Title: Facilitating a 3D granular flow with an obstruction.*

It was shown some years ago [1] that a well-positioned obstacle can surprisingly suppress clogging at the outlet of a silo. While this effect has been extensively explored in 2D geometries, there has not been similar work in the industrially and naturally important 3D case. We report experiments on the discharge of spherical glass grains from a cylindrical hopper terminating in a conical outlet, with a clamped rod inserted axially down the hopper. As the height of the tip of the rod above the outlet is increased, we find that the flow rate and the clogging probability vary nonmonotonically, with clogging suppressed greatly at an optimal height. The optimal height and the level of suppression relative to an unobstructed flow vary with the rod diameter, with the biggest facilitation of flow when the rod obstructs as little as 2% of the area of the outlet! We also report the effect of the rigidity of the rod and the clamping mechanism on the flow-facilitation.  The tip of the rod plays an important part in destabilizing the clog, with the jamming arch forming close to the end of the rod at the optimal height.

References:

1) Zuriguel, Iker & Janda, Alvaro & Garcimartín, Angel & Lozano, Celia & Arévalo, Roberto & Maza, Diego. (2011). Silo Clogging Reduction by the Presence of an Obstacle. Physical review letters. 107. 278001. 10.1103/PhysRevLett.107.278001.