Controlling rheology via boundary conditions in dense granular flows

TL;DR

This study investigates how boundary conditions, especially roughness, influence dense granular flow behavior and develops a nonlocal model to predict flow profiles based on boundary properties.

Contribution

It introduces a nonlocal model that incorporates boundary effects to accurately describe slow dense granular flows with varying wall roughness.

Findings

Flow profiles can be predicted with a single set of model parameters.

Rougher boundaries lead to increased internal stress fluctuations.

Wall slip velocity can be directly measured and used in modeling.

Abstract

Boundary shape, particularly roughness, strongly controls the amount of wall slip in dense granular flows. In this paper, we aim to quantify and understand which aspects of a dense granular flow are controlled by the boundary condition, and to incorporate these observations into a cooperative nonlocal model characterizing slow granular flows. To examine the influence of boundary properties, we perform experiments on a quasi-2D annular shear cell with a rotating inner wall and a fixed outer wall; the later is selected from among 6 walls with various roughness, local concavity, and compliance. We find that we can successfully capture the full flow profile using a single set of empirically determined model parameters, with only the wall slip velocity set by direct observation. Through the use of photoelastic particles, we observe how the internal stresses fluctuate more for rougher boundaries, corresponding to lower wall slip, and connect this observation to the propagation of nonlocal effects originating at the wall.