Contact force calculation

Submitted by SHUBHAM AGARWAL on Tue, 10/13/2020 - 09:06

Hi
I am new to LIGGGHTS and modeling an impact problem. As shown in attached figure, impact is happening between a web and fixed wall. I want to compute the total impact force with time (sum of contact force for all contact points). Please note that the contact points change as the simulation progresses. Please help me by suggesting a suitable command for that.

Thanks

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mschramm | Sun, 10/18/2020 - 20:04

Hello,
You can use compute reduce sum to sum an array. This would allow you to sum all forces acting on a group of atoms (group can be all). This would not sum it over time though...
If you need to sum all forces over all time, you will need to wright your own fix.

SHUBHAM AGARWAL | Tue, 10/20/2020 - 16:36

Thanks
The particle group on which I am interested to calculate the contact force is 'freezed'. Will summation of 'all' the forces always be zero?

mschramm | Tue, 10/20/2020 - 22:43

Hello,
Before the forces are zeroed, they are stored and summed. From the documentation:
This fix computes a global 3-vector of forces, which can be accessed by various output commands. This is the total force on the group of atoms before the forces on individual atoms are changed by the fix. The vector values calculated by this fix are “extensive”.

So if your freeze command is called frozen, you can get the forces via
variable fx equal f_frozen[0]
variable fy equal f_frozen[1]
variable fz equal f_frozen[2]

Again, theses are not running totals. The values are reset each step.

SHUBHAM AGARWAL | Sun, 10/25/2020 - 18:08

Hey thanks
I am having difficulty to dump the data. It is throwing error saying dump ID not found. Can you please help me about dump script I should use

SHUBHAM AGARWAL | Mon, 10/26/2020 - 20:39

Hi
I want the force on the blade group, I have tried following commands:
(1) dump blade_force all local ${fileprint_step} post/crepe_*.dat f_blade
(2) dump blade_force all custom/vtk ${fileprint_step} post/crepe_*.vtk f_blade

Both of them did not work
Errors:
(1) Dump local fix does not compute local info
(2)Dump particle fix does not compute per-atom info

Thanks

mschramm | Tue, 10/27/2020 - 19:53

dump commands are used to obtain per particle or per triangle data.
f_blade is a call to a fix compute vector command. These are not compatible.
Going back to my example with the frozen command, I would obtain those values as follows.

variable file_print_step equal 1000 # This tells the print command how often to print the results
variable fx equal f_freeze[0]
variable fy equal f_freeze[1]
variable fz equal f_freeze[2]
fix freeze_print all print ${file_print_step} "${fx}, ${fy}, ${fz}" file my_freeze_output.csv screen no title "fx, fy, fz"