1,947 research outputs found
Friction law for dense granular flows: application to the motion of a mass down a rough inclined plane
The problem of the spreading of a granular mass released at the top of a
rough inclined plane was investigated. We experimentally measure the evolution
of the avalanche from the initiation up to the deposit using a Moir\'e image
processing technique. The results are quantitatively compared with the
prediction of an hydrodynamic model based on depth averaged equations. In the
model, the interaction between the flowing layer and the rough bottom is
described by a non trivial friction force whose expression is derived from
measurements on steady uniform flows. We show that the spreading of the mass is
quantitatively predicted by the model when the mass is released on a plane free
of particles. When an avalanche is triggered on an initially static layer, the
model fails in quantitatively predicting the propagation but qualitatively
captures the evolution.Comment: 19 pages, 10 figures, to be published in J. Fluid Mec
Long surface wave instability in dense granular flows
In this paper we present an experimental study of the long surface wave
instability that can develop when a granular material flows down a rough
inclined plane. The threshold and the dispersion relation of the instability
are precisely measured by imposing a controlled perturbation at the entrance of
the flow and measuring its evolution along the slope. The results are compared
with the prediction of a linear stability analysis conducted in the framework
of the depth-averaged or Saint-Venant equations. We show that when the friction
law proposed in Pouliquen (1999a) is introduced in the Saint-Venant equations,
the theory is able to predict quantitatively the stability threshold and the
phase velocity of the waves but fails in predicting the observed cutoff
frequency. The instability is shown to be of the same nature as the long wave
instability observed in classical fluids but with characteristics that can
dramatically differ due to the specificity of the granular rheology.Comment: 29 pages, 20 figures, to be published in Journal of Fluid Mechanic
Longitudinal Vortices in Granular Flows
We present a new instability observed in rapid granular flows down rough
inclined planes. For high inclinations and flow rates, the free surface of the
flow experiences a regular deformation in the transverse direction.
Measurements of the surface velocities imply that this instability is
associated with the formation of longitudinal vortices in the granular flow.
From the experimental observations, we propose a mechanism for the
longitudinal vortex formation based on the concept of granular temperature.Comment: 4 pages, 4 figure
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
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
Recommended from our members
Generating Helices in Nature
Macroscopic helical structures formed by organisms include seashells, horns, plant tendrils, and seed pods (see the figure, panel A). The helices that form are chiral; like wood screws, they have a handedness. Some are helicoids, twisted helices with saddle-like curvature and a straight centerline; others are cylindrical helices with cylindrical curvature and a helical centerline. Studies of the mechanisms underlying the formation of helicoid or helical ribbons and of the transitions between these structures (1â4) have left an important question unanswered: How do the molecular organization of the material and its global geometrical features interact to create a diversity of helical shapes? On page 1726 of this issue, Armon et al. (5) explore the rich phenomenology associated with slender strips made of mutually opposing âmolecularâ layers, taking a singular botanical structureâthe Bauhinia seed podâas their inspiration. They show that a single component, namely a flat strip with a saddle-like intrinsic curvature, is sufficient to generate a wide variety of helical shapes.Organismic and Evolutionary Biolog
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
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
Rapid granular flows on a rough incline: phase diagram, gas transition, and effects of air drag
We report experiments on the overall phase diagram of granular flows on an
incline with emphasis on high inclination angles where the mean layer velocity
approaches the terminal velocity of a single particle free falling in air. The
granular flow was characterized by measurements of the surface velocity, the
average layer height, and the mean density of the layer as functions of the
hopper opening, the plane inclination angle and the downstream distance x of
the flow. At high inclination angles the flow does not reach an x-invariant
steady state over the length of the inclined plane. For low volume flow rates,
a transition was detected between dense and very dilute (gas) flow regimes. We
show using a vacuum flow channel that air did not qualitatively change the
phase diagram and did not quantitatively modify mean flow velocities of the
granular layer except for small changes in the very dilute gas-like phase.Comment: 10 pages, 16 figures, accepted to Phys. Rev.
- âŠ