Velocity correlations in dense granular flows observed with internal imaging

TL;DR

This study reveals that dense granular flows inside silos exhibit velocity correlations similar to those in elastic liquids, with measurable spatial and temporal correlations indicating complex internal dynamics.

Contribution

It demonstrates that velocity correlations in dense granular flows resemble hydrodynamic responses of elastic liquids, using internal imaging techniques to reveal detailed flow behaviors.

Findings

Velocity correlations are similar to those in elastic liquids.

Weak spatial correlations extend over several grain diameters.

Caging dynamics are indicated by mean square displacement inflections.

Abstract

We show that the velocity correlations in uniform dense granular flows inside a silo are similar to the hydrodynamic response of an elastic hard-sphere liquid. The measurements are made using a fluorescent refractive index matched interstitial fluid in a regime where the flow is dominated by grains in enduring contact and fluctuations scale with the distance traveled, independent of flow rate. The velocity autocorrelation function of the grains in the bulk shows a negative correlation at short time and slow oscillatory decay to zero similar to simple liquids. Weak spatial velocity correlations are observed over several grain diameters. The mean square displacements show an inflection point indicative of caging dynamics. The observed correlations are qualitatively different at the boundaries.