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Dimensionless equations, variables, mesh etc

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Old   July 10, 2009, 09:56
Default Dimensionless equations, variables, mesh etc
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bhh
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Bjorn H. Hjertager
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Hi,

All my equations, variables and coordinates etc. are dimensionless. OF require dimensions on all variables, coordinates etc.. How can I do my modelling in OF without dimensions?

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Bjorn
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Old   July 10, 2009, 11:33
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Antonio Martins
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bhh,

Even with normalized dimensions, you still have the points relative position. From there you create the mesh. Concerning the variables, it is a question of defining dimensioned scalar, or volVectorField, or equivalent, in the createFields.H fil.

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Titio
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Old   July 12, 2009, 23:56
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Bjorn H. Hjertager
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Hi,

How and where may I turn off the dimension checking?

Bjørn
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Old   July 13, 2009, 04:14
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I need to turn off the dimension checking. Does anybody know how to do it in OpenFOAM?
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Old   July 13, 2009, 09:22
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Henrik Rusche
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Dear All,

google("dimension checking openfoam") yields

http://openfoamwiki.net/index.php/Ma...al_checking.3F

as the first hit - not that I think it's a good idea to switch it off!

Henrik
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Old   July 13, 2009, 10:41
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Bjorn H. Hjertager
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Henrik,

Thanks for the info!

I agree that the dimension checking should generally not be turned off. However, when doing a non-dimensional problem it should be OK, or?

Bjorn
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Old   July 13, 2009, 11:31
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Henrik Rusche
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Dear Bjorn,

Why do you want to implement the equation set in dimensionless-form in the first place? You loose one of the best features of OpenFOAM while you need it most, i.e. while implementing a new set of equations and solution algorithmn.

Without knowing you and your skills, my bet is that you will make mistakes which could have been easily flagged through dimension checking. I cannot tell you how much I hated the void fraction for being dimensionless, but that one cannot be helped ;-)

I would rather read the dimensionless parameters from dictionary and set the properties in the code. Post-processing can do the reverse and everybody is happy.

Henrik
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Old   August 5, 2009, 12:21
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Sven Winkler
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Hey henrik,

where can I set dimensions, when I implement new equations, that means in which file can I set the dimensions of constants that come with the implementation of a new equation? Thank you!
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Old   August 5, 2009, 12:41
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Henrik Rusche
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Dear Sven,

the only dimensions that are not read in from file are those that arise from the discretisation (fvc::* and fvm::* methods). Those can not be changed - only the errors can be switched off.

Not sure this answers your question ...

Henrik
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Old   August 5, 2009, 13:09
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Hey Henrik,

thanks for your answer. If I understand you right, you say that the dimensions of constants (for example Cmu in some turbulence models) are read in from file. I agree with that. But in the files where these constants are defined, there is no information about the dimensions of these constants. Thus, I guess there must be another file in which the dimensions of these constants are set.
I also tried to switch of the dimensions checking of OF. Unfortunately I can not find the controlDict file. According to the user Guide it should be located in $WM_PROJECT_DIR/OpenFOAM-1.5/.OpenFOAM-1.5. But in WM_PROJECT_DIR/OpenFOAM-1.5 there is no hidden folder named .OpenFOAM-1.5.
Thanks for your help!
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Old   August 5, 2009, 14:01
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Henrik Rusche
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Sven,

Cmu, C1, C2, alphaEps are dimensionless.

The guide is outdated: The controlDict is now in $WM_PROJECT_DIR/etc

Please file a bug on this or make a note on the Wiki.

Henrik
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