Rotating Drums

Rotating drums are widely used in industry for the batch mixing of granular materials. Despite their mechanical simplicity, they exhibit rich and complex particle dynamics, including radial and axial segregation driven by differences in particle size, density, or shape. MultiFlow's Discrete Element Method (DEM) solver is well suited to studying these systems, resolving individual particle trajectories and collisions across drums containing hundreds of thousands of particles.

Spherical Particle Mixing and the Effect of Side-Walls

The study by Jain, Evrard & van Wachem (Particuology, 2022) uses DEM simulations to investigate how the shape of the side-walls of a rotating drum affects the mixing and segregation of a ternary particle mixture. While the literature contains extensive work on radial and axial segregation, the role of curved side-walls had remained largely unexplored.

The drum geometry consists of a cylindrical section of length \(L_c\) and diameter \(D = 2.642\) m, capped with spheroidally shaped side-walls characterised by a semi-axis length \(a\). Four side-wall curvatures (\(a/D\) = 0, 0.125, 0.25, and 0.5) and three drum lengths (\(L_c/D\) = 1, 1.35, and 2) were studied, each loaded with an initially segregated ternary mixture of particles with diameters 25.4 mm, 31.75 mm, and 38.1 mm at a fill volume of 31%. Simulations were run for 100 s of physical time at a rotation speed of 1.72 rad/s (Froude number 0.39), placing the flow firmly in the cascading regime.

Key Findings

Mixing quality is quantified using an entropy-based mixing index, where a value of 1 corresponds to a perfectly homogeneous mixture. Three phases of mixing are consistently observed across all drums: a rapid initial mixing phase (0–20 s), a slower intermediate phase (20–60 s), and a stable final state (60–100 s).

The central finding is that curved side-walls paradoxically hinder mixing and promote segregation, despite inducing strong local circulation patterns near the side-walls. The mechanism is a size-dependent axial velocity: larger particles migrate towards the curved side-walls, while smaller particles are displaced towards the drum centre. As the drum length increases, the influence of the side-walls diminishes but remains observable even in the largest drums studied. Flat side-wall drums consistently achieve the highest mixing indices (~0.95), while drums with the most pronounced curvature reach the lowest (~0.82).

Spherical particle mixing in a large rotating drum, showing radial and axial segregation dynamics.

Non-Spherical Particles: Superquadrics

Beyond spheres, MultiFlow supports superquadric particle shapes, which allow smooth interpolation between spheres, cylinders, and cube-like forms using a small number of shape parameters. This makes it possible to study how particle angularity and aspect ratio affect mixing, segregation, and packing behaviour in rotating drums — without the computational cost of resolving truly arbitrary geometries.

Mixing of tablet-shaped superquadric particles in a rotating drum.

Complex Particle Shapes

For applications where particle morphology plays a critical role — such as pharmaceutical blending, food processing, or mineral handling — MultiFlow supports fully arbitrary particle shapes described by surface meshes. The immersed boundary and surface-intersection contact detection algorithms handle the resulting complex geometry, capturing the interlocking, bridging, and orientation-dependent flow behaviour that spherical models cannot reproduce.

Rotating drum simulation with complex, non-convex particle shapes.