Spreading of a granular mass on a horizontal plane

Abstract

The transient surface flow occurring when a cylindrical pile of dry granular material is suddenly allowed to spread on a horizontal plane is investigated experimentally as a function of the released mass M, the initial aspect ratio a of the granular cylinder pile, the properties of the underlying substrate (smooth or rough, rigid or erodible) and the bead size. Two different flow regimes leading to three different deposit morphologies are observed as a function of the initial aspect ratio a, whatever the substrate properties and the bead size. For a≲3, the granular mass spreads through an avalanche on its flanks producing either truncated cone or conical deposits. For a≳3, the upper part of the column descends conserving its shape while the foot of the pile propagates radially outward. The obtained deposit looks like a “Mexican hat” and the slope angle at the foot of the deposit is observed to saturate at a value of the order of 5°. For a given ground and bead size, the flow dynamics and the deposit morphology are found to be independent of M and to vary only with the initial aspect ratio a. Further investigation indicates that the deposit morphology depends only slightly on the substrate properties and the bead size, except when a becomes large. In particular the same dynamical regimes and deposit morphologies are recovered for the same range of a, independent of the substrate properties or the bead size. Moreover the rescaled deposit radius, the rescaled spreading velocity, and the fraction of energy dissipated during the flow do not depend on M, the substrate properties, or the bead size, but vary only with a. We believe this to be the signature of the fact that the flow develops near the free surface of the granular pile so that the dynamics is essentially controlled by grain/grain interactions.