Cannot see the contacts between wall and single with fix wall/gran wiggle command

[Tom_lammps]

Dear all, hope you are well and had a happy break for the Labour day
I am trying to use fix wall/gran wiggle command to compress just one single sphere by rigid walls.

Following the very useful suggestions from experts:

How to move a rigid wall in lammps

thanks for your response and help, i have tried this variable way, and this did not work. I believe that wiggle could be alternative method for me to compress the single particle, but I then face another problem about the force of wall. I wish to obtain the relationship between “generated force” and “displacement". I checked the doucmentation, i cited “If the contacts option is used, this fix generates a per-atom array with 8 columns as output” but this command seems did not work well: fix 3 …

I tried to use this wiggle to shear the particle.

however, i have a weired problem that i cannot actually see the contacts between my clump and the wall. I made a mistake for not including time integration.

Why cannot see the contacts between wall and spheres with fix wall/gran command

Dear all, hope you are well and wiil have a nice holiday soon. I am trying to do a calibration test for my clump, therefore hope to use a rigid wall to compress my clump to obtain the force - displacement relationship. Following the very useful suggestions from experts: i decided to use fix wall/gran with wiggle to shear my clump. however, i have a weired problem that i cannot actually see the contacts between my clump and the wall. i generated this clump with top boundary 0.01261; bottom…

i therefore added
fix 5 all nve/sphere for one finite-size particle.
fix 5 all nve/bpm/sphere for bpm-created clump.
but i still cannot identify the contact between sphere and the wall.

to find the reason, i changed the bpm clump to just one simple sphere, however, i still cannot really identify the contact.

I generated this sphere with top boundary 0.01261; bottom boundary -0.00501. as illusrated in the image, they are close to clump but not really contacting the clump

the boundary is p p f, the rigid boundary in z direction;
i used

fix 3 all wall/gran hooke/history 1e20 NULL 50.0 NULL 0.5 0 zplane -0.00501 NULL contacts
fix 4 all wall/gran hooke/history 1e20 NULL 50.0 NULL 0.5 0 zplane NULL 0.01261 wiggle z 0.1 0.1 contacts
fix 5 all nve/sphere

The top wall should show wiggle behaviour and contact the single since the amplitude is 0.1 m; it shoule be really large to the contact the between wall and sphere.
however i cannot really identify the contacts between clump and wall.
this is confirmed by

dump        7 all custom 100000 r100d2_contact/dump*.contact f_4[1] f_4[2] f_4[3] f_4[4] f_4[5] f_4[6] f_4[7] f_4[8]

for a long time simulation.

i always get 0 for everything.

dose anyone have a idea that why i have this problem? Does I make anything wrong?

i attached the data file and input script for anyone’ reference.
single.data (175 Bytes)
r100d2.in (1.8 KB)
hope you have a nice day

best regards

[Tom_lammps]

Besides, for the clump case, i can see that the position of these constitute spheres did not change, which also suggest that no external force from wall.

[Tom_lammps]

[akohlmey]

@jtclemm would you mind having a look at this? I made some tests and it looks like there may be a logic error somewhere, but this is also a rather unusual setup, so I may have missed something obvious.

[jtclemm]

I think this is just due to the fact that in fix wall/gran the amplitude is signed. In the documentation, fix wall/gran command — LAMMPS documentation, it says wiggle moves the box height as:
position = coord + A - A cos (omega * delta)
So for a wall above the particle to come into contact, you’d need a negative amplitude. Swapping the sign in the original input script (and changing some of the odd values so it doesn’t run for billions of timesteps), I see the wall push the particle away.

[Tom_lammps]

Dear akohlmey

Thanks for your help for that and i really appreciate that, following your suggestions, i have sucessfully conducted this compression test for my bonded structure. the results look reasonable.
hope you have a nice day

best regards

[Tom_lammps]

Dear jtclemm

many thanks for your comment on this code, I finally find the reason for not that. it is really helpful. hope you have a nice day

best regards