how to generate a very loose packing

Submitted by ma on Sun, 05/10/2015 - 08:19

Hi
I am trying to generate a very loose packing by radius expansion method. I pack smaller particles randomly and then assign a friction coefficient (i.e 0.24 in my case) and allow the particles to grow up to a desired radius. consequently, the consolidation procedure is modeled. The method does not result in a very loose pack and the void ratio does not differ significantly from that in a specimen with a friction coefficient of 0.0. the code is as follows

##############################################################################################
#Material properties required for new pair styles
fix m1 all property/global youngsModulus peratomtype 1.e8 1.e8 1.e8
fix m2 all property/global poissonsRatio peratomtype 0.22 0.22 0.22
fix m3 all property/global coefficientFriction peratomtypepair 3 0.24 0.0 0.9 0.0 0.0 0.0 0.9 0.0 0.0
fix m4 all property/global coefficientRestitution peratomtypepair 3 0.87 0.87 0.87 0.87 0.87 0.87 0.87 0.87 0.87

pair_style gran/hertz/history
pair_coeff * *

#distributions for insertion/ Desired final particle radius is 0.00054
fix pts1 all particletemplate/sphere 1 atom_type 1 density constant 2650 radius constant 0.00035
fix pdd1 all particledistribution/discrete 1. 1 pts1 1.0

#parameters for gradually growing particle diameter
variable alphastart equal 0.4
variable alphatarget equal 0.58
variable growts equal 100000
variable growevery equal 10000

#particle insertion
fix ins nve_group insert/pack seed 5330 distributiontemplate pdd1 &
maxattempt 10000 insert_every once overlapcheck yes all_in yes vel constant 0. 0. 0. &
region bc particles_in_region 65000

#calculate grow rate
variable Rgrowrate equal (${alphatarget}/${alphastart})^(${growevery}/(3.*${growts}))
print "The radius grow rate is ${Rgrowrate}"

#do the diameter grow
compute rad all property/atom radius
variable dgrown atom ${Rgrowrate}*2.*c_rad
fix grow all adapt ${growevery} atom diameter v_dgrown

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

So:
1- how can I model a loose packing by radius expansion method (I cannot increase the friction coeffficient)?
2- As I need a uniform loose specimen, is there any way other than radius expansion? pluviation method generates a looser packing but the void ratio distribution is not uniform.

Thank you in advance
mojgan

Forums:

JoshuaP | Thu, 05/14/2015 - 01:38

Hi,

I would try to use the rolling resistance model epsd2, otherwise particles have excessive rolling behaviour. To get a very loose packing you need to use fill by mass, and fill it in a bit higher region than you want and let it settle.

regards
Joshua

ma | Sat, 05/23/2015 - 09:46

Thank you for your reply,
I have modeled in this way but the packing is not uniform and the void ratio distributes nonuniformly.
best regards
ma

ARPITA RAY | Sat, 01/13/2024 - 13:36

Hi, I am trying to model an undrained triaxial Constant Volume test using LIGGGHTS. I am trying to generate the sample by randomly placing particles in a larger space and compressing them to a smaller size (equal to that of the required specimen). My particles are monosized of the radius 2 mm, and are cohesionless. I am using a high interparticle friction (value 10.0) while insertion and also performing an overlap check. But, I cannot generate a loose sample (I check the porosity of the sample by simply negating the volume of the particles from the total volume). Hence, I would be obliged to get a little help in generating a loose sample in this context.
Thankyou in advance.
Arpita

blueguy | Mon, 03/25/2024 - 13:34

Recent findings indicate that utilizing spherical particles does not yield the desired outcomes in producing loose samples. Therefore, it is strongly recommended to employ particles with irregular shapes, such as superquadrics or multi-sphere configurations, to achieve better results.

For further information and detailed insights on this subject, please refer to the following reference, which has been identified as particularly useful.

Best regards,

https://ascelibrary.org/doi/10.1061/%28ASCE%29GT.1943-5606.0001181