Abstract
To find the status of multiple collisions in transitional granular flow, moderate to densely packed monosized plastic disks were sheared in experimental 2D shear flow apparatus by applying shear strain rate in the range of 14.8 s−1 to 34.6 s−1. Application of high speed video camera and subsequent image processing techniques precisely measure the spatial positions of the particles involved in making the flow. Collision detection and contact duration finding algorithms were formulated to detect inter particle collision in each time step. The proportion of binary and multiple collisions was quantified in each time step. It is found that the contribution of multi particle collision is 13% for the lowest normalized solid fraction ( ν * = 0.50) and 68% for the highest normalized solid fraction ( ν * = 0.85) under consideration. The inter particle collision time is also found greater than binary collision time in all the flows under consideration. The group size is determined for each time step as an additional length scale associated with multi-particle interactions. Number of groups was made of more than two particles in all the flows under consideration, and the amount of such groups increased on increasing the solid fraction irrespective of the shear rate. In the dense cases (solid fraction > 0.60), rapid increase in the group size and the formation of occasional force chains should be attributed for the change in the stress generation pattern.
Highlights
A granular material is an assembly of a large number of discrete solid particles where the interstices are generally filled with fluids like air or water
The transitional granular flow with normalized solid fraction in the range of 0.50 to 0.80 and shear rate of 14.8 s−1 to 34.6 s−1 is studied in the experiment using a 2D shear flow apparatus and with the help of image processing technique
It is found that the average contact duration, defined as average duration of encounter or total duration for which particles move together since their first encounter, decreases on increasing the shear rate and increases on increasing the solid fraction
Summary
A granular material is an assembly of a large number of discrete solid particles where the interstices are generally filled with fluids like air or water These materials could be quite inhomogeneous and generally characterized by similarities or dissimilarities in properties with fluids under different conditions of loading and packing. Bagnold’s approach considers the collisions to be instantaneous binary The assumption in this approach is that in the grain inertia region, stress is dominantly generated from particle-particle interactions and it results in the momentum transfer. Both the change in momentum during collision and the rate at which collisions occur, are proportional to the relative velocity of particles, which in turn is proportional to the shear rate, γ. Both the normal and shear stresses are proportional to the square of the shear rate, γ. 2
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