The Rocky DEM-CFD two-way coupling uses a neighbor-cell averaging procedure (section Lagrangian to Eulerian mapping) to avoid high volume fractions in cells of the CFD domain, distributing the exceeding particle volume among nearby cells of the mesh. Thus this methodology is suitable for dealing with dense particle flows where the volume of particles inside the cell is comparable to the cell volume, but performs less well when particles are larger than the cells.

Therefore, it is recommended that the mesh size be larger than the largest particle size throughout the simulation domain. Some regions with detailed geometry and strong gradients can have smaller cells but the coupling results in these regions tend to be less accurate.

Traditional CFD recommendations of mesh size for accurate boundary-layer capturing can usually be ignored in this case. The two-way coupling will generally be applied to cases with dense particle flows where the particle-fluid interaction will usually be more significant than the boundary-layer effects.