[Dune] gridcheck.cc : Integral over outer normals is not always zero.

[Aleksejs Fomins]

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Dear Dune,

When running the gridcheck.cc I notice I encounter the following warning:

- -- Checking Intersection Iterator
Warning: Integral over outer normals is not always zero.
         This behaviour may be correct for entities with nonzero curvature.
Warning: Integral over outer normals is not always zero.
         This behaviour may be correct for entities with nonzero curvature.

Could the person who wrote this test please explain what this means.

I assume that this refers to the integral Int(vec{n} dS) over the surface of an element.
If this is indeed the case, then this integral should be zero by divergence theorem.
In particular, could you explain the case in which the integral would not be zero

Greetings,
Aleksejs
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