie, B.P. 146, 13384 Marseille cédex 13, 13003 Marseille, France. Tel: (33) 04 96 13 97 79

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    Last modified: February 28, 2008.

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