Experimental Assessment Of The Effective Friction At The Base Of Granular Chute Flows On A Smooth Incline

We report on direct measurements of the basal force components for granular material flowing down a smooth incline. We investigate granular flows for a large range of inclination angles from theta = 13.4 degrees to 83.6 degrees and various gate openings of the chute. We find that the effective basal friction coefficient mu(B), obtained from the ratio of the longitudinal force to the normal one, exhibits a systematic increase with increasing slope angle and a significant weakening with increasing particle holdup H (the depth-integrated particle volume fraction). At low angles, the basal friction is slightly less than or equal to tan theta. The deviation from tan theta can be interpreted as a contribution from the sidewall to the overall friction. At larger angles, the basal friction mu(B) saturates at an asymptotic value that is dependent on the gate opening of the chute. Importantly, our data confirm the outcomes of recent discrete numerical simulations. First, for steady and fully developed flows as well as for moderately accelerated ones, the variation of the basal friction can be captured through a unique dimensionless number, the Froude number Fr, defined as Fr = (U) over bar/(gHcos theta)(1/2), where (U) over bar is the mean flow velocity. Second, the mean velocity scales with the particle holdup H with a power exponent close to 1/4, contrasting with the Bagnold scaling ((U) over bar similar to H-3/(2)).