Re: [ESPResSo-users] Lattice Boltzmann boundaries in Espresso 3.1

[Owen Hickey]

Hi Wolfgang,

We modified the implementation of the LB nodes in order to allow for the use of verlet lists together with the LB fluid.  This means that LB nodes are now at multiples of half of the agrid.  Thus if agrid is 1 LB nodes are at 0.5, 1.5, 2.5, etc.  The boundaries are inserted using a bounce back scheme.  This is usually thought of as creating a hydrodynamic barrier which is half way between the bounce-back node and the free fluid node.  Thus if node 0.5 is a bounce-back node and 1.5 is a regular fluid node the hydrodynamic boundary is, to first order, at 1.

In your specific case the wall at 32 should make the node at 32.5 a bounce-back node and thus have velocity zero.  The boundary wall at 0.0 doesn't actually do anything but the node at 39.5 is a bounce-back node because all nodes greater than 32 are bounce back nodes due to the other boundary.  You should get reasonable results putting walls at 1 and 32 instead, creating 0.5 being a bounce-back node and all nodes above 32 being bounce-back too (e.g. 32.5 and up).  I hope that is at least clearer than the documentation.

Owen

p.s. you might notice a slight change in how the velocity profile looks as it now interpolates linearly between the fluid node and the bounce-back node.

On Tue, Mar 13, 2012 at 2:14 PM, Wolfgang Riefler <address@hidden> wrote:

Hi All,

I have recently switched from Espresso 3.0 to 3.1 (Congrats to the new version by the way).

To test if everything is going well, I wanted to simulate a simple Poiseuille flow between two walls, thus defining

setmd box_l 40 40 40
lbboundary wall dist 0.0 normal 0.0 1.0 0.0
lbboundary wall dist -32.0 normal 0.0 -1.0 0.0

which should give me two plane walls in the x-z plane at y=0 and y=32.

According to the Espresso manual (and simulations done with Espresso 3.0), it will result in two effective boundaries at y~0.5 and y~31.5.

I set the fluid to

lbfluid agrid 1.0 dens 1.0 visc 3.0 tau 0.001 ext_force 0.001 0.0 0.0 friction 20.0
thermostat lb 0.0

and start integrating until the solution has converged well enough.

Surprisingly I get effective boundaries at y~-0.5 and y~31.5, which means that y=0 is no boundary node.

I have read in the NEWS file that the boundaries have been worked on for Espresso 3.1, so am I missing something?

Thank you very much in advance,

best,

Wolfgang