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Bruit et fluctuations dans les écoulements de fluides complexes

Abstract : This manuscript gathers experimental results of three different experiments, all involving quasistatic flows of complex fluids. A first experiment tackles the creep motion of a granular pile induced by thermal cycling. We report a time-resolved study of the dynamics associated with the slow compaction of a granular column submitted to thermal cycles. The column height displays a non trivial behavior: for a large amplitude of the temperature cycles, the granular column settles continuously, experiencing a small settling at each cycle; By contrast, for small-enough amplitude, the column exhibits a discontinuous and intermittent activity: successive collapses are separated by quiescent periods whose duration is exponentially distributed. Interpretations involving the relative thermal expansion of the container and previously proposed by others, are ruled out by our results. A plausible scenario for the compaction is given: the grains are subjected to a periodic vertical-shear due to the imposed radial thermal-gradient, which induces local rearrangements, and, accidentally, internal avalanches. The finite-temperature transition observed is a finite-size effect which involves the grains surface properties (roughness). The role of the frequency of the cycles of temperature is also qualitatively discussed. A second experiment focuses on a plane shear flow of an immersed slurry. We bring to the fore a large set of experimental results which indicate that, over a few decades of values, the effective dynamical friction coefficient depends neither on the viscosity of the interstitial fluid nor on the size of beads in the sheared layer, which bears out the analogy with the solid-solid friction in a wide range of experimental parameters.We also discuss our results in the frame of the Inertial parameter and demonstrate that this number might not be relevant to describe immersed granular flows, in the quasi-static limit. Finally, in a third experiment, We report an experimental study of the intermittent dynamics of a gas flowing through a column of a non-Newtonian fluid. In a given range of the imposed constant flow rate, the system spontaneously alternates between two regimes: bubbles emitted at the bottom either rise independently one from the other or merge to create a winding flue which then connects the bottom air entrance to the free surface. The observations are reminiscent of the spontaneous changes in the degassing regime observed on volcanoes and suggest that, in the nature, such a phenomenon is likely to be governed by the non-Newtonian properties of the magma. We focus on the statistical distribution of the lifespans of the bubbling and flue regimes in the intermittent steady state. The bubbling regime exhibits a characteristic time whereas, interestingly, the flue lifespan displays a decaying power-law distribution. The associated exponent, which is significantly smaller than the value 1.5 often reported experimentally and predicted in some standard intermittency scenarios, depends on the fluid properties and can be interpreted as the ratio of two characteristic times of the system.
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Contributor : Thibaut Divoux <>
Submitted on : Thursday, July 2, 2009 - 5:47:27 PM
Last modification on : Friday, November 20, 2020 - 2:56:08 PM
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  • HAL Id : tel-00401274, version 1

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Thibaut Divoux. Bruit et fluctuations dans les écoulements de fluides complexes. Analyse de données, Statistiques et Probabilités [physics.data-an]. Ecole normale supérieure de lyon - ENS LYON, 2009. Français. ⟨tel-00401274⟩

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