How to add viscous and damp?

Submitted by laudari on Sun, 05/02/2021 - 17:17

Hello LIGGGHTS forum, I am trying to run a simulation in an inclined surface. Till now my script is working nicely.
I want to add viscous and damp command in my script.

When ever I added it as ` fix 1 viscous 0.1 ` or `fix 1 damp viscous 0.5 scale 3 2.5` I got following message.
```
ERROR: Could not find fix group ID

```

I also tried to add as `fix m6 viscous...` got the same error.

How do I add them correctly?

Any help would be appreciated.

Thanks a lot.

My script

```

# Initialize

atom_style granular
atom_modify map array
###########################################################################################
#Describe boundary

boundary f f f
newton off
communicate single vel yes
units si

####################################################################################################################
#Declare domain number of materials as domain

region domain block -0.5 0.1 -0.2 0.2 -0.4 0.15 units box
create_box 1 domain

#####################################################################################################################
#Neighbor listing

neighbor 0.001 bin
neigh_modify delay 0

#####################################################################################################################
## Material and interaction properties

fix m1 all property/global youngsModulus peratomtype 5.e6
fix m2 all property/global poissonsRatio peratomtype 0.4
fix m3 all property/global coefficientRestitution peratomtypepair 1 0.6
fix m4 all property/global coefficientFriction peratomtypepair 1 0.4
fix m5 all property/global k_finnie peratomtypepair 1 1.0
fix m6 all property/global coefficientRollingFriction peratomtypepair 1 0.02
#fix m7 viscous 0.1
#######################################################################################################################
#New pair style, define physics

pair_style gran model hertz tangential history #Hertzian without cohesio
pair_coeff * *

#############################################################################################
#Time step

timestep 0.00001

##############################################################################################
# Integrator and gravity

fix integrate all nve/sphere
#fix damp nve_group viscous 0.189
fix gravi all gravity 9.81 vector 0.0 0.0 -1.0

#############################################################################################
#Particle insertion

#fix cad all mesh/surface/stress file meshes/plane.stl type 1 wear finnie
#fix cad all mesh/surface/stress file meshes/simple_chute.stl type 1 wear finnie
fix cad all mesh/surface/stress file meshes/down.stl type 1 wear finnie
fix inface all mesh/surface file meshes/insertion_face.stl type 1
fix granwalls all wall/gran model hertz tangential history mesh n_meshes 1 meshes cad
fix mass all massflow/mesh mesh inface count once point_at_outlet 0. 0. 0.

#######################################################################################################
# Particle properties

fix pts1 all particletemplate/sphere 15485863 atom_type 1 density constant 2500 radius constant 0.01

#Describe the composition of the particle distribution
fix pdd1 all particledistribution/discrete 32452843 1 pts1 1.0

##############################################################################################################

#Define insertion region, velocity for particle

group nve_group region domain
region bc cylinder z 0.0 0.0 0.015 0.05 0.12 units box

#fix 1 all nve velocity all pts1 NULL 0.5 NULL units box

##############################################################################################################

#particle insertion

fix ins nve_group insert/pack seed 32452867 distributiontemplate pdd1 insert_every 1 overlapcheck yes all_in yes particles_in_region 1 region bc

################################################################################################################
compute 1 all erotate/sphere
thermo_style custom step atoms ke c_1 vol
thermo 1000
thermo_modify lost ignore norm no
compute_modify thermo_temp dynamic yes
```

Forums:

Daniel Queteschiner | Mon, 05/03/2021 - 09:54

>>When ever I added it as ` fix 1 viscous 0.1 ` or `fix 1 damp viscous 0.5 scale 3 2.5` I got following message.
>>ERROR: Could not find fix group ID
fix 1 viscous 0.1
As the error message tells you, this command is missing the group-ID (group of particles) that the fix should be applied to (or more precisely LIGGGHTS is looking for a group named 'viscous')
fix 1 damp viscous 0.5 scale 3 2.5
Similarly, in this case LIGGGHTS is looking for a group of atoms named 'damp' but you have not defined such a group in your script, thus LIGGGHTS cannot find it
A working syntax should be
fix 1 all viscous 0.1
which applies the fix to the default group of atoms (named 'all').
Generally, the first argument of a fix command is always the ID of the fix, the 2nd argument is always the group-ID, and the 3rd argument is always the style of the fix (e.g. 'viscous' in this case), the rest of the arguments depend on the fix style.

Also, be aware that this fix is simple modifying the particle force as
fi = fi - const*vi