How to distinguish the difference between fix_couple/cfd/force/implicit and fix_couple/cfd/force

Submitted by Rachel on Wed, 12/06/2017 - 02:12

Dear all,

I am doing a case study shown in tutorials of CFDEM and feel confused about "fix couple/cfd/force" and "fix couple/cfd/force/implicit" commands. The online documentation says the "fix couple/cfd/force" command contains contributions from all force models active in the CFD calculation, while the "fix couple/cfd/force/implicit" seems only contain the drag force coz it says "At every coupling time step the drag coefficient and the fluid velocity is transferred to LIGGGHTS".

I am wondering if I want to exert all the drag force, viscous force, hydraulic gradient force on Particles, does that mean I can only use the "fix couple/cfd/force" including all the necessary forces while cannot use the implicit one which only include drag forces? The example given in tutorial, i.e. cfdemSolverPiso/ErgunTestMPI use the "fix couple/cfd/force/implicit" command, does that incorrect for modelling all the forces on particles? Could anyone tell me the difference between these two commands if convenient?

Thanks ahead!
Rachel

Forums:

AndresMM | Fri, 12/15/2017 - 13:20

Hi Rachel,

do you get different results if you exchange one line for the other?

Regards,
Andrés

Ermek Asylbekov | Tue, 03/27/2018 - 09:49

Hi Rachel,

as far as I've understood this the difference is that by using "fix couple/cfd/force/implicit" the drag coeff is calculated every DEM timestep (regarding the current U Field) while by using "fix couple/cfd/force" its only calculated after every CFD timestep. I didn't test it yet, but if this is the case then the results should be different if tDEM << tCFD.

Regards,
Ermek