Hi everyone,
I am a new user to LIGGGHTS and I am trying to simulate a packing including both non-cohesive and cohesive particles and the interaction between them. I already tried to simulate the in.cohesion example in LIGGGHTS/ Tutorial_public but when I tried to add a larger size particle (non-cohesive) together with the existing cohesive particles, I saw no cohesive behaviour between them. Is there anybody has any ideas on how to insert a packing of non-cohesive and cohesive particles? I really really appreciate any helps.
I also saw an animation of cohesive mixing in CFDEM forums. Does anyone know how to get the codes for this example?
Thank you all so much.
Thao
| Attachment | Size |
|---|---|
 2_atoms_sim.txt | 2.88 KB |
mschramm | Wed, 10/21/2020 - 17:23
input file
Hello,
To simulate a cohesive and non cohesive system, you should only need to adjust the number of atom types and determine what the interaction matrix should be for your atom types.
What are you currently doing in your input script?
You should have something like the following (assuming that atom type 1 is your cohesive material and your atom type 2 is your non-cohesive material).
variable coh_den_1_1 equal val1
variable coh_den_1_2 equal val1
variable coh_den_2_2 equal 0.0
fix m6 all property/global cohesionEnergyDensity peratomtypepair 2 ${coh_den_1_1} ${coh_den_1_2} ${coh_den_1_2} ${coh_den_2_2}
Thao | Thu, 10/22/2020 - 07:04
Output of the pull-off force and separation distance
Hi mschramm,
I followed your advice and the packing seemed to include both the cohesive and non-cohesive particles now but I still need to modify it.
I have another question. In order to see the cohesive force clearly, I plan to just simulate 2 cohesive particles sticking together. Then I plan to apply a pull-off force to pull these particles away from each other.
Firstly, can you advise me how to apply a pull-off force in LIGGGHTS? Secondly, I would like to see the relationship of the pull-off force and the separation distance. I reckon the pull-off force will depend on the separation distance as the smaller the separation distance, the higher the attractive cohesion force so the higher pull-off force is required to pull these away.
If my question is unclear, please let me know.
Thanks again for your help.
mschramm | Thu, 10/22/2020 - 23:44
fix add force
The fix addforce (https://www.cfdem.com/media/DEM/docu/fix_addforce.html) is what you will be wanting to use.
You will need to add the particle to a group in order to apply it. As for "seeing" the force relationship, you can simply
print off the forces acting on the two spheres. They should be equal but opposite in direction.
NOTE: LIGGGHTS does not model attractive forces (like jkr) only cohesive forces while the two spheres have overlap.
As soon as that overlap goes away, so does the cohesive force. (See surface close and surface intersect functions in the cohesive classes).
Thao | Wed, 11/25/2020 - 06:10
Output of contact forces
Hi mschramm,
Thank you for your help. As following your advice closely, I have attached herewith my script of '2 atoms simulation', simulating 2 particles contacting and then moving away from each other under added forces. I have two questions:
1) As I want to see the relationship between the contact forces and the overlap distance delta, I used the compute pair/gran/local to compute force and contactArea and then attempted to print these by using dump local. But the results from dump local remained zeros. Can you suggest what have I done wrong?
2) For the dump results, even though this is a small simulation with only 50 dump files, to extract the values of forces, do I have to go through 50 files file-by-file? Are there any more efficient ways to extract the outputs of forces automatically?
I hope to hear from you. If my questions are not clear, please let me know.
mschramm | Sun, 11/29/2020 - 21:38
print command
Hello,
First, the file you uploaded seems to not we working (at least for me).
I get the following 404 error:
Not Found
The requested URL "/system/files/2_atoms_sim_0.txt" was not found on this server.
As for printing out the info that you want, I would recommend simply using the fix print command (https://www.cfdem.com/media/DEM/docu/fix_print.html)
and print the following info into a csv file
time, x1, y1, z1, f1, x2, y2, z2, f2
This will provide a single file and any other information you would want can easily be calculated.
Example
variable sim_time equal step*${dt}
variable x1 equal x[1]
variable y1 equal y[1]
variable z1 equal z[1]
variable f1 equal f[1]
variable x2 equal x[2]
variable y2 equal y[2]
variable z2 equal z[2]
variable f2 equal f[2]
fix outfile all print 100 "${sim_time}, ${x1}, ${y1}, ${z1}, ${f1}, ${x2}, ${y2}, ${z2}, ${f2}" file output.csv screen no