[lammps-users] velocity set for interface study

Dear all,

I’m trying to simulate the displacement between two infinite blocks. Therefor I’m using the command velocity and style set in both blocks in oposite directions with the same velocity module. The problem is that when I plot the center of mass displacement from the lower block, I get a periodic function. Could you please tell me what this command really does? In the way I’m applying it, can it mask any interaction effect as adhesion, cristalline defects or vorticies if they are to appear?
The input is as follows:
"

2d friction simulation

dimension 2
boundary p s p
atom_style atomic
neighbor 0.3 bin
neigh_modify delay 5
units lj

create geometry

lattice hex 0.9
region box block 0 30 1.1 80 -0.25 0.25
create_box 4 box

mass 1 1.0
mass 2 1.0
mass 3 1.0
mass 4 1.0

atom regions

region lo-fixed block INF INF 1.1 3.1 INF INF
region lo-slab block INF INF 3.2 15.9 INF INF
region hi-fixed block INF INF 28 30 INF INF
region hi-slab block INF INF 16 27.9 INF INF

create 2 surfaces with asperities

create_atoms 1 region lo-slab
create_atoms 2 region hi-slab
create_atoms 3 region lo-fixed
create_atoms 4 region hi-fixed

LJ potentials

pair_style lj/cut 2.5
pair_coeff * * 1.0 1.0 2.5

define groups

group lo region lo-slab
group lo type 1
group hi region hi-slab
group hi type 2
group lo-fixed region lo-fixed
group hi-fixed region hi-fixed
group mobile union lo hi
group boundary union lo-fixed hi-fixed
group upper-group union hi-fixed hi
group lower-group union lo-fixed lo
set group lo-fixed type 3
set group hi-fixed type 4

initial velocities

compute temperatura boundary temp/partial 0 1 0
compute temperatura2 mobile temp/partial 0 1 0

compute 1 lo com

velocity all create 0.033 482748 temp temperatura

#Upper-group displacement

displace_atoms upper-group move 0 20.0 0 units box

#Fixes

fix 2 all nve

#Minimize

minimize 0.000001 0.001 1826000 1826000

run details

thermo 5000
timestep 0.0001
run_style verlet

dump 1 all atom 500 dump.p01
run 1826000

#move atoms back
displace_atoms upper-group move 0 -20.0 0 units box

write_restart restart.p01

run 20000

#main simulation

variable Temperatura atom “mass*(vy^2)/4”

unfix 2

fix 3 boundary nvt 0.033 0.033 0.75
fix_modify 3 temp temperatura
fix 4 mobile nve

fix 5 all ave/spatial 1 1 1000 y 0.0 0.6 v_Temperatura x y file temp.p01

#velocity command

velocity hi set 0.1095 0 0 sum no
velocity lo set -0.1095 0 0 sum no

thermo 5000
thermo_modify temp temperatura2
thermo_style custom step temp etotal c_1[1] c_1[2] c_1[3]

undump 1
dump 2 all atom 1000 dump.p01_a
dump 3 all xyz 1000 dump.p01..xyz
dump 4 all custom 1000 dump.p01_c.
id type x y vx vy fx fy

timestep 0.0001
run 300000
"

Best regards,
Débora M M Marques

The velocity set command does something very simple.
It sets the velocity of the specified atoms to the value
you tell it. What your model does from that point on,
is up to you. If you have a group of atoms moving
thru a periodic boundary, then their center-of-mass
will move thru the boundary as well, unless you compute
it in some fashion that accounts for the fact it has
passed thru the box several times.

Steve

2010/3/30 Débora Marques <[email protected]>: