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

[Ulf Schiller]

Hi,

I don't intend to create any confusion here, but the fact that the 
lattice nodes now lie at half integer positions means that any boundary 
aligned with the main lattice directions has effectively an integer 
position. So the implementation should now be able to generate stick 
boundary conditions at positions 0.0 and 32.0 in Wolfgang's example. The 
shift of the lattice nodes has the rather nice feature that the 
half-grid offset of boundaries can be hidden under the hood and, in my 
opinion, the lbboundary command should produce boundaries at integer 
positions as specified by the user.

Interpolating the velocity field between fluid and wall nodes does not 
make sense, since the velocity at a bounce-back node is ill-defined. To 
linear order, it could be extrapolated to -u where u is the velocity of 
the fluid node closest to the wall. This would generate a smooth 
extrapolation for Couette flow but not for Poiseuille. The location of 
the boundary at half-grid spacing holds for both though.

So I'd suggest to revisit the implementation such that in the example 
the wall nodes lie at y=-0.5 and y=32.5, and the zero-velocity is 
located effectively at y=0.0 and y=32.0. If the user specifies a 
non-integer position, there is a choice to put the wall location to 
ceil() or floor(), possibly depending on the orientation of the wall. In 
this case, multi-reflection BCs would be better anyway.

Hope this helps and best wishes,
Ulf

On 03/13/2012 04:36 PM, Owen Hickey wrote:
> 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 <mailto: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

--
Dr. Ulf D. Schiller                        Building 04.16, Room 3006
Institute of Complex Systems (ICS-2)       Phone:   +49 2461 61-6144
Forschungszentrum Jülich, Germany          Fax:     +49 2461 61-3180

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