questions about f_dragforce

Dear all,

I am using Model A with following code lines, where implicit method is used. For the fluid force on a particle (Fpf), there are mainly three kinds of components, fd, f_pg (pressure gradient force), f_v (viscous force).

1) what is the true meaning of f_dragforce, does it only referee to the pressure gradient force f_pg + viscous force f_v, namely (Fpf-fd) ?

2) If f_cfd2 is the sum of Fpf for all particles, why I find f_cfd2 < sum (f_dragfroce) << pressure drop * area? The case with cylindrical bed is used.

Looking forward to your help. Many thanks.

Best wishes
Nan

########### code examples ################
fix cfd all couple/cfd couple_every 10 mpi
fix cfd2 all couple/cfd/force/implicit
........
# compute explicit dragforce
compute explDrag all reduce update_on_run_end yes sum f_dragforce[1] f_dragforce[2] f_dragforce[3]
........
# sum of explicit and implicit drag force given from CFD to DEM
variable totalDragX equal f_cfd2[1]
......