wiggling a primitive wall

Submitted by benassi.a on Fri, 01/29/2016 - 16:04

Dear users and developers,
I am trying to familiarize with Liggghts starting from some simple example. I started filling a small cylinder with a mono-disperse cohesive powder. In the attached screenshot you should notice that the rest configuration of the particles after pouring is pretty odd. I see squared powder aggregates that resemble the processor grid, i.e. a 3x4x1 mesh. I thought this might come from the fact that the particle generation is independent for each core and, together with the strong adhesion force, the new particles are immediately attracted close to each other in the very first instants after the insertion. TO verify this hypothesis I run a calculation with no cohesion between particles and indeed the squared aggregates disappear. So far so good, now to simulate certain rheology experiments of mine I have to live with a strong adhesion, thus I thought to apply a small wiggle to the cylinder bottom to homogenize a little bit the powder during pouring (see attached input file) and here comes the problem: according to the on-line manual for the wall/gran fix it is not possible to apply a wiggle to a primitive wall, only a shear. I am a little used to LAMMPS so I tried to use the wiggle keyword in the LAMMPS way but I got a parsing error message:

ERROR: Fix wall/gran (id zwalls1): Unknown argument or wrong keyword order: 'wiggle' (../fix_wall_gran_base.h:118)

So how can I apply a wiggle to a primitive flat wall?
Thanks in advance for your help.

Image icon screenshot.png138.3 KB
Plain text icon in.cohesion_shake.txt2.21 KB
ckloss's picture

ckloss | Mon, 02/08/2016 - 14:48

Hi Andrea,

the "odd" particle configuration comes from the way LIGGGHTS inserts particles, they are not placed along processor boundaries. THis allows the insertion to scale much better compared to LAMMPS. You can't wiggle primitive walls, I'd suggest you use a mesh wall. You can impose any kind of motion to a mesh wall, see fix move/mesh

Best wishes

jagan1mohan | Mon, 06/08/2020 - 20:50

Hello Dr. Kloss, Greetings. I hope, you are doing good. I'm working on homogeneous simple shear simulation using the following two methods.

In first method, I'm taking a cube of side 7.5 and filling it with 483 particles each of diameter 1 and density 1000 so that packing / volume fraction is 0.6. There is only 1 atom_type and all six faces are periodic. Particle-particle friction is 0.1 and there is no gravity for time being. I apply fix deform xy with a known value of dimensioned shear rate, say gamaDOT = 1/sec, so that the top plane (ymax plane) moves along positive x-direction with a velocity of 7.5. Script writes out VTK files and I observe that the simulation box / region tilts correctly and at steady state, using spatial averaging I compute x - velocity gradient which closely matches with applied shear rate. This I have tried with low as well as high shear rates and also for different friction values. It works well.

In second method, I want to mimic same phenomenon by moving top and bottom walls in opposite directions (and hence not tilting box and not using fix deform). Rest all settings are same. I first used fix wall/gran on top (ymax) and bottom (ymin) planes to create walls of same atom_type and used shear keyword along with +3.75 and -3.75 units as velocity, respectively. The particles, close to the wall move very very slowly with velocity several orders of magnitude less. This could be due to the less friction between the wall and the particles. In next attempt, I created walls of particles of same diameter, density in a staggered arrangement such that the particles just touch each other. I'm using fix move on these two walls and I observe that a bit more momentum is transferred as compared to fix wall/gran but falling short by 40%.

1. If I use only fix wall/gran and declare walls as type 2, then what should be the wall-particle friction value? Perhaps, LIGGGHTS can take any value and can this value be as high as possible?

2. In particle wall method, I'm not integrating the wall particles using nve/sphere as I'm already moving them with fixed velocity. How can I improve these setups to achieve the correct shear rate?

Please let me know if you have any additional questions. I can share the scripts if you want more details.

Thank you,
Jagan Mohan