58 research outputs found
Crucial role of side walls for granular surface flows: consequences for the rheology
In this paper we study the steady uniform flows that develop when granular
material is released from a hopper on top of a static pile in a channel. We
more specifically focus on the role of side walls by carrying out experiments
in setup of different widths, from narrow channels 20 particle diameters wide
to channels 600 particle diameters wide. Results show that steady flows on pile
are entirely controlled by side wall effects. A theoretical model, taking into
account the wall friction and based on a simple local constitutive law recently
proposed for other granular flow configurations (GDR MiDi 2004), gives
predictions in quantitative agreement with the measurements. This result gives
new insights in our understanding of free surface granular flows and strongly
supports the relevance of the constitutive law proposed.Comment: a forgotten square root in Appendix B (Eq B4), and corrected
coefficients in Appendix C; 25 pages, 17 figures, published in J. Fluid Mec
Segregation in sheared granular flows: forces on a single coarse particle
International audienceIn order to better understand the mechanism governing segregation in granular flows, the force experienced by a large particle embedded in a granular flow made of small particles is studied using discrete numerical simulations. An empirical expression of the segregation force is proposed as a function of the stress distribution
Mouvements rapides chez les plantes : mesures mécaniques et hydrodynamiques à l'échelle cellulaire
Les plantes offrent un exemple de système vivant capable de produire des mouvements mécaniques en l'absence de muscle. Parmi ces mouvements, la fermeture rapide du piège de la plante carnivore Dionée fait figure de paradigme. Nous avions mis en évidence le rôle de l'élasticité et de la géométrie dans le mécanisme de fermeture (instabilité de flambage). Nous étudions maintenant l'origine interne de ce mouvement grâce à un dispositif micro-fluidique qui permet de mesurer en temps réel et in vivo des paramètres mécaniques à l'échelle cellulaire (pression, élasticité, perméabilité)
Depth-Independent Drag Force Induced by Stirring in Granular Media
Publisher version: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.110.138303 5 pagesInternational audienceThe drag force experienced by a horizontal cylinder rotating around the vertical axis in a granular medium under gravity is experimentally investigated. We show that for deeply buried objects, the drag force dramatically drops after half a turn, as soon as the cylinder crosses its own wake. Whereas the drag during the first half turn increases linearly with the depth, the drag after several rotations appears to be independent of depth, in contradiction with the classical frictional picture stipulating that the drag is proportional to the hydrostatic pressure. We systematically study how the saturated drag force scales with the control parameters and show that this effect may be used to drill deeply in a granular medium without developing high torques
Interparticle friction leads to non-monotonic flow curves and hysteresis in viscous suspensions
Hysteresis is a major feature of the solid-liquid transition in granular
materials. This property, by allowing metastable states, can potentially yield
catastrophic phenomena such as earthquakes or aerial landslides. The origin of
hysteresis in granular flows is still debated. However, most mechanisms put
forward so far rely on the presence of inertia at the particle level. In this
paper, we study the avalanche dynamics of non-Brownian suspensions in slowly
rotating drums and reveal large hysteresis of the avalanche angle even in the
absence of inertia. By using micro-silica particles whose interparticle
friction coefficient can be turned off, we show that microscopic friction,
conversely to inertia, is key to triggering hysteresis in granular suspensions.
To understand this link between friction and hysteresis, we use the rotating
drum as a rheometer to extract the suspension rheology close to the flow onset
for both frictional and frictionless suspensions. This analysis shows that the
flow rule for frictionless particles is monotonous and follows a power law of
exponent , in close agreement with the previous
theoretical prediction, . By contrast, the flow rule for
frictional particles suggests a velocity-weakening behavior, thereby explaining
the flow instability and the emergence of hysteresis. These findings show that
hysteresis can also occur in particulate media without inertia, questioning the
intimate nature of this phenomenon. By highlighting the role of microscopic
friction, our results may be of interest in the geophysical context to
understand the failure mechanism at the origin of undersea landslides.Comment: 10 pages, 8 figure
Deformation upon impact of a concentrated suspension drop
We study the impact between a plate and a drop of non-colloidal solid
particles suspended in a Newtonian liquid, with a specific attention to the
case when the particle volume fraction, , is close to - or even exceeds -
the critical volume fraction, , at which the steady effective viscosity
of the suspension diverges. We use a specific concentration protocol together
with an accurate determination of for each drop and we measure the
deformation for different liquid viscosities, impact velocities and
particle sizes. At low volume fractions, is found to follow closely an
effective Newtonian behavior, which we determine by documenting the low
deformation limit for a highly viscous Newtonian drop and characterizing the
effective shear viscosity of our suspensions. By contrast, whereas the
effective Newtonian approach predicts that vanishes at , a
finite deformation is observed for . This finite deformation
remains controlled by the suspending liquid viscosity and increases with
increasing particle size, which suggests that the dilatancy of the particle
phase is a key factor of the dissipation process close to and above .Comment: Submitted to JF
Lift forces in granular media
Published version: http://scitation.aip.org/content/aip/journal/pof2/26/4/10.1063/1.4869859International audienceThe paper presents an experimental and numerical study of the forces experienced by a cylinder moving horizontally in a granular medium under gravity. Despite the symmetry of the object, a strong lift force is measured. Whereas the drag force increases linearly with depth, the lift force is shown to saturate at depths much greater than the cylinder diameter, and to scale like the buoyancy with a large amplification factor of order 20. The origin of this high lift force is discussed based on the stress distribution measured in discrete numerical simulations. The lift force comes from the gravitational pressure gradient, which breaks the up/down symmetry and strongly modifies the flow around the obstacle compared to the case without pressure gradient
Physique des mouvements rapides chez les plantes
National audienceDépourvues de muscles, certaines plantes mettent en œuvre des mouvements dont la fulgurance est comparable à celle des animaux. Nous montrons dans cet article que beaucoup de ces mouvements, nécessités par la reproduction ou la nutrition, ont la même base physique : une instabilité mécanique qui libère de l’énergie élastique stockée. Deux grands types d’instabilités mécaniques sont utilisés par les plantes pour amplifier la vitesse de leur mouvement : les ruptures solides ou liquides (cavitation) pour la propulsion des graines ou des spores de fougères, et les instabilités de flambage élastique pour les pièges des plantes carnivores, telles que la Dionée ou l’utriculaire
Forces subies par un objet en mouvement dans un milieu granulaire
Les forces s'exerçant sur un objet en mouvement dans un milieu granulaire ont été assez peu étudiées, en particulier pour les forces perpendiculaires à la direction de déplacement. Nous présenterons une expérience permettant la mesure de ces forces pour un objet en translation horizontale, et montrerons la présence d'une force de portance un ordre de grandeur supérieure à la poussée d'Archimède. L'influence de différents paramètres (vitesse, forme, enfoncement etc.) sera étudiée. Ces résultats seront reliés à des modélisations récemment proposées pour la rhéologie des milieux granulaires
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