Une petite histoire du mulet

Concerne e.p. la Suisse, le Valais

La faune des sites mésolithiques et néolithique de la zone du Nil Bleu (Soudan Central) La fauna de los yacimientos mesolíticos y neolítico del área del Nilo Azul (Sudán Central)

Se presentan los análisis zooarqueológicos de los restos de fauna de los yacimientos mesolíticos de Sheikh Mustafa y El Mahalab y del yacimiento neolítico inicial de Sheikh el Amin, en la zona del Nilo Azul al sureste de Jartum. Los restos óseos mesolíticos muestran un modelo opuesto, de caza y pesca intensivas en SM cerca del Nilo Azul y de casi solo caza en AM en la llanura de la Butana lejos del río. La ausencia de moluscos en SM seguramente indica que el sitio se abandonó antes del fin de la estación seca. El tamaño grande de los restos piscícolas de AM sugiere que fueron pescados en aguas profundas del wadi, probablemente durante la estación húmeda. En Sheikh el Amin los restos de moluscos y pescado son muy escasos, tal vez indicando la menor importancia de la economía de predación. Los restos de bóvidos y ovicápridos son, sin embargo, bastante menos abundantes que en otros yacimientos neolíticos, insinuando una economía fundamentalmente de caza que aprovechó las condiciones húmedas de la sabana durante el Neolítico inicial.The results are presented of the analysis of the faunal remains collected during the excavations of the Mesolithic sites of Sheikh Mustafa and El Mahalab and the Early Neolithic site of Sheikh el Amin in the Blue Nile area south-east of Khartoum. Remains from the first two sites show a contrasting model, with intensive fishing and hunting at SM near the Blue Nile, similar to the scenario at the Saggai site on the main Nile, and a primary hunting orientation at AM in the Butana plain far from the river. The absence of molluscs at SM probably indicates the abandonment of the site before the end of the dry season. The big size of fish in the small sample of AM suggests that they were captured in the wadi deep waters, probably during the rainy season. At Sheikh el Amin the remains of fish and molluscs are fairly scarce, possibly indicating the lower weight of predatory economy. Bones from large and small livestock, however, are quite lower than in most other Neolithic sites of the area, insinuating a fundamentally hunting economy that benefited of the savanna humid conditions at the Early Neolithic period

Milieu granulaire soumis à des vibrations horizontales : simulations numériques

National audienceDans cette étude, une boîte rectangulaire contenant un milieu granulaire est soumise à des vibrations horizontales sinusoïdales. On simule ce phénomène en utilisant un logiciel basé sur la Méthode des Eléments Discrets. Le logiciel calcule la position et la vitesse de chaque grain à chaque instant, c'est-à-dire avec un point de vue Lagrangien. Après un post-traitement des données, cette approche est transformée en une approche Eulérienne où chaque variable (vitesse, compacité, pression) est associée à une zone fixe dans une repère lié à la boîte. Nous présentons tout d'abord les mouvements globaux (convection) dans le milieu soumis à des vibrations. L'existence d'un état stationnaire où le champ de vitesse demeure constant est ensuite mise en évidence. Enfin, nous étudions l'influence de l'accélération sur la vitesse de déplacement des grains et la compacité du milieu. Quand cela est possible, nous comparons nos résultats numériques avec des résultats expérimentaux issus de la littérature afin de déterminer la pertinence et les limites de notre modèle

Experimental study of a 3D dry granular medium submitted to horizontal shaking

International audienceThis experimental work consists in a study of the influence of horizontal sinusoidal vibrations on the behaviour of a 3D sand granular medium. A transition from jammed or glassy states to a fluidized state has been observed for critical acceleration increasing with the frequency. Convective patterns have been encountered and characterised: two cylindrical counter-rotating rolls have been observed in the top of the packing. In addition, for high acceleration values, more complex patterns appear. The free surface at the top of the packing presents different shapes called “dome”, “dromedary” or “camel” depending on acceleration. One or three humps can be found. Analysis of grain movements and velocities along the walls of the container has been carried out and show that a stationary state can be reached after a transient regime. Finally the mechanisms of material transport through convective motion have been discussed

Etude de procédés de compaction par vibrations horizontales en milieu ouvert ou fermé - Approches expérimentales et numériques.

