Scale dependence of segregation patterns in the filling of silos

TL;DR

This study investigates how particle size segregation patterns in silos depend on scale and fluid conditions, revealing key parameters that unify laboratory and industrial observations.

Contribution

It identifies the silo slenderness and Stokes number as critical parameters governing segregation patterns, providing a unified scaling framework for different scales and conditions.

Findings

Segregation patterns depend on silo geometry and fluid viscosity.

Slenderness and Stokes number determine whether large particles migrate to walls or center.

Fluid presence influences segregation dynamics and pattern formation.

Abstract

Size segregation in granular flows is a well-known phenomenon: laboratory experiments consistently show that large particles migrate toward silo walls during filling, while smaller particles concentrate near the center. Paradoxically, field observations in large-scale industrial silos often report the opposite pattern, challenging these findings. We demarcate these patterns through a systematic experimental study spanning a range of dimensionless numbers relevant to bidisperse granular flows in quasi-2D silos under both dry and immersed conditions, varying container geometry and fluid viscosity. Image analysis reveals that the observed patterns are governed by two key dimensionless parameters: the slenderness of the silo and the Stokes number, which encapsulates the balance between particle inertia and viscous drag. Our results demonstrate the role of fluids on segregation dynamics and provide a unified scaling framework that reconciles laboratory- and field-scale observations in air.