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["B. Emek Abali" <question155482@xxxxxxxxxxxxxxxxxxxxx>]

Question #155482 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/155482

    Status: Open => Answered

B. Emek Abali proposed the following answer:
I started to work on that, sorry about the guess, ALL Function objects
are evaluated with the BasisFunctions so also your vel function needs to
be restricted, so this

bound_mem =  conc('+')*vel('+')[i]*normal('+')[i] +
conc('-')*vel('-')[i]*normal('-')[i]

will work. In the assembly, the iterator builds a list over the elements
and then over the faces, so that every face is listed once. But a face
(or correctly facet for FEniCS jargon) has two sides ('+') for the
element for which you write your form and the neighbour one ('-') the
face normal is outwards n('+') or the neighbour normal n('-') is inwards
to your element for which you should write your form.

so a function a has a continuity over the face if a('+') * n('+') -
a('-') * n('+') = 0 or even the same a('+') * n('+') + a('-') * n('-') =
0

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