Parameters units from 2D to 3D

faab · November 16, 2015, 16:53

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.

Nicole · November 18, 2015, 05:24

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!

faab · November 18, 2015, 06:22

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).

Nicole · November 18, 2015, 06:49

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

faab · November 18, 2015, 07:20

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.

Nicole · November 18, 2015, 07:40

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.

faab · November 18, 2015, 07:57

Hi again,

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

November 16, 2015, 16:53	Parameters units from 2D to 3D	#1
faab Member fouad abi Join Date: Nov 2015 Location: Geneva, Switzerland Posts: 42 Rep Power: 10	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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November 18, 2015, 05:24		#2
Nicole Member Nicole Andrew Join Date: Sep 2014 Location: Pretoria, South Africa Posts: 58 Rep Power: 11	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!

November 18, 2015, 06:22		#3
faab Member fouad abi Join Date: Nov 2015 Location: Geneva, Switzerland Posts: 42 Rep Power: 10	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).

November 18, 2015, 06:49		#4
Nicole Member Nicole Andrew Join Date: Sep 2014 Location: Pretoria, South Africa Posts: 58 Rep Power: 11	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

November 18, 2015, 07:20		#5
faab Member fouad abi Join Date: Nov 2015 Location: Geneva, Switzerland Posts: 42 Rep Power: 10	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.

November 18, 2015, 07:40		#6
Nicole Member Nicole Andrew Join Date: Sep 2014 Location: Pretoria, South Africa Posts: 58 Rep Power: 11	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.

November 18, 2015, 07:57		#7
faab Member fouad abi Join Date: Nov 2015 Location: Geneva, Switzerland Posts: 42 Rep Power: 10	Hi again, No worries, so far I think it is fine. Thank you again for your help.