LAMMPS experts,
I am trying to perform a 2D simulation of granular flow down an inclined plane. The 2D ‘granules’ I used for this simulation are discs of diameter 0.0005m (units si). I begin by defining the region a want to populate with disks via the region command whose spatial extent in the x and y directions are in units of the disc diameter.
lattice sq 0.0005
region hill block 0 50 0 100 0 1 units lattice
create_box 1 hill
create_atoms 1 region hill
set region hill diameter 0.0005 density/disc 2500
These lines of code create and populate a rectangular region in the xy-plane with 5000 0.5mm disks. To create a roughened inclined surface, I froze the atoms situated at the base of this rectangular region (50 disks) and permitted the flow of only the overlying 4,950 disks after tilting the chute at a specified angle via the fix gravity chute command.
My overall goal in this simulation is to determine the diffusion coefficient (over time , timestep 0.0001) of the granular material as it cascades down the inclined surface and I do this by
(a) using the compute msd command for calculating the mean squared displacement. this is the 4th element of the 4-element vector c_msd[4]
(b) accumulating the computed values of c_msd[4] over time via the fix vector command.
© computing the diffusion coefficient via the slope(x) function.
So far I have been successful in computing the diffusion coefficients and the values of the diffusion coefficients I obtained were in 10e-06 meters squared per second (as I expected).
My problem: ALL of the 4,950 atoms flow down (visualized in OVITO) the inclined surface once I tilt the chute at a given angle. In contrast, I expect some residual layer of grains (in addition to the ones I froze at the bottom) to remain in my simulation domain since this is what was obtained in the paper by Steve Plimpton, et. al. Phys. Rev. E 64, 051302-1-051302-14 (2001).
Thank you for taking the time to read my post. Your guidance on this issue would be greatly appreciated.
-Elleesse NP
this is a discussion you should have with your adviser/supervisor as
this is about doing your science correctly and now about how to use
LAMMPS.
axel.
A couple of points that I hope help you out in your quest for diffusion - if that is the goal.
“So far I have been successful in computing the diffusion coefficients and the values of the diffusion coefficients I obtained were in 10e-06 meters squared per second (as I expected).”
I don’t know how you came to that conclusion. This protocol for calculating diffusion makes very little sense to me - namely that a single diffusion coefficient should not exist in this problem (if at all for granular systems - but not important). You’ll also notice that despite reporting a component of an MSD matrix that they do not report a diffusion coefficient.
Granular materials are soft matter, whose temperature, and hence diffusion constant depend intimately on the balance of viscous heating from local shear rate, dissipation from the dashpot interaction, and even heat transfer. Effectively, as a particle moves up and down in the different layers they are sampling different shear rates and hence diffusion constants. From the most basic standpoint think about this, if you expect that some layers of particles will not move - how should they have a diffusion constant?
“(a) using the compute msd command for calculating the mean squared displacement. this is the 4th element of the 4-element vector c_msd[4]”
Why on earth would you do that? (Select the 4th component) Write out the local mean portion of this equation (mean flow rate) - Does this make sense for calculating a diffusion coefficient? Selecting COM isn’t going to do much for you here.
“In contrast, I expect some residual layer of grains (in addition to the ones I froze at the bottom) to remain in my simulation domain since this is what was obtained in the paper by Steve Plimpton, et. al. Phys. Rev. E 64, 051302-1-051302-14 (2001).”
Your guess is as good as anyone’s - you’ll notice that the portion of the regime map where this happens is quite small and given only for very specific set of circumstances. I’d side with Axel here - this is something for you and your adviser to figure out.