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

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

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 (1–4) 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 “molecular” layers, taking a singular botanical structure—the Bauhinia seed pod—as 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

SLOW DENSE GRANULAR FLOWS AS A SELF INDUCED PROCESS

International audienceA simple model is presented for the description of steady uniform shear flow of granularmaterial. The model is based on a stress fluctuation activated process. The basic ideais that shear between two particles layers induces fluctuations in the media that maytrigger a shear at some other position. Based on this idea a minimum model is derivedand applied to different configurations of granular shear flow

Transient flows and migration in granular suspensions: key role of Reynolds-like dilatancy

We investigate the transient dynamics of a sheared suspension of neutrally buoyant particles under pressure-imposed conditions, subject to a sudden change in shear rate or external pressure. Discrete Element Method simulations show that, depending on the flow parameters (particle and system size, initial volume fraction), the early stress response of the suspension may strongly differ from the prediction of the Suspension Balance Model based on the steady-state rheology. We show that a two-phase model incorporating the Reynolds-like dilatancy law of Pailha & Pouliquen (2009), which prescribes the dilation rate of the suspension over a strain scale $\gamma_0$, quantitatively captures the suspension dilation/compaction over the whole range of parameters investigated. Together with the Darcy flow induced by the pore pressure gradient during dilation or compaction, this Reynolds-like dilatancy implies that the early stress response of the suspension is nonlocal, with a nonlocal length scale $\ell$ which scales with the particle size and diverges algebraically at jamming. In regions affected by $\ell$, the stress level is fixed, not by the steady-state rheology, but by the Darcy fluid pressure gradient resulting from the dilation/compaction rate. Our results extend the validity of the Reynolds-like dilatancy flow rule, initially proposed for jammed suspensions, to flowing suspension below $\phi_\mathrm{c}$, thereby providing a unified framework to describe dilation and shear-induced migration. They pave the way for understanding more complex unsteady flows of dense suspensions, such as impacts, transient avalanches or the impulsive response of shear-thickening suspensions.Comment: 24 pages, 9 figure

Force de ségrégation sur une particule dans un écoulement granulaire cisaillé

Lorsqu'un milieu granulaire constitué de particules de diamètres différents est mis en écoulement, il y a généralement séparation des particules selon leur taille. Cette ségrégation des grosse particules vers la surface implique qu'une force perpendiculaire à l'écoulement s'exerce sur elles. Nous étudions ici au moyen de simulations bidimensionnelles de dynamique moléculaire cette force de ségrégation s'exerçant sur une grosse particule dans un écoulement cisaillé de petites. Nous mettons en évidence que les gradients de pression et de contraintes de cisaillement génèrent tous deux des forces de ségrégation, mais qui sont opposées en direction. En outre, l'intensité de la force de ségrégation, proportionnelle à l'intensité du gradient, dépend aussi du coefficient de friction local, avec une augmentation importante de la force pour des écoulements proches du régime quasi-statique. Cette étude ouvre la voie à une interprétation du phénomène de ségrégation granulaire basée sur la dynamique des forces en jeux sur les particules

Couplages poroélastiques dans des branches naturelles et artificielles en lien avec la mécano-perception des plantes

Dans la nature les plantes sont soumises à des sollicitations mécaniques externes qui ont un impact important sur leur croissance. De façon remarquable, cette réponse en croissance n'est pas seulement locale mais aussi non-locale, suggérant un transport de l'information. Récemment, il a été suggéré que ce signal pourrait être une onde de pression générée par la flexion mécanique des branches. Afin de tester cette idée, nous avons élaboré une branche artificielle en élastomère de silicone (PDMS). De façon surprenante, la flexion d'une telle branche génère une surpression dont l'amplitude varie quadratiquement avec la déformation imposée. Pour comprendre l'origine de cette réponse non-linéaire, nous proposons un modèle énergétique simple. Des expériences sur des branches naturelles suggèrent la robustesse de ce mécanisme

Suspensions rhéo-épaississantes - Principes et applications

International audienceShear-thickening is observed in dense particulate suspensions and consists in a severe increase of the suspension viscosity above an onset stress. This behavior, which is very useful for certain technological applications, can also be an issue in some industrial processes. Shear thickening was considered a puzzle for a long time. It can now be explained as a frictional transition thanks to a recent theoretical model. This paper presents this model and its numerical and experimental validations. Different applications which may emerge from the understanding of this phenomenon are then discussed.Le rhéo-épaississement est un phénomène observé dans certaines suspensions denses de particules. Il consiste en une augmentation parfois brutale de leur viscosité lorsqu'elles sont soumises à une forte contrainte. Ce comportement, très utile pour certaines applications tech-nologiques, peut aussi s'avérer problématique dans certains processus industriels. Longtemps resté une énigme, le rhéo-épaississement est désormais décrit de façon cohérente par le modèle de transition frictionnelle. Cet article présente ce modèle ainsi que les études numériques et expérimentales qui le valident. Sont ensuite abordées différentes perspectives d'applications offertes par la compréhension de ce phénomène. Résumé Shear-thickening is observed in dense particulate suspensions and consists in a severe increase of the suspension viscosity above an onset stress. This behavior, which is very useful for certain technological applications, can also be an issue in some industrial processes. Shear thickening was considered a puzzle for a long time. It can now be explained as a frictio-nal transition thanks to a recent theoretical model. This paper presents this model and its numerical and experimental validations. Different applications which may emerge from the understanding of this phenomenon are then discussed

A viscoelastic deadly fluid in carnivorous pitcher plants

Background : The carnivorous plants of the genus Nepenthes, widely distributed in the Asian tropics, rely mostly on nutrients derived from arthropods trapped in their pitcher-shaped leaves and digested by their enzymatic fluid. The genus exhibits a great diversity of prey and pitcher forms and its mechanism of trapping has long intrigued scientists. The slippery inner surfaces of the pitchers, which can be waxy or highly wettable, have so far been considered as the key trapping devices. However, the occurrence of species lacking such epidermal specializations but still effective at trapping insects suggests the possible implication of other mechanisms. Methodology/Principal Findings : Using a combination of insect bioassays, high-speed video and rheological measurements, we show that the digestive fluid of Nepenthes rafflesiana is highly viscoelastic and that this physical property is crucial for the retention of insects in its traps. Trapping efficiency is shown to remain strong even when the fluid is highly diluted by water, as long as the elastic relaxation time of the fluid is higher than the typical time scale of insect movements. Conclusions/Significance : This finding challenges the common classification of Nepenthes pitchers as simple passive traps and is of great adaptive significance for these tropical plants, which are often submitted to high rainfalls and variations in fluid concentration. The viscoelastic trap constitutes a cryptic but potentially widespread adaptation of Nepenthes species and could be a homologous trait shared through common ancestry with the sundew (Drosera) flypaper plants. Such large production of a highly viscoelastic biopolymer fluid in permanent pools is nevertheless unique in the plant kingdom and suggests novel applications for pest control