Intermittency and Velocity Fluctuations in Hopper Flows Prone to Clogging

TL;DR

This study investigates granular flow dynamics in hoppers, revealing continuous clogging probability without a sharp critical size and linking flow intermittency to grain exit speed, challenging the notion of a well-defined clogging threshold.

Contribution

It provides experimental evidence that clogging is always possible and introduces a measure of intermittency based on velocity distribution comparisons over time.

Findings

No discontinuity in velocity measures at critical outlet size.

Intermittency time scale correlates with grain exit speed.

Supports clogging as a probabilistic process without a sharp threshold.

Abstract

We experimentally study the dynamics of granular media in a discharging hopper. In such flows, there often appears to be a critical outlet size $D_c$ such that the flow never clogs for $D > D_c$. We report on the time-averaged velocity distributions, as well as temporal intermittency in the ensemble-averaged velocity of grains in a viewing window, for both $D < D_c$ and $D > D_c$, near and far from the outlet. We characterize the velocity distributions by the standard deviation and the skewness of the distribution of vertical velocities. We propose a measure for intermittency based on the two-sample Kolmogorov-Smirnov $D_{KS}$-statistic for the velocity distributions as a function of time. We find that there is no discontinuity or kink in these various measures as a function of hole size. This result supports the proposition that there is no well-defined $D_c$ and that clogging is always possible. Furthermore, the intermittency time scale of the flow is set by the speed of the grains at the hopper exit. This latter finding is consistent with a model of clogging as the independent sampling for stable configurations at the exit with a rate set by the exiting grain speed [Thomas and Durian, Phys. Rev. Lett. (2015)].