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Re: [ESPResSo-users] lbboundary

From: Stefan Kesselheim
Subject: Re: [ESPResSo-users] lbboundary
Date: Thu, 4 Dec 2014 20:25:35 +0100

Hi Roya,

On Dec 4, 2014, at 8:04 PM, roya moghaddasi <address@hidden> wrote:

> Hi Espresso users,
> I want to use a cylinder boundary for my lbfluid. I want to ask some 
> questions:
> 1. can I use penetrable or direction or reflecting for this boundary?
> 2. If I use "reflecting 2", will the boundary condition be bounce-back 
> condition which is nice for a lbboundary? What is your suggestion?

Both options exist only for constraints and not for lbboundaries. 

> 3. Espresso manual says "This has important implications for the location of 
> hydrodynamic boundaries which are generally considered to be halfway between 
> two nodes to first order". my main cylinder which contains all the particles 
> has a radius of 25 and I want my lbboundary to have the same radius. Do you 
> see any contradictions of this and the Espresso manual statement?

lbboundaries use the "link bounce back" method, that is described a lot in the 
LB literature. If you create an lbboundary cylinder with a certain radius 
around a certain point all nodes inside this cylinder will be flagged "wall". 
If the walls were plane, The the hydrodynamic boundary appears halfway between 
the outermost "fluid" and the innermost "wall" node.  The lbboundary node 
positions are half-integer, i.e. 0.5, 1.5, 2.5 …
Thus, if you make e.g. a cylinder with radius 10 around position x=15, node 
x=4.5 will be "wall", and node x=5.5 will be  "fluid". The hydrodynamic 
boundary will be at 5, thus you get what you expect. You should, however, check 
the results i.e. by applying a pressure gradient against the analytical 
solution. From you other mail, I see you might be using a viscosity that is not 
suited for using the link bounce back method because it is too high. 

Note that in most soft matter applications, the absolute value of the viscosity 
does not matter anyways, because the fluid is described by the linear Stokes 
equation and changing the viscosity only changes the absolute timescale, but 
that can be corrected for at the end.

Cheers and good luck

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