Granular fingering patterns in horizontal rotating cylinders

We performed systematic experiments involving a thin layer of a granular medium flowing within a horizontally rotating cylinder. The cylinder aspect ratio, the particle size, and the percentage of cylinder volume occupied by particles were varied. The flow phenomena observed in these experiments were diverse and complicated. In particular, in most cases, frontal fingers were found. Dimensional analysis was used to gain insight regarding the mechanisms underlying the formation of these fingers. This analysis shows that the dimensionless granular wavelength can be expressed as a function of cylinder aspect ratio, granular Reynolds number, granular Froude number, and granular filling percentage.