particle cross the stl wall frequently

Submitted by asheesh22 on Tue, 05/05/2015 - 18:50

I want to simulate shear mixing in a twin-screw geometry, but I am facing two problem:
1. after particle insertion, they don’t see the STL walls, so they often cross kneading discs, and other time the barrel, which I have not been able to understand why. Doubting the role of smaller time step requirement, I check time step suitability using check/timestep/gran to be sure that time step is not undesirable, changed location of particle introduction to avoid wall interference, but still not able to figure it out.
2. to quickly achieve steady state with the high volume fraction, I wanted to define the particle insertion region using VTK files for the geometry (implemented but commented in my script). If I do so, the simulation stays frozen. I thought maybe it is because the create_atoms requires a lattice definition, the number of particles is too much and it requires much more time, but then leaving for days, it didn’t work neither the error message appears.

Can some someone suggest, what I might be doing wrong?

Forums:

carlesbp | Thu, 05/07/2015 - 09:32

Hi Asheesh,

The Young modulus of your first atom type (9.8e4) may be too low. Try to increase it and see if this is the cause of your problem, I have seen it a few times.

I hope that helped.

Regards,

Carles

ckloss's picture

ckloss | Mon, 06/01/2015 - 11:26

Hi,

>>The Young modulus of your first atom type (9.8e4) may be too low
It definitely is! Minimum for a "better than showcase" simulation in 1e7 Pa

Best wishes
Christoph

asheesh22 | Mon, 05/11/2015 - 14:57

Hi Carles,

Thank you for your help. Indeed, it was due to Young modulus. I was earlier simulating with Young modulus of 1e7 as particles used to cross the wall, so I reduced it hoping that having softer particles of low Young modulus will help, but seems I should have increased. Now when I used 9.8e9 and it goes alright.
I replied late because I wanted to test all cases on my little PC before letting you know.

Only the second question related to the particle insertion in VTK defined insertion region is unsolved now.

Regards,

Ashish

aaaachenjian | Tue, 05/19/2015 - 12:07

I have the same problem with the first example in LIGGGHTS_Tutorial.pdf, bin flow. my code is shown as follows.

#### initialization #

units si
atom_style sphere
atom_modify map array
boundary f f f
newton off
communicate single vel yes
#processors * * 2

region domain block -0.138 0.138 -0.138 0.138 -0.0545 0.43 units box
create_box 2 domain

neighbor 0.003 bin
neigh_modify every 1 check no

# setup #

fix m1 all property/global youngsModulus peratomtype 2.5e6 2.5e6
fix m2 all property/global poissonsRatio peratomtype 0.25 0.25
fix m3 all property/global coefficientRestitution peratomtypepair 2 0.5 0.5 0.5 0.5
fix m4 all property/global coefficientFriction peratomtypepair 2 0.2 0.175 0.175 0.5

fix stopper all wall/gran model hertz/history primitive type 2 zplane 0.0
fix cad all mesh/surface file mesh/hopper.stl type 2 scale 0.001 #move 0. 0. -0.2

fix geometry all wall/gran model hertz/history mesh n_meshes 1 meshes cad #top_box flap_minus flap_plus opening_minus opening_plus staticdrawdown

fix pts all particletemplate/sphere 1 atom_type 1 density constant 1000 radius constant 0.0015
fix pdd all particledistribution/discrete 63243 1 pts 1.0

region factory cylinder z 0 0 0.10 0.4 0.41 units box

fix ins all insert/rate/region seed 123481 distributiontemplate pdd nparticles 150000 particlerate 150000 insert_every 1000 overlapcheck yes vel constant 0. 0. -1.0 region factory ntry_mc 10000

pair_style gran model hertz
pair_coeff * *

fix integrate all nve/sphere
fix grav all gravity 9.81 vector 0.0 0.0 -1.0
timestep 0.00001
thermo_style custom step atoms ke cpu
thermo 10000
thermo_modify norm no lost ignore

fix ctg all check/timestep/gran 1 0.01 0.01
run 1
unfix ctg

dump dumpstl all stl 10000 post/dump*.stl
dump dmp all custom 10000 post/dump*.binflow id type type x y z ix iy iz vx vy vz fx fy fz omegax omegay omegaz radius

run 150000 upto
#unfix stopper
#run 300000

I modified the Young's modulus but it was no use. I also drew a new stl file and it didn't make a difference. Could someone help me with this problem?