High speed imaging of traveling waves in a granular material during silo discharge

TL;DR

This study uses high-speed imaging and acoustic measurements to analyze sound wave propagation in granular materials during silo discharge, revealing variable wave velocities and flow dynamics.

Contribution

It provides the first detailed experimental visualization of sound wave behavior and flow transition effects in granular silo discharge.

Findings

Wave velocity increases towards the silo's lower end.

Sound waves propagate upward without phase synchronization.

Grain oscillations exhibit stick-slip behavior only in the upper silo.

Abstract

We report experimental observations of sound waves in a granular material during resonant silo discharge called silo music. The grain motion was tracked by high speed imaging while the resonance of the silo was detected by accelerometers and acoustic methods. The grains do not oscillate in phase at neighboring vertical locations, but information propagates upward in this system in the form of sound waves. We show that the wave velocity is not constant throughout the silo, but considerably increases towards the lower end of the system, suggesting increased pressure in this region, where the flow changes from cylindrical to converging flow. In the upper part of the silo the wave velocity matches the sound velocity measured in the same material when standing (in the absence of flow). Grain oscillations show a stick-slip character only in the upper part of the silo.