National audienceLes procédés de compaction par vibrations sont largement utilisés dans l'industrie (génie civil par exemple pour densifier les soubassements de routes). Plus récemment, dans le domaine des fusibles, les vibrations horizontales ont permis d'améliorer le remplissage de sable dans les récipients contenant les éléments fusibles. L'objectif scientifique est de développer la compréhension des mécanismes mis en jeu dans un milieu granulaire vibré horizontalement permettant d'optimiser le procédé industriel. Deux approches ont été utilisées en parallèle, l'une expérimentale, l'autre par simulation numérique. L'approche expérimentale a été réalisée sur des grains de silice de diamètre moyen 500 mm. Un récipient de dimensions allant de 1.5 centimètre à 80 cm est soumis à un mouvement sinusoïdal de fréquence comprise entre 20 et 200 Hz, avec des accélérations allant jusqu'à 10 g. Le dispositif instrumenté permet la mesure instantanée de la force, de l'accélération, de la vitesse des grains aux parois (PIV) et de la densité globale du milieu. L'approche numérique est basée sur la méthode des éléments discrets (DEM). Nous avons développé des méthodes spécifiques d'analyse des résultats pour assurer la comparaison avec l'expérience. Elles permettent d'obtenir des informations qui ne sont pas accessibles expérimentalement comme les densités, vitesses et contraintes locales dans l'ensemble de l'empilement. Dans le cas d'un récipient ouvert, la simulation permet de retrouver les résultats expérimentaux : rouleaux de convections, seuils de comportement, influence de l'accélération... Des résultats originaux ont été établis dans les expériences et les simulations comme une croissance de la vitesse des grains avec la longueur du récipient. La simulation a aussi permis d'obtenir des résultats comme l'influence du coefficient de friction sur le sens des rouleaux et la caractérisation des contraintes au sein de l'empilement. Le comportement du sable dans un récipient fermé (milieu confiné) a été étudié expérimentalement au cours d'un remplissage progressif. De nettes différences par rapport au milieu ouvert ont été constatées lorsque le taux de remplissage devient élevé. Des mesures d'accélération et de force sur l'ensemble du dispositif ont permis de définir et mesurer une masse apparente et l'énergie dissipée par le dispositif. This work on the densification of a granular medium under horizontal vibrations was realised in the context of an industrial study on sand in fuses. The scientific purpose is the understanding of the mechanisms involved in horizontally vibrated granular media, either in open box as in closed systems. Both experiments and computer simulations were used to follow the behaviour of the sand packing. Experiments were conducted with 500 μm silica grains. Sinusoidal accelerations up to 10 g were applied to a container of a few centimetres using frequencies between 20 and 200 Hz. The experimental device enables instantaneous force, acceleration, grain velocity on the walls (PIV) and mean bulk density to be measured. Discrete element method (DEM) was used for the computer simulations. Specific techniques were developed to analyse the results and compare them with experiments. Computer simulations provided data which are not experimentally available such as local values of density, velocities and stress inside the particle packing. In the case of an open container, simulation results are in good agreement with experimental ones (convection rolls, thresholds, effect of acceleration...). Original results are obtained in both experiments and simulations, such as the grain velocity increasing with the container length. Simulation provided specific results such as the effect of friction coefficients on the direction of rotation of convection rolls and the haracterization of stress inside the particle packing. The behaviour of sand in a closed container (confined medium) was studied during a progressive filling. Significant differences were observed when the filling rate becomes high. The dissipated energy and the apparent mass of the vibrated device were defined and measured using acceleration and force measurements. The highest densities were reached for very high relative acceleration

Analysis of the densification of a vibrated sand packing

International audienceTwo techniques have been used to analyse the densification of a silica sand by horizontal sinusoidal vibrations of frequency f=50Hz for relative accelerations between 0 and 6: the quantitative analysis of motion observed through the transparent wall, and the altitude map of the free top surface of the sample. The first technique was used to analyse the transient regime: during the first 10 seconds, slight densification occurs at the bottom of the powder bed, while the upper part enters convective motion and the intermediate part reaches densities higher than 66%. The second technique allowed to quantify the evolution of the overall density vs. acceleration during the steady regime (dynamic density) and after the vibrations (relaxed density): a maximum of density is observed in both cases for an optimal acceleration which depends on the initial height of the powder bed. These results are analyzed and discussed

Study by X-Ray microtomography of the horizontal vibration effects on sand densification

International audienceX ray microtomography experiments were performed in order to evaluate the densification of silica sand submitted to horizontal sinusoidal vibrations carried out at constant frequency (50 Hz) with controlled acceleration and deceleration Γ. Packing homogeneity was characterized using relative density distribution through 3D images of the relaxed samples. Information obtained from the images allowed us to evaluate data at grain scale: porosity and pore size distribution, number of contacts per particle, particle shape and size distribution were evaluated and linked to the densification process. Based on the internal analysis of samples, the results confirm and extend the conclusions of previous works regarding the 3-layer densification under vibration and the proposed optimized vibration cycle to get dense and homogeneous samples. They extend them to different initial packings. Additionally, significant correlations are found between density and local particle packing characteristics such as pores size distribution, or the number of contacts per particle

Modelling the influence of process parameters on the densification of granular media under horizontal vibrations.

National audienceNumerical and experimental studies have been undertaken to analyze the compaction of a granular media submitted to sinusoidal horizontal vibrations. We characterize especially the influence of the dimensionless acceleration Γ of the vibrations and of the size of the box. For low value of Γ, the surface layers are compacted and the bottom layers remain at their initial density. The jamming effect of the bottom layers is explained by the influence of the pressure in these layers, which constraints the grains in their displacements. For high values of Γ, the bottom layers get compacted, the surface layers are fluidized and the density is decreased. We have also noticed that the compaction intensity depends on the size of the box: when the size of the box is lowered, the compaction becomes smaller

Numerical Simulation Of Granular Media Under Horizontal Vibrations.

International audienceSinusoidal horizontal vibrations have been applied to a parallelepiped containing rounded sand grains. To model the granular medium behavior, a commercial software based on Molecular Dynamics has been used. The influence on the rheologic behavior of many process parameters, such as the dimensionless acceleration and the frequency has been studied. The velocity, density, pressure and mass flow fields have been computed, both in dynamic and static modes and compared with experimental results. The correlation between these different parameters has been also examined

Convective Flow in a Horizontally Vibrated 3D Granular Packing

International audienceWe report experimental observations of a horizontally vibrated granular medium made out of sand grains. For large enough acceleration, two counter-rotating convection rolls appear in the upper part of the granular packing, whereas the bed remains unperturbed in the lower part. For increasing acceleration, the free surface exhibits different shapes: slight dome, two ridges and single roof. A quantitative characterization is performed by mean of PIV and image analysis. It shows that both relaxed and dynamic densities present a maximum at =3.8 and that the measured thickness of the fluidized region scales with a square root law (as a function of the dimensionless acceleration). In the discussion, a convection mechanism is proposed, as well as an analogy with classical Newtonian fluids