Flow of wet granular materials: a numerical study

We simulate dense assemblies of frictional spherical grains in steady shear flow under controlled normal stress $P$ in the presence of a small amount of an interstitial liquid, which gives rise to capillary menisci, assumed isolated (pendular regime), and to attractive forces. The system behavior depends on two dimensionless control parameters: inertial number $I$ and reduced pressure $P^*=aP/(\pi\Gamma)$, comparing confining forces $\sim a^2P$ to meniscus tensile strength $F_0=\pi\Gamma a$, for grains of diameter $a$ joined by menisci with surface tension $\Gamma$. We pay special attention to the quasi-static limit of slow flow and observe systematic, enduring strain localization in some of the cohesion-dominated ($P^*\sim 0.1$) systems. Homogeneous steady flows are characterized by the dependence of internal friction coefficient $\mu^*$ and solid fraction $\Phi$ on $I$ and $P^*$. We record fairly small but not negligible normal stress differences and the moderate sensitivity of the system to saturation within the pendular regime. Capillary forces have a significant effect on the macroscopic behavior of the system, up to $P^*$ values of several units. The concept of effective pressure may be used to predict an order of magnitude for the strong increase of $\mu^*$ as $P^*$ decreases but such a crude approach is unable to account for the complex structural changes induced by capillary cohesion. Likewise, the Mohr-Coulomb criterion for pressure-dependent critical states is, at best, an approximation valid within a restricted range of pressures, with $P^*\ge 1$. At small enough $P^*$, large clusters of interacting grains form in slow flows, in which liquid bonds survive shear strains of several units. This affects the anisotropies associated to different interactions, and the shape of function $\mu^*(I)$, which departs more slowly from its quasistatic limit than in cohesionless systems.Comment: 20 pages, 29 figures with 39 subfigure

Annular shear of cohesionless granular materials: from inertial to quasistatic regime

Using discrete simulations, we investigate the behavior of a model granular material within an annular shear cell. Specifically, two-dimensional assemblies of disks are placed between two circular walls, the inner one rotating with prescribed angular velocity, while the outer one may expand or shrink and maintains a constant radial pressure. Focusing on steady state flows, we delineate in parameter space the range of applicability of the recently introduced constitutive laws for sheared granular materials (based on the inertial number). We discuss the two origins of the stronger strain rates observed near the inner boundary, the vicinity of the wall and the heteregeneous stress field in a Couette cell. Above a certain velocity, an inertial region develops near the inner wall, to which the known constitutive laws apply, with suitable corrections due to wall slip, for small enough stress gradients. Away from the inner wall, slow, apparently unbounded creep takes place in the nominally solid material, although its density and shear to normal stress ratio are on the jammed side of the critical values. In addition to rheological characterizations, our simulations provide microscopic information on the contact network and velocity fluctuations that is potentially useful to assess theoretical approaches

Influences des paramètres micromécaniques dans la simulation numérique discrète des matériaux granulaires : assemblage, déformation quasi-statique, écoulements

27 pagesWe review the influence of micromechanical parameters on the macroscopic mechanical behaviour of granular materials, as numerically simulated in discrete element approaches, both in quasistatic conditions an din dense flow. We insist in particular on the role of suitably defined dimensionless numbers apt to provide a classification of rheological regimes of quite general validity

Simulation numérique discrète et comportement mécanique des matériaux granulaires

Article in French. A free English translation is available on the web site of the journal : http://www.lcpc.fr/en/sources/blpc/index.phpInternational audienceAs a complementary tool to laboratory experiments, discrete numerical simulation, applied to granular materials, provides valuable information on the grain and contact scale microstructure, thereby enabling one to better understand the microscopic origin of macroscopic mechanical behaviours. We first introduce different simulation methods, which we regard as techniques for numerical experimentation, in connection with micromechanical models for intergranular contacts. We lay special emphasis on the important issue of sample representativity and stress the usefulness of dimensional analysis in the definition of relevant control parameters. We then apply this approach to two important rheological regimes of granular systems: solid-like, slowly strained granular materials, which might be ruled by elastoplastic constitutive relations; and liquid-like, dense granular flows, either confined or with a free surface, described by suitable friction and dilatancy law

L’ergotoxicologie est-elle soluble dans les politiques de prévention du risque chimique professionnel ? Le cas de l’ 'enquête arsénite de soude et de ses (in)-conséquences réglementaires'

L’ergotoxicologie, qui cherche à appliquer la démarche ergonomique à la compréhension et à la prévention des risques liés aux expositions à des substances chimiques sur le lieu de travail, est aujourd’hui un savoir relativement marginal dans les politiques de contrôle des toxiques professionnel. Ces dernières demeurent largement dominées par des approches d’hygiène industrielle classiques, reposant sur la définition de valeurslimites d’exposition et des préconisations relatives au port d’équipements de protection. Pour comprendre cette situation, nous proposons ici de restituer la manière dont les savoirs ergotoxicologiques ont été mobilisés, formalisés et interprétés dans le cadre d’une enquête conduite par la Mutualité Sociale Agricole au début des années 2000 et qui a conduit à l’interdiction de l’arsenite de soude en viticulture. Nous montrons comment ces savoirs ont été retraduits par les acteurs des politiques de prévention du risque professionnel agricole dans des termes compatibles avec leurs logiques d’action

Marine protected areas: from conservation to sustainable development

International audienceAfter a period during which MPAs were instrumentalised as a palliative against the inadequacy of a fisheries management conventional approach, the multi-use character of these areas prevailed, conflicts were pacified by instilling a participative process and a plurality of goals is now fully recognised. Thus MPAs are becoming both a tool and a preferred context for sustainable development in its ecological, economic and social dimensions and applied through a participatory approach that entitles local communities to both create and manage this type of area. This article will discuss the full range of these changes and how MPAs' goals have evolved with time, from a priority granted to conservation to the recognition of a plurality of goals. This article will then examine to what extent the results obtained by MPAs working toward each one of these goals make them valuable examples of areas where the requirements of sustainable development are met

Comment ignorer ce que l’on sait ?:La domestication des savoirs inconfortables sur les intoxications des agriculteurs par les pesticides

Les recherches actuelles sur la construction sociale de l’ignorance soutiennent que cette dernière est soit le fruit de stratégies conscientes, soit l’effet involontaire d’un mode d’organisation de la production de connaissances. Cet article propose de dépasser cette opposition en introduisant la question de la réflexivité des acteurs des systèmes organisés qui produisent de l’ignorance : que se passe-t-il lorsque ces acteurs prennent conscience des limites des routines qui structurent leur propre action ? Quelles dynamiques de changement résultent de cette prise de conscience ? Le cas étudié ici est celui du dispositif de prévention des intoxications professionnelles induites par les pesticides en France. En prenant appui sur l’interdiction de l’arsenite de soude (2001), nous montrons comment ce dispositif parvient à s’accommoder des savoirs « inconfortables » susceptibles de remettre en cause ses arrangements institutionnels ordinaires. Nous mettons en évidence les mécanismes par lesquels les organisations qui produisent ces savoirs offrent à leurs membres de « bonnes raisons » de les ignorer, en désamorçant leur sens critique et en évitant d’œuvrer aux changements institutionnels qui devraient découler de leur prise en considération