fix smd or fix drag for finite size particles

Dear Lammps users

I tried to pull one selected particle which is at a position (2.0,-7.5,8.0) in the medium of finite size particles (granular media), using fix smd command as follows

fix pull test smd cfor 100.0 tether -8.0 0.0 0.0 0.0.

What I observed is that the particle moves a distance equivalent to particle diameter and doesn’t move from there.

Since it is granular media, normally to make them flow we will apply either shear or vibration.

The problem which we are interested is to pull the selected particle in the stationary media (gravity will be present)

The direction of pull is perpendicular to gravity.

Please find here the part of input script here

pair_style gran/hooke/history 400000.0 NULL 100.0 NULL 0.5 0
pair_coeff * *

timestep 0.0001

group test id 5001
fix 1 all nve/sphere
fix 2 all gravity 1.0 spherical 0.0 -180.0

fix zlower all wall/gran 200000.0 NULL 50.0 NULL 0.5 0 &
zplane 0.0 NULL

fix pull test smd cfor 100.0 tether -8.0 0.0 0.0 0.0

I am thinking that fix smd or fix drag commands shouldn’t have any problem for systems of finite size particles as well.

Could you please let me know if some of you have used these commands to pull a spherical particle

Thank you very much

best regards

Anki

Please find here the part of log file

Step Atoms KinEng 1 Volume
0 5001 2.0815488 1.1539811 6600
10000 5001 19.920717 8.450075 6600
20000 5001 4.6466062 2.6050034 6600
30000 5001 1.692772 1.2215294 6600
40000 5001 1.1634224 0.89929606 6600
50000 5001 0.66904094 0.54022745 6600
60000 5001 0.40175209 0.34152204 6600
70000 5001 0.1763575 0.18494485 6600
80000 5001 0.063989349 0.079050236 6600
90000 5001 0.040091054 0.087449436 6600
100000 5001 0.020513442 0.065473162 6600
110000 5001 0.026494835 0.08490638 6600
120000 5001 0.021493694 0.03707685 6600
130000 5001 0.0064139961 0.010305947 6600
140000 5001 0.0033736717 0.0050430249 6600
150000 5001 0.0015689132 0.0025060104 6600

It’s difficult to say what’s happening without more complete information. You should be able to drag intruder/tethered particles (doesn’t matter if it’s spherical) in this fashion.

Just remember that the velocity that the spring’s tether position (and hence particle) moves at, from being pulled by a moving spring maintaining a constant force, is subject to a force balance. If the particle’s weight and the weight of the particles above the tethered particle are greater than your constant force, the particle’s not going to move. (Plus in dense packings the darned things have to fight friction to rearrange to allow an intruder to move) If its the same order of magnitude, its going to move slowly.

Why not try some constant velocity and monitor the force? This will also avoid getting trapped in a jammed configuration.