Could not cancel gradPFoce and viscForce

Hi all,

I try to do a simple CFD-DEM coupling simulation so as to validate our new drag force model. Thus I canceled the "gradPForce" and "viscForce" force models, however, the solver complains that the two forces are not defined.

My questions is that the two force models are essential and could not be cancelled? Do you have some suggestions on fixing the problem?

Regards,
Yutian