Dimensionless equations, variables, mesh etc

bhh · July 10, 2009, 10:56

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?

Regards

Bjorn

titio · July 10, 2009, 12:33

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.

Regards,

Titio

bhh · July 13, 2009, 00:56

Hi,

How and where may I turn off the dimension checking?

Bjørn

isabel · July 13, 2009, 05:14

I need to turn off the dimension checking. Does anybody know how to do it in OpenFOAM?

henrik · July 13, 2009, 10:22

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

bhh · July 13, 2009, 11:41

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

henrik · July 13, 2009, 12:31

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

sven · August 5, 2009, 13:21

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!

henrik · August 5, 2009, 13:41

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

sven · August 5, 2009, 14:09

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!

henrik · August 5, 2009, 15:01

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

July 10, 2009, 10:56	Dimensionless equations, variables, mesh etc	#1
bhh Member Bjorn H. Hjertager Join Date: Mar 2009 Posts: 69 Rep Power: 4	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? Regards Bjorn

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July 10, 2009, 12:33		#2
titio Senior Member Antonio Martins Join Date: Mar 2009 Location: Porto, Porto, Portugal Posts: 113 Rep Power: 4	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. Regards, Titio

July 13, 2009, 00:56		#3
bhh Member Bjorn H. Hjertager Join Date: Mar 2009 Posts: 69 Rep Power: 4	Hi, How and where may I turn off the dimension checking? Bjørn

July 13, 2009, 05:14		#4
isabel Senior Member isabel Join Date: Apr 2009 Location: Spain Posts: 153 Rep Power: 4	I need to turn off the dimension checking. Does anybody know how to do it in OpenFOAM?

July 13, 2009, 10:22		#5
henrik Senior Member Henrik Rusche Join Date: Mar 2009 Location: Braunschweig, Niedersachsen, Germany Posts: 254 Rep Power: 5	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

July 13, 2009, 11:41		#6
bhh Member Bjorn H. Hjertager Join Date: Mar 2009 Posts: 69 Rep Power: 4	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

July 13, 2009, 12:31		#7
henrik Senior Member Henrik Rusche Join Date: Mar 2009 Location: Braunschweig, Niedersachsen, Germany Posts: 254 Rep Power: 5	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

August 5, 2009, 13:21		#8
sven Member Sven Winkler Join Date: May 2009 Posts: 70 Rep Power: 4	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!

August 5, 2009, 13:41		#9
henrik Senior Member Henrik Rusche Join Date: Mar 2009 Location: Braunschweig, Niedersachsen, Germany Posts: 254 Rep Power: 5	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

August 5, 2009, 14:09		#10
sven Member Sven Winkler Join Date: May 2009 Posts: 70 Rep Power: 4	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!

August 5, 2009, 15:01		#11
henrik Senior Member Henrik Rusche Join Date: Mar 2009 Location: Braunschweig, Niedersachsen, Germany Posts: 254 Rep Power: 5	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