This occurs because the angle of repose keeps gravity from filling the concavity initially until rotation sufficiently increases the material slope in the cavity. When this expanding avalanche within the cavity occurs underneath the core, the core location can shift into the mixing zone. This occurs because grains tend to only partially fill these concavities at first. The bulk of the material settles as the concavity later fills, and as a result, the core is moved away from its original position and thereafter participates in mixing. This complication can be included in a straightforward way.
Interactions represent the most serious challenge for our model.
Nevertheless, experiments with weakly cohesive particles reveal that geometrical structures persist. We are hopeful that approaches of this kind, modeling only the most basic mechanisms and including complications as needs arise, may lead to successful analyses of complex granular flows.