Leidenfrost Granular Flows
This paper presents numerical findings on rapid 2D and 3D granular flows on a bumpy base. In the supported regime studied here, a strongly sheared, dilute and agitated layer spontaneously appears at the base of the flow and supports a compact packing of g
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Universit´e de Lyon, Laboratoire de Physique, ENS Lyon & UFR de Physique, UCBL, 46 all´ee d’Italie, 69 007 Lyon, France [email protected] GMCM, Universit´e Rennes 1, Campus Beaulieu, 35 000 Rennes, France
Summary. This paper presents numerical findings on rapid 2D and 3D granular flows on a bumpy base. In the supported regime studied here, a strongly sheared, dilute and agitated layer spontaneously appears at the base of the flow and supports a compact packing of grains moving as a whole. In this regime, the flow behaves like a sliding block on the bumpy base. In particular, for flows on a horizontal base, the average velocity decreases linearly in time and the average kinetic energy decreases linearly with the travelled distance, those features being characteristic of solid-like friction. This allows us to define and measure an effective friction coefficient, which is independent of the mass and velocity of the flow. This coefficient only loosely depends on the value of the micromechanical friction coefficient whereas the influence of the bumpiness of the base is strong. We give evidence that this dilute and agitated layer does not result in significantly less friction. Finally, we show that a steady regime of supported flows can exist on inclines whose angle is carefully chosen.
1 Introduction Granular flows can display a wide variety of behaviors ranging from solidlike to gas-like and a great deal of work has been devoted to steady and fully-developed dense flows [1–4]. Here, using 2D and 3D molecular dynamics simulations we study transient granular flows on a horizontal base as well as the steady regime obtained on inclines. We focus on the supported regime, in which a strongly sheared, dilute and agitated layer spontaneously forms at the base of the flow and supports a compact packing of grains moving as a whole. This regime has been described in the geophysical literature [5–8] and in the kinetic theory framework [9], but these authors did not study either the dynamical properties of the flow or the arrest of the flow. Moreover, the influence of the bumpiness and inclination of the base remains unexplored both experimentally and numerically. The density inversion observed in supported flows is similar to the Leidenfrost effect observed when a drop of water is poured onto a hot plate. A thin layer of gas develops underneath the drop and considerably reduces the apparent friction between the drop and the plate.
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Nicolas Taberlet, Patrick Richard, and Renaud Delannay
Therefore, one can wonder if supported granular flows are self-lubricated. Let us mention that a density inversion has also been observed in the case of vertically shaken granular media [10, 11]. Note that an impressive number of parameters (among which grain mass, size, density, shape, polydispersity, Young modulus, restitution and friction coefficients as well as total number of grains, initial velocity direction and amplitude, initial granular temperature, initial packing fraction, bumpiness, polydispersity, order/disorder and inclination of the base)
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