# [Getdp] Partial Fluxes

**"Tobias Nähring"**
i_inbox at tn-home.de

*Thu Mar 7 07:57:35 CET 2013*

Dear gmsh/getdp community,
The minimal example test.{geo,pro} demonstrates a problem I have with
the calculation of fluxes through partial surfaces.
The Laplacian equation is solved for the potential v on the unit square
[0,1]^2 with natural boundary conditions at x=0 and x=1, with v=0 on
y=0, and v=1 on y=1.
To get partial fluxes iA and iB through line segments x\in[0,split] and
x\in[split,1] at y=1 I am using two node groups and corresponding
global quantities.
The problem is that I always get the superposition of the boundary
potentials at the intersection point of the intervals, i.e., v=2 at x=split,
y=1.
All attempts to exclude the intersection point from the physical
regions of the partial surfaces failed. (Some of these attempts are indicated by
comments in
the .pro-file.)
If I introduce a small gap between the line segments A and B I get
almost the expected fluxes.
But the introduction of the small gap does not work for the real
application. The target is a 3D domain defined by a STL grid translated to the
gmsh-geo format. The
boundary is defined by surfaces and the partial boundaries are defined
by separation curves. There, I have the same effect.
With best regards,
Tobias Naehring
PS: Thank you for the tool.
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