Bouncing, piling, spreading: Impact of a dense granular jet on a rough surface*

Handling of granular materials often involves the formation of axisymetric sandpiles. Their growth as material is being poured, and in particular the radial extension of the grains, are an important parameter to predict. We experimentally study the continuous impact of a dense stream of grains on a rough surface. Initially, a central granular jump is observed between the incoming grains and prior grains that have been deposited. As more mass is provided, the radius of the jump is observed to reduce. Once the granular jump reaches its minimum radius, incoming grains are observed to start piling symmetrically. The presence of the pile removes the individual effects of bouncing grains that are instead pushed into the pile's apex, leading to a growing center and avalanches on the free surface as the pile spreads. We describe these different regimes and the transition between them in terms of the mass flux of the granular jet and the height over which the grains are allowed to accelerate in the air prior to impact. The range of avalanche behavior in the second regime is then characterized. This system illustrates the complexity of the transition between the grain scale and the pile scale.