Experimental investigation of high speed granular flows down inclines

We report on laterally confined granular flow experiments on steep slopes. We provide evidences for the existence of di_erent flow regimes with secondary flows. At moderate mass flow, we observe a first flow regime with a pair of longitudinal vortices which are localized close to the lateral walls and span progressively over the whole flow width with increasing flow rate. They are counter-rotative and induce a vertical upward motion of the grains at the wall. Upon a further increase of the mass flow rate, a transition is evidenced by a reversal of the rotation direction of the vortices which trigger in contrast a downward motion of the grains close to the lateral walls and an upward motion at the center of the channel. We argue that these flows bear some resemblance with the flow regimes reported recently in discrete element simulations.