Using the Lubrication Approximation to Model the Effects of Viscosity in DEM Simulations for Complex Flow Regimes

Abstract

This study explores an efficient approach to modelling the flow of particles in wet granular systems using the Discrete Element Method (DEM). Typically, when particles move in a viscous fluid, DEM is coupled with Computational Fluid Dynamics (CFD) or Smooth Particle Hydrodynamics (SPH) methods to capture both particle and fluid motion. However, the computational expense and time required for one- or two-way coupled simulations, such as DEM–CFD or DEM–SPH, can be significant. In this research, a lubrication approximation is introduced to address fluid viscosity within DEM, which is particularly suitable for dense granular systems where viscous forces play a dominant role. DEM simulations incorporating the lubrication approximation were compared with in situ data obtained from Positron Emission Particle Tracking (PEPT) experiments. These experiments involve a cylindrical setup of radius R=230mm and length L=200mm, filled with 10mm spherical glass beads at a 50% fill fraction, and various mixtures of water and glycerol (60%, 75%, and 90% by weight) as the fluid phase. Simulations are conducted within the LIGGGHTS–DEM framework, and detailed comparisons with PEPT data assess the suitability of the lubrication approximation for rotating drum systems. The analysis of tangential velocity profiles across different Stokes numbers reveals the applicability of the same constitutive equation for modelling both the PEPT and DEM data within a specified viscosity range, with the exception of the highest viscosity. This understanding is crucial for interpreting the flowing layer dynamics and optimising the simulation parameters for accurate predictions.