Photoelastic study of dense granular free-surface flows

We use a novel experimental method to reveal the distribution of dynamic forces in the bulk of granular free-surface flows. We release photoelastic discs from an incline to create steady 2D monodisperse and bidisperse avalanches. These gravity-driven dry granular flows are in the slow to intermediate regime (I=0.1-1), dense and thin (h≈10d). We measure constant density and quasi-linear velocity profiles through particle tracking at several points down the chute, for two different basal topographies. The photoelastic technique allows the visualisation and quantification of instantaneous forces transmitted between particles during collisions. We use this information to obtain force measurements and coarse-grained profiles of the stress tensor components at various points along the chute. We first analyse how particle size distribution affects the force network. Furthermore, we specify the ways in which granular flow behaviour is analogous to that of a continuous fluid flow. We observe a hydrostatic increase of the mean pressure with depth, and show that the dynamic average force diagram is the same for discrete and continuous flows. We also show that the highly fluctuating force chains within the flow form preferentially in the directions of the forces acting on the bulk.