One of the most studied classes of granular flows is the hopper flow. The interest for this kind of flow is mostly academic, because of its geometric simplicity and because the granular material is continuously submitted to shearing. The on-going work aims at investigating the micromechanical behavior of a sand submitted to this kind of flow. The chosen approach is the discrete modelling, so far only in two dimensions.
Two important aspects are studied more closely: the implications of the non-circularity of the grains regarding the flow kinematics and the contact forces network, and the appearance and propagation of structured fluctuations in this flow.
Beyond the theoretical aspects related to the understanding of micromechanicals mechanisms, this work may provide interesting results about the sollicitations and the strength of silos and hoppers submitted to this kind of flow.
Figure 1. Granular mass during the flow. The sample is composed of roughly 6000 grains with complex shapes, each grain being modelled by roughly 20 overlapping discs in order to approach typical shapes of Toyoura sand grains.
Figure 3. Contact forces network during the flow. The non-circularity of the grains seems to trigger longer force chains from the lateral walls of the hopper, and to increase their inclination and their intensity.
Figure 2. Influence of the non-circualrity of the grains and of the opening width of the hopper. When compared to the flow of perfect discs, accounting for complex shapes leads to narrower velocity fields ("funnel" flow), to localized grains rotations, and to a larger average number of contacts per particle.
Figure 4. Temporal fluctuations along the vertical axis of the flow, normalized by their standard deviations. One clearly observes a periodicity and a propagation of these fluctuations in the direction opposite to the flow, except for the void ratio which does not seem to exhibit any structured fluctuation.
Figure 5. Spatial correlations of the fluctuations in the whole sample. These correlations are intense, anisotropic (more correlations in the ortho-radial direction than in the radial one), and increase with height. These observations suggest a progressive structuration of these waves during their travel upwards.