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Re: [ESPResSo-users] Proper use of command lbboundary

From: Stefan Kesselheim
Subject: Re: [ESPResSo-users] Proper use of command lbboundary
Date: Tue, 6 Mar 2012 10:29:51 +0100

Hi Salvador,

Am 05.03.2012 um 23:09 schrieb Salvador H-V:

> To constraint the spheres (diam = 1), I am using the following parameters:
> set blz_wall [expr {2.0 * $diam}]
> set wall1 [expr {$blz*0.5 - 1.5} ]
> constraint wall  normal 0 0 1  dist $wall1 type 1
> set wall2 [expr {-1.0*($wall1 + $blz_wall) } ]
> constraint wall  normal 0 0 -1 dist $wall1 type 1

There is probably a mistake in you last line, it should be $wall2 instead of 

Are you explicitly setting periodic 1 1 0?

Then particles can eventually leak to z > box_l, where there is no lb fluid any 
more. Then however depending on your espresso version, you should see the error 
message from map_position_to_lattice in lattice.h. 

But we did changes on the lattice mapping, so if you are using the current git 
version, it might crash instead of giving this error message.

> I was wondering if it is possible to add fluid constraints/boundaries at the 
> same position/coodinates that those of the wall constraints or should exist a 
> gap between them? Is there any restriction in the distance-separation between 
> the boundaries for the fluid?

In principle you can set constraint and lbboundarys independent of each other. 
You should not however, that we are using the link bounce back method for 
walls. These walls appear mid-grid (plus corrections for relaxation time). In 
all older releases, the lattice points sit at 0*a, 1*a, 2*a, ..., while in the 
current git master the lattice is shifted to sit at 0.5*a, 1.5*a, ...

Thus the location of the effective boundary condition for the LB depends on the 
espresso version :-).

If this does not fix your problem, please send us more information on your 
espresso version and a larger part of you script.

Cheers and good luck

Stefan Kesselheim
Institute for Computational Physics
University of Stuttgart

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