output the dragforce

Submitted by openfoam on Thu, 03/28/2013 - 10:20

In the DEM inpustscript file, I use the comand:
compute dragtotal all reduce sum f_dragforce[1] f_dragforce[2] f_dragforce[3]
#screen output
compute 1 all erotate/sphere
thermo_style custom step atoms ke c_1 vol c_centerOfMass[3] c_dragtotal[1] c_dragtotal[2] c_dragtotal[3]
thermo 10
thermo_modify lost ignore norm no
compute_modify thermo_temp dynamic yes
#insert the first particles so that dump is not empty
dump myDump all stl 1 ../DEM/post/dump_*.stl
#run 1
dump dmp all custom 100 ../DEM/post/dump*.meshGran id type type x y z vx vy vz fx fy fz f_dragforce[1] f_dragforce[2] f_dragforce[3] radius

to output the drag force, the screen output is:

Step Atoms KinEng 1 Volume centerOf dragtota dragtota dragtota
401 18 0.31959064 1.349667e-05 0.045144 0.19798336 367.85555 77.949508 -42889.077

but in the dump file, the drag force is :
ITEM: ATOMS id type type x y z vx vy vz fx fy fz f_dragforce[1] f_dragforce[2] f_dragforce[3] radius
6 1 1 -0.0185977 0.0180572 0.126699 1.85043 0.129289 7.91131 8.47019e-05 5.38816e-06 0.00032971 8.47019e-05 5.38816e-06 0.000330113 0.002

What's the difference between the c_dragtotal and the f_dragforce? which one is the particle's drag force?

ckloss's picture

ckloss | Thu, 03/28/2013 - 19:08

c_dragtotal is the total dragforce on all particles, f_dragforce is each particle's force


openfoam | Sat, 03/30/2013 - 17:00

Thanks for your reply, and I have another question. In the couplingProperties file, I did not choose any drag force models, but I found the total drag force is not equal 0, just as following:

Memory usage per processor = 14.2411 Mbytes
Step Atoms KinEng 1 Volume centerOf dragtota dragtota dragtota
1801 171 0.02411855 1.5004949e-05 0.045144 0.56308747 -0.00016906595 -0.00036594012 0.055927227
1810 171 0.024456422 1.5003719e-05 0.045144 0.56945905 -0.00016906595 -0.00036594012 0.055927227

cgoniva's picture

cgoniva | Mon, 04/08/2013 - 18:34


did you choose any force model? (I guess so - otherwise the solver should complain)


openfoam | Tue, 04/09/2013 - 03:02

Thanks, I choose the gradp force model and viscous force model, but the c_dragtotal is the total dragforce on all particles and I did not use any drag force model. I think c_dragtotal's value should be zero.

cgoniva's picture

cgoniva | Wed, 04/17/2013 - 14:44


here the name is a bit misleading - the term also includes the gradP and viscous force.


xiuhan | Fri, 10/11/2013 - 12:58

Hi, I have another question concerning this.
Is that "dump stl" wrong? cause I only see "dump mesh/stl" in the manual. In this case, the walls are set primitive, not mesh. so these walls should not be dumped into output. Right??

And I find f_dragforce, f_couple_cfd, these variables in the script. I cannot find the description of these variables in the Liggghts or CFDEM manual. So where can I find a description about how and when to use these variables, is there any document?


jtvanlew | Fri, 02/27/2015 - 02:25

i didn't find any description in documentation anywhere, but you can find the source in the LIGGGHTS-PUBLIC/src directory with the filename "fix_cfd_coupling_force.cpp"

if you follow through the file you can see what's going on with it, that might help.