Clusters in dense - inertial granular flows : two new views of the conundrum

There is a conundrum in the rheological behavior of Dense-inertial granular flows. Dense inertial flows are dominated by clusters of particles which provide a path for internal force transmission through the elasticity of the interparticle contacts. Yet arguably the presence or absence of the clusters have no quantitative effect on the stresses. One possible explanation is that, even though the clusters exist they are mechanically inactive so the flow behaves as if it were a collisional flow. This paper addresses that possibility in two ways. First, computer simulations that assess both the actual contact stress tensor and a parallel collisional stress tensor show that the collisional stresses can be a small fraction of the total stresses until the flow approaches a fully collisional state, supporting the idea that clusters are indeed important. The second method was to compute force correlations between particles that inhabit the same cluster, as a way of assessing the efficiency of the cluster in force transmission. But once again, the results indicate that forces can be well correlated within a cluster and thus do provide an elastic path for force transmission. Both indicate that clusters are important in internal transport within a dense-inertial granular flow and thus shed no light on why clusters do not quantitatively affect the stresses.