Extended CFD-DEM for free-surface flow with multi-size granules

Computational fluid dynamics and discrete element method (CFD-DEM) is extended with the volume of fluid (VOF) method to model free-surface flows. The fluid is described on coarse CFD grids by solving locally averaged Navier-Stokes equations, and particles are modelled individually in DEM. Fluid-particle interactions are achieved by exchanging information between DEM and CFD. An advection equation is applied to solve the phase fraction of liquid, in the spirit of VOF, to capture the dynamics of free fluid surface. It also allows inter-phase volume replacements between the fluid and solid particles. Further, as the size ratio (SR) of fluid cell to particle diameter is limited (i.e. no less than 4) in coarse-grid CFD-DEM, a porous sphere method is adopted to permit a wider range of particle size without sacrificing the resolution of fluid grids. It makes use of more fluid cells to calculate local porosities. The developed solver (cfdemSolverVOF) is validated in different cases. A dam break case validates the CFD-component and VOF-component. Particle sedimentation tests validate the CFD-DEM interaction at various Reynolds numbers. Water-level rising tests validate the volume exchange among phases. The porous sphere model is validated in both static and dynamic situations. Sensitivity analyses show that the SR can be reduced to 1 using the porous sphere approach, with the accuracy of analyses maintained. This allows more details of the fluid phase to be revealed in the analyses and enhances the applicability of the proposed model to geotechnical problems, where a highly dynamic fluid velocity and a wide range of particle sizes are encountered. © 2015 John Wiley & Sons, Ltd.