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## Re: [ESPResSo-users] lbboundary

 From: roya moghaddasi Subject: Re: [ESPResSo-users] lbboundary Date: Fri, 5 Dec 2014 05:41:37 +0000 (UTC)

Dear Stefan,

Thank you for your time and consideration.
I changed my parameters to this:
lbfluid dens 0.362  visc 3.8  agrid 1.0  tau 0.05
lbfluid friction 14

What is the relationship between viscosity and absolute timescale?
Best,
Roya

On Thursday, 4 December 2014, 22:55, Stefan Kesselheim <address@hidden> wrote:

Hi Roya,

> 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
Stefan