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Parameters units from 2D to 3D

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Old   November 16, 2015, 17:53
Default Parameters units from 2D to 3D
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fouad abi
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Dear Foamers,
I am working on a 3D case using a modified version of the chtmultiregion solver, and for each region I obviously define equation constants and appropriate properties. My question is, do I have to modify these terms (units wise) when I go from a 2D case to a 3D model? I am quite new with Openfoam, my apologies if this question sounds basic.

Thanks a lot in advance for your help.
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Old   November 18, 2015, 06:24
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Nicole Andrew
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Hi Fauad,

Strictly speaking, OpenFOAM does not know about 2D cases. To run a 2D case you simply make a mesh once cell thick and make the front and back patches "empty".

So there should be no changes when going from 2D to 3D, except in your mesh and boundaries.

I do not understand your question though - what units exactly are you talking about?

I am a novice myself so I apologise if I am misunderstanding you. I hope this helps!
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Old   November 18, 2015, 07:22
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fouad abi
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Hi Nicole,

Thank you for your reply. I was talking about the properties you define in the /constant folder. But I actually figured that it won't affect the results, the calculated values would be "incorrect" unit-wise but would simply have to be interpreted accordingly (for instance reading 5m2/s instead of 5m3/s for a flux).
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Old   November 18, 2015, 07:49
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Nicole Andrew
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Hi Fouad,

As far as I understand it OpenFOAM defines a flux as a surfaceScalarField or the m2/s passing through each face in the direction normal to the face. This is different to a volumetric flow rate (m3/s).

Perhaps you need to explain more about the equations you are using and the properties you are talking about
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Old   November 18, 2015, 08:20
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fouad abi
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Yes but I think you did not get my point, sorry maybe I was not clear enough. Say, in a 2D case, you will still define a pressure term for instance as a volscalarfield, even though you only work with a planar domain, the speed U you will define as a volvectorfield and state 0m/s for the 3rd dimension, etc. So going from 2D to 3D does not imply any change in the definition of the problem and therefore no need to adjust the units of the transportProperties, or EquationConstants etc.
As for the case I'm working on myself, it is a heat transfer case, where I try to compute the profile temperature inside a narrow channel filled with liquid Helium. To do so I need to compute equations where I define constant properties, like heat transfer coefficient for the Kapitza conductance etc. And it is the units of these properties that bothered me initally, hence my question.
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Old   November 18, 2015, 08:40
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Nicole Andrew
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Hi Fouad,

Thanks for clarifying. If you are still concerned, I think you will have to get someone (smarter than me) to look at all of your equations in more detail.
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Old   November 18, 2015, 08:57
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fouad abi
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Hi again,

No worries, so far I think it is fine. Thank you again for your help.
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