Hi All,
I have a question about the fix freeze command in CFDEM coupling, as I found it's behavior is different from what's described in the docs (or, I'm doing something wrong, please help me with this:-() . I define particles in a region near the bottom to be type 2, and the other particles to be type 1. in the DEM part, I used the following line to freeze the particles of type 2:
......
group bottom type 2
group active type 1
fix 2 bottom freeze
velocity bottom set 0.0 0.0 0.0 units box
fix cfd all couple/cfd couple_every 100 mpi
fix cfd2 all couple/cfd/force/implicit
......
You may notice that I even set the velocities of particles (type 2) to be zero. However, when I check the numerical result, I realize that the particles of type 2 are moving upward as a whole body, i.e. the particle velocity is not zero, and they are not freezed.
I summary, my question is: 1. What causes the particles to move, even I used fix freeze. 2. How should I fix the particles to be not moved, i.e. the voidfraction is not changed with time, and the particle velocity Us becomes zero.
Thank you in advance!
j-kerbl | Mon, 08/10/2015 - 10:22
Hi Cheng,
Hi Cheng,
I haven't used fix freeze myself. This sounds like the particle-fluid forces are not turned off.
What I usually do, if a part of the particles should not move, is to define the integration fix only for the particles (groups) supposed to move.
fix name GROUP nve/sphere
Cheers
Josef
cheng1988sjtu | Mon, 08/10/2015 - 19:13
Thank you, it helps, and now
Thank you, it helps, and now it's working, :-)
richti83 | Mon, 08/10/2015 - 11:00
As far as I remember the
As far as I remember the order is important:
fix freeze:
Zero out the force and torque on a granular particle.
so this fix should come after fix couple/cfd bc. otherwise fix couple adds new forces/torques which will be integrated by nve/sphere and results in velocity and motion.
cheng1988sjtu | Mon, 08/10/2015 - 19:18
Thank you, you are right, now
Thank you, you are right, now I understand the script of LIGGGHTS better, combining your reply and Josef's, now it's working perfectly!