Mechanisms for recirculation cells in granular flows in rotating cylindrical rough tumblers

TL;DR

This study uses discrete element method simulations to analyze how wall roughness and curvature induce recirculation cells in granular flows within rotating cylindrical tumblers, revealing the interplay of endwall and wall effects on particle trajectories.

Contribution

It provides a detailed analysis of how endwall and wall roughness influence particle recirculation and drift in granular tumblers, highlighting the competition between these effects.

Findings

Endwall roughness increases trajectory curvature.

Smooth cylindrical walls enhance axial drift near endwalls.

Counter-rotating cells form in long tumblers due to wall effects.

Abstract

Friction at the endwalls of partially-filled horizontal rotating tumblers induces curvature and axial drift of particle trajectories in the surface flowing layer. Here we describe the results of a detailed discrete element method study of the dry granular flow of monodisperse particles in threedimensional cylindrical tumblers with endwalls and cylindrical wall that can be either smooth or rough. Endwall roughness induces more curved particle trajectories, while a smooth cylindrical wall enhances drift near the endwall. This drift induces recirculation cells near the endwall. The use of mixed roughness (cylindrical wall and endwalls having different roughness) shows the influence of each wall on the drift and curvature of particle trajectories as well as the modification of the free surface topography. The effects act in opposite directions and have variable magnitude along the length of the tumbler such that their sum determines both direction of net drift and the recirculation cells. Near the endwalls, the dominant effect is always the endwall effect, and the axial drift for surface particles is toward the endwalls. For long enough tumblers, a counter-rotating cell occurs adjacent to each of the endwall cells having a surface drift toward the center because the cylindrical wall effect is dominant there. These cells are not dynamically coupled with the two endwall cells. The competition between the drifts induced by the endwalls and the cylindrical wall determines the width and drift amplitude for both types of cells.