The complexity of the fluid-solid flow present in these systems makes modeling them a challenging task. The primary source of difficulties is due to differences in order of magnitudes amongst the characteristic scales existing in the problem.

First of all, there is the device scale, which is naturally respected. Secondly, the typical fluid-flow scales are captured in a CFD solution by solving the flow at the mesh scale, which is generally much bigger than the particle but yet quite small when compared to the device scale. Finally, there is the fluid-particle interactions scale, which has the magnitude of the smaller particles, therefore making it computationally prohibitive to solve the flow in a sub-particle resolution for most industrial applications. These difficulties are what makes the coupled DEM-CFD approach so promising: it provides an intermediate level between using the sub-particle resolution for the fluid and the mesh resolution for both fluids and particles.