Experimental confirmation of secondary flows in granular media

As collections of grains flow, their free surface rarely remains flat. We can observe this first-hand during laboratory flow experiments or infer from undulating deposits left after landslides on Earth or Mars. The free-surface disturbances that develop suggest the presence of secondary flows, smaller in magnitude than the primary motion but driving complex three-dimensional structures inside the flow. While one can infer such behaviour from the boundaries or simulations, until now we have not been able to directly observe secondary flows experimentally. In this paper we present the first experimental confirmation of secondary kinematics in granular media using dynamic x-ray radiography, without needing to stop the motion for tomography. Specifically, we create a bulldozing mechanism by driving granular material with a conveyor belt. This sets up a unidirectional primary flow, but also generates a non-uniform, indented free-surface, hinting that secondary mechanisms are at play. Modelling of the system using discrete element method simulations is consistent with this secondary-flow explanation. We then probe further experimentally using two perpendicular pairs of x-ray sources and detectors to measure the velocity field inside the bulk. This indeed unveils a complex three-dimensional flow pattern that must include vortices and convection rolls. This advancement is pertinent for many industrial and natural scenarios where grains impact obstacles, and has even broader relevance for secondary rheology of other amorphous soft materials such as dense emulsions, pastes and colloidal suspensions.