[Gmsh] post-processing: how to integrate over physical surface?

[Ingmar Rauschert]

Hi,

I am using GMSH for flow simulation and am interested in the pressure forces at
boundary surfaces. I got pretty familiar with meshing now, but I seem to not
get a good grip onto the post-processing module.

My question: 

I have a scalar (pressure) view of a 3D object and I want to integrate the
pressure over a particular (physical) surface. Now, I know there is the
"integrate" plugin, but I could not find a way how to extract only a particular
boundary surface. The surfaces I deal with are non-planar and quite irregular,
so that cutting out a plane or even trying to parametrize a plane is not
feasible. I tried all kinds of things, none worked out! 

Is there a way to specify (or cut out) a physical surface from an iView just
like it works for a geometry via Tools->Visibility?

I appreaciate any suggestions.

Thanks,
Ingmar