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what does the empty boundary condition mean?

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Old   May 30, 2017, 17:39
Default what does the empty boundary condition mean?
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Hi all
we are preparing a finite volume code to simulate flow over a airfoil with FORTRAN. we have a mesh which is extruded one layer (Nz=1). we need to impose EMPTY boundary condition in two faces in Z direction. but we don't know how to apply this. in other word, we don't know what are the velocity and pressure values on the boundaries nodes (what is the meaning of the EMPTY boundary condition???)
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Old   May 31, 2017, 01:27
Default it does nothing
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Svetlana Tkachenko
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It does nothing.

It is specifically designed for cases with reduced dimensions because OpenFOAM has no other way to reduce dimensions.

Quoting henrik from here,

«OpenFOAM does not have special machinery do deal with 2D domains. Therefore, a 2D mesh has twice as many faces on the front and back as it has cells.

Therefore it is a good idea to do nothing for those faces and that is what "empty" does.»

To quote OpenFOAM documentation from here,

«This boundary condition provides an ’empty’ condition for reduced dimensions cases, i.e. 1- and 2-D geometries. Apply this condition to patches whose normal is aligned to geometric directions that do not constitue solution directions.»

If you're writing your own code, simply make sure your code avoids calculating the dimensions which you do not need. That would be equivalent to the empty boundary condition provided in OpenFOAM.

If you must calculate the third dimension, impose a symmetry boundary condition.
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Old   June 1, 2017, 07:43
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Dear Светлана

Thanks a lot for your consideration.

what I found from your answer, if I remove all of the equations in Z direction in entire code, we impose EMPTY b.c yes? or we should remove equation in Z direction just in front and back faces (the boundary faces)?
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empty boundaries, fortran 90

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