Madani Ammi

SILO HICCUPS : DYNAMIC EFFECTS OF DILATANCY IN GRANULAR FLOW

The granular flow through an open silo is investigated experimentally. A mechanism based both on the dilation of the granular medium and an interaction with the interstitial gas causes the flow to stop at regular intervals. The experiments are carried out at different surrounding pressures P0, and it is found that the intermittent flow becomes continuous at sufficiently low P0, showing that the intermittency is linked to the interaction the gas. The scaling of the average flow rate with particle size further supports our view of the gas-grain interaction.

Model of Sand Transport by Consecutive Particle Collisions with Wind Interaction

We study successive collisions of incident energetic beads with granular packing in the context of Aeolian saltation transport. We investigate the collision process for the case where the incident bead and those from the packing have identical mechanical properties. We analyze the features of the consecutive collision processes. We used a molecular dynamics method known as DEM (soft Discrete Element Method) with an initial static packing of 20,000 2D particles. The packing is created randomly in a box of dimension 250 × 60 disk diameters. The incident disks are launched at a given velocity ranging between 25 and 100 (normalized by \(\sqrt{g.d}\)) and an angle of 15∘ with random position on top of the flowing zone. Few disks are launched to initiate the dilation of the upper surface. This situation due to the previous collisions is responsible for a need of less input energy for maintaining the flow by ejecting new disks from the static bed. This new behavior is obtained by the presence of the wind which can accelerates the flowing disks during their ballistic trajectories.

Granular Flow in Hoppers and Tubes

We have studied the effect of gas-grain interaction on the flow of sand for two different experiments. The first experiment deals with the intermittent flow observed in a closed hour-glass. The intermittency results from a coupling between the flow of sand and convection of air through the sand. The second experiment deals with the oscillatory flow observed in an open silo. In this case a local mechanism is caused by dilation of the granular material and interaction with the interstitial air is responsible for the observed oscillations.

Granular flow: Some experimental results

The relative influence the friction forces and interstitial air is analyzed under two experimental conditions. First, the motion of one ball on a rough surface built by gluing a monolayer of spheres on an inclined plane is studied as function of the inclination angle ≡ and the ratio Θ of the diameter of the rolling ball to glued spheres diameter. An analysis of the friction force is proposed, and an expression is given for the “mean path length” of one grain moving on this surface. The second part of the paper enlights the strong influence of air in the working of hourglasses when the size of the grains (and thus the permeability of the sand) becomes small. The question of stability of arches appears to be a very central one.