Dear LAMMPS Users,

I am doing a research project on granular materials and I am using the in.pour file to output information. I am using the information from the dump commands to create the simulation of dumping granular particles into a container in Python.

My questions are the following:

1. when my particles output as an image, some of the particles are overlapping, which makes the simulation not look very realistic. I believe that this is set with pair_style command but when i change the values for the normal repulsion force, my output remains the same. Here is what my pair_style line looks like : pair_style gran/hooke/history 2000.0 NULL 50.0 NULL 0.5 0
What i did was changing the 2000.0 to 2.0e5 and also changing the timestep, but that does not seem to fix the problem.

I also looked at the special_bonds and bond_ style command, but since I am new to LAMMPS Im not sure what I need to do.

Any ideas on what I could do so that my particles don’t overlap?

Here is what my in.pour file looks:

atom_style sphere
boundary p p fm
newton off
communicate single vel yes

region reg block -10 10 -10 10 -0.5 16 units box
create_box 1 reg

neighbor 0.2 bin
neigh_modify delay 0

# IMPORTANT NOTE: these values of stiffness (2000) and timestep (0.001)
# are used in this example file to produce a quick simulation and movie.
# More appropriate values for realistic simulations would be
# k = 2.0e5 and dt = 0.0001, as in bench/in.chute.

pair_style gran/hertz/history 2.0e5 NULL 50.0 NULL 0.5 0
pair_coeff * *

timestep 0.0001

fix 1 all nve/sphere
fix 2 all gravity 1.0 spherical 0.0 -180.0
fix zlower all wall/gran 2000.0 NULL 50.0 NULL 0.5 0 &
    zplane 0.0 2000.0

region slab block -9.0 9.0 -9.0 9.0 10.0 15.0 units box
fix ins all pour 3000 1 300714 vol 0.13 50 region slab

compute 1 all erotate/sphere
#compute 2 all coord/atom 3.0 * *
#compute 2 all property/local patom1 patom2
#compute 2 all pair/local fx fy fz
thermo_style custom step atoms ke c_1 vol
thermo 1000
thermo_modify lost ignore norm no
compute_modify thermo_temp dynamic yes

dump id all custom 10000 dump.pour6 id type x y z radius
#dump 2 all image 1000 image.*.jpg type type &
# axes yes 0.8 0.02 view 80 -30
#dump_modify 2 pad 5

#dump 3 all movie 1000 movie.mpg type type &
# axes yes 0.8 0.02 view 80 -30
#dump_modify 3 pad 5

run 250000

unfix ins
fix 2 all gravity 1.0 chute 26.0
run 250000

Thank you so much for you time,

You could try doing things more scientifically, such as creating a histogram of particle overlaps normalized by the radius. Or better yet, find a way to predict what the static equilibrium overlap should be due to gravity or maximum overlap due to a collision using a characteristic impact velocity (e.g. thermal speed of particles). Hint: these are all standard in the granular community. In the end, spring stiffness relative to other dynamic variables is the only parameter to change overlaps with. You shouldn’t be closer than random close packing (solid vol frac 0.64) if you are pouring particles in.

Or visualize the particles using a smaller radius.

If they are actually overlapping in the simulation,

they would fly apart rapidly, assuming

you have parameterized the granular potential