Role of vibrations in the jamming and unjamming of grains discharging from a silo

TL;DR

This study investigates how gentle vertical vibrations influence the flow of grains in silos, revealing that vibrations mainly break existing arches rather than prevent their formation, thus improving flow.

Contribution

It provides experimental evidence that vibrations mainly facilitate flow by breaking arches, not by reducing their formation probability, supported by a probabilistic model.

Findings

Vibrations primarily break blocking arches, not prevent their formation.

Flow improvement correlates with arch breakage rather than arch formation.

A probabilistic model confirms arch breakage as the main mechanism.

Abstract

We present experimental results of the jamming of non-cohesive particles discharged from a flat bottomed silo subjected to vertical vibration. When the exit orifice is only a few grain diameter wide, the flow can be arrested due to the formation of blocking arches. Hence, an external excitation is needed to resume the flow. The use of a continuous gentle vibration is a usual technique to ease the flow in such situations. Even though jamming is less frequent, it is still an issue in vibrated silos. There are, in principle, two possible mechanisms through which vibrations may facilitate the flow: (i) a decrease in the probability of the formation of blocking arches, and (ii) the breakage of blocking arches once they have been formed. By measuring the time intervals inside an avalanche during which no particles flow through the outlet, we are able to estimate the probability of breaking a blocking arch by vibrations. The result agrees with the prediction of a bivariate probabilistic model in which the formation of blocking arches is equally probable in vibrated and non-vibrated silos. This indicates that the second aforementioned mechanism is the main responsible for improving the flowability in gently vibrated silos.