When I look at the DEM code, I see that the drag coefficient calculated at that location is related to whether or not the coupling is explicit. If the coupling is explicit, the drag coefficient is calculated on the fluid side. Conversely, the drag coefficient is calculated on the particle side. I don’t know if I understand correctly, and if so, why the difference?

When explicit coupling is turned off (`DES_EXPLICITLY_COUPLED=.FALSE.`

), the drag forces are calculated for DEM particles at each solids time-step. When explicit coupling is turned on (`DES_EXPLICITLY_COUPLED=.TRUE.`

), the drag forces are calculated for DEM particles only once per fluid time step, not at each solids time-step.

So, whether it is explicit coupling or implicit coupling, there is no difference in the form of the drag force in the fluid equation, only in the calculation of particle motion?

No difference in the drag, just how often it is called.