Experimental confirmation of secondary flows within granular media

As collections of grains flow, free-surface deformations often develop. These typically suggest the presence of secondary flows, smaller in magnitude than the primary motion but driving complex three-dimensional internal structures. While one can infer such behaviour from boundaries or simulations, we have not previously been able to directly observe secondary flows experimentally. In this paper we present an experimental confirmation of secondary kinematics within granular media using dynamic x-ray radiography, without needing to stop motion for tomography. Specifically, we create a bulldozing mechanism of conveyor-driven grains. This generates a non-uniform, indented free-surface, hinting that secondary mechanisms are at play alongside the primary regime. Discrete element method simulations are shown to be consistent with this secondary-flow explanation. We then probe further experimentally using two perpendicular x-ray source/detector pairs to measure the velocity inside the bulk. This indeed unveils a complex three-dimensional flow pattern that deviates from the primary vertical planes and must include vortices and convection rolls. This advancement is pertinent for industrial and natural scenarios where grains impact obstacles, and has broader relevance for studying the rheology associated with secondary flows in other amorphous materials such as emulsions, pastes and colloids.