Flow and mixing of granular material over a single blade

Flow of liquid over a weir is important in a variety of contexts and has been productively studied for a number of decades. Granular flow over a blade (or weir) is essential for mixing operations in various industries but has not been studied to the same extent as in liquid systems. We have carried out experiments to study the velocity field and mixing of granular material as it flows over a single blade. We observe that like in fluids it is possible to have material flow against the main flow direction, leading to a recirculation region in front of the blade/weir. However unlike in fluid flows we observe that such a recirculation can occur for granular flows if the Froude number is less than a critical Froude number, i.e., for subcritical flows. We study the flow by varying the particle size and bed height and we characterize the results based on the Froude number and a dimensionless shear rate. Mixing is quantified using a dispersion coefficient and we find that our results agree qualitatively with a simple model for granular mixing. Finally we examine the mixing behavior when moisture is added to the grains and observe an increase in the mixing rate compared to dry flows.