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## Re: [ESPResSo-devel] [ESPResSo-users] Simulation of fluid using LB-metho

 From: Stefan Kesselheim Subject: Re: [ESPResSo-devel] [ESPResSo-users] Simulation of fluid using LB-method Date: Wed, 29 Jan 2014 10:59:47 +0100

```Hi everybody,

On Jan 27, 2014, at 6:35 PM, Ulf Schiller <address@hidden> wrote:

> Hi,
>
> can anybody explain the rationale of the following lines to me? I'd have
> expected an exponent of either -1 or 2 for the grid spacing, depending on
> whether ext_force is a force or a force density. In lattice units there is no
> difference.
>
> #ifdef EXTERNAL_FORCES
>    // unit conversion: force density
>      lbfields[index].force = lbpar.ext_force*pow(lbpar.agrid,4)*tau*tau;
>      lbfields[index].force = lbpar.ext_force*pow(lbpar.agrid,4)*tau*tau;
>      lbfields[index].force = lbpar.ext_force*pow(lbpar.agrid,4)*tau*tau;
> #else

I'm also surprised. I however checked that part relatively carefully a couple
of years ago. The unit of the force density is energy/length^4, or
mass*time^2/length. It somehow looks like the two were mixed up in these lines.

We have a test case which is in principle sensitive to that. The pressure
tensor test case checks the diagonal elements of the pressure tensor by
applying a force density normal to a wall (=hydrostatic pressure). In principle
it is possible that several unit conversion problems are present which entirely
cancel out as long as Stokes flow is concerned. This can be possible because
the flow field is linear in this case, and a factor in the force density might
well be compensated for by an inverse factor e.g. in the output of the flow
field. Problems like these are quite difficult to debug, because one would in
principle need a nonlinear flow field case for which reference data is somewhat
difficult to get.

I'll be looking deeper into that in the next couple of days.

Cheers
Stefan

```