  The flow of dry granular material in a half-filled rotating drum is studied.
The thickness of the flowing zone is measured for several rotation speeds, drum
sizes and beads sizes (size ratio between drum and beads ranging from 47 to
7400). Varying the rotation speed, a scaling law linking mean velocity vs
thickness of the flow, $v\sim h^m$, is deduced for each couple (beads, drum).
The obtained exponent $m$ is not always equal to 1, value previously reported
in a drum, but varies with the geometry of the system. For small size ratios,
exponents higher than 1 are obtained due to a saturation of the flowing zone
thickness. The exponent of the power law decreases with the size ratio, leading
to exponents lower than 1 for high size ratios. These exponents imply that the
velocity gradient of a dry granular flow in a rotating drum is not constant.
More fundamentally, these results show that the flow of a granular material in
a rotating drum is very sensible to the geometry, and that the deduction of the
``rheology'' of a granular medium flowing in such a geometry is not obvious.
