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[ESPResSo-users] Fence-like density distribution MD+LB

From: Aleksei Kabedev
Subject: [ESPResSo-users] Fence-like density distribution MD+LB
Date: Tue, 10 Mar 2015 14:24:43 +0000

Dear all,

I observe some strange behaviour of MD particles, dissolved in the fluid, modelled by LB.
1)What I do is
- create a box with periodicity 1 1 0 with boundaries for both md and lb
- put some particles into it
- turn on the lb thermostat
- write down the positions of the particles (that don't interact to each other directly, just hydrodynamically) along the z-axis
- make a histogram of these positions, which is basically a density distribution function.
2) What I expect to see is
- equal distribution in the middle of the channel, drastic declines by the sides
- flatness of surface in the middle is getting higher upon the growth of statistics amount.

Instead of that I see fence-like curves with "top edges" exactly at the positions of cell edges (or exactly in the middle between them). That works even if I don't use any external flow. After I changed the agrid parameter from 1.0 to 2.0 it seemed to disappear. If I do exactly the same simulation with langevin thermostat, I see a flat surface without any "hills" on that. I am just wondering if someone knows why it happens? Thank you in advance!
See the plot examples and a simple code attached.

Actually, there is another question I should ask - what is going to happen if I simulate the particles inside the channel and the boundaries for fluid and particles are not exactly at the same place? Say, lbboundary is 0.5*agrid further from the particles, than md-constraint or vice versa? Thank you!


Kind regards,
Aleksei Kabedev

Attachment: dens_distr_agrid_1.eps
Description: PostScript document

Attachment: dens_distr_agrid_2.eps
Description: PostScript document

Attachment: Fence_test_LB.tcl
Description: Tcl script

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