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Avalanches granulaires en milieu fluide

Abstract : Take a box full of grains with a horizontal free surface and incline it smoothly. At a critical angle, a surface flow starts. This avalanche, whose amplitude and time duration are finite, takes the pile angle relax to a few degrees smaller angle. This process is often observed in nature, such as debris flows that occur on Earth surface or on submarine grounds. Nevertheless, not many studies have concerned dense granular flows when the pile is immersed in a liquid. We have thus performed an extensive series of experiments to investigate the influence of the interstitial fluid, gas or liquid, on the packing stability and the avalanche dynamics. Three regimes of avalanches in fluids are put in light depending on the grain/fluid density ratio r, and on the Stokes number St which prescribes the relative importance of grain inertia to fluid viscous effects. In gas (large r and St values) the amplitude and time duration of avalanches do not depend on any fluid effect and a surprising free fall regime is observed. In liquids (small r values), for decreasing St, the amplitude decreases and the time duration increases, exploring an inertial and a viscous regime. These three regimes are described by the simple analysis of the elementary motion of one grain. In a second part, we investigate the effect of confining lateral walls on pile angles. Maximum for small gap width, angles decrease on a characteristic length B with an increasing gap. This is explained by a model based on the redirection of stresses through the granular media to the walls, generating friction forces that prevent or stop the flow. Two different scaling laws are put in light for B. For large beads, wall effect is geometric as B is proportional to the bead size, whereas a cohesive regime leading to a constant B value is observed for small beads. Finally, we report first measurements of the grain velocity in air. Although the flow does not reach any stationary state, velocity profiles quickly adopt a stationary shape.
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https://tel.archives-ouvertes.fr/tel-00004216
Contributor : Sylvain Courrech Du Pont <>
Submitted on : Monday, January 19, 2004 - 5:13:51 PM
Last modification on : Wednesday, December 9, 2020 - 3:06:28 PM
Long-term archiving on: : Friday, April 2, 2010 - 8:01:55 PM

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  • HAL Id : tel-00004216, version 1

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Sylvain Courrech Du Pont. Avalanches granulaires en milieu fluide. Dynamique des Fluides [physics.flu-dyn]. Université Paris Sud - Paris XI, 2003. Français. ⟨tel-00004216⟩

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