CONICET | Buscador de Institutos y Recursos Humanos

Abstract Granular matter is present in all parts of our practical lives; we can see or use it every day without knowing the different problems linked to their presences. Several major problems co-exist with grains: how to obtain them (crushing, extracting ?), how to transport them (trucks, air flow, ?) and how to mix them (mechanically, gravitationally,?). So some basic studies to characterize grains properties are needed and done but generally with simple practical rules. These studies combine experimental and numerical works.We are looking on how transport can be modified with a key parameter: humidity. The numerical approach uses the classical Discrete Element Methods (well known as DEM) with soft model. A series of experiments were made by Arriarn (2010) by doing a progressive tilting with variable humidity rate from 10% 95%. Several avalanches have occured dépending of the tilting angles and the humidity rates.So in order to model these experiments, we are using a classical DEM (Discrete element Method) with soft spheres. The mechanical interaction are also taken from the literature and is defined as "Spring-Dashpot" model. The preparation of the sample is obtained by two consecutive procedure: a box filling then a gravity field presence. The box filling is partially defined as under gravity (directional effect)(Powell1980). The first layer is made by touching spheres siting on the bottom wall. Then each new sphere is placed when it has three underlying stable contacts chosen at random between all the already placed spheres close to the upper surface and its center is inside a given layer. So, the packing is build layer by layer and the layer thickness is on the order 1/10th of the sphere radius. This Powell´s algorithm will produce a dense packing fraction around 0.58-0.60. The second step consists of a switching of the gravity field to generate the standard compression gradient due to the amount of grains present on top of the packing. The sample contains 16000 spheres with a mean radius and a linear distribution from 5 to 15%. The box has a square bottom with a dimension close to 54 times the radius size R and the height goes up to 40 times R. First the packing has to reach an equilibrium checked by looking at the instantaneous velocities of all the particles of the sample: all have to be very small; i.e. they cannot be perfectly null in the DEM approach. Then we start the basic tilting of the sample by only the rotation of the gravity field at a given rate. The adjustment of this rate has to be controlled according to the real experimental value but also to the ability of the DEM code to handle some "non-static" conditions such as this dynamical tilting studies. We have made several tests by varying the tilting rate, by comparing also the different available results such as the slope, the surface displacement and the surface velocity evolution.