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-   -   Do I need to change mechnical properties after using transformpoint scale utility? (http://www.cfd-online.com/Forums/openfoam/108808-do-i-need-change-mechnical-properties-after-using-transformpoint-scale-utility.html)

Sargam05 November 1, 2012 23:31

Do I need to change mechnical properties after using transformpoint scale utility?
 
Hello everyone,

I am solving a case for 3D cube geometry. First, I have created cube mesh for 1mX1mX1m dimension then transferred the geometry to smaller scale (0.00005, 0.00005, 0.00005) by using transformPoint scale utility. I am using mechanical and thermal properties (e.g. Youngs modulus, conductivity, specific heat etc.) for the case.

Do I need to change the dimensions of mechanical and thermal properties after reducing mesh size.

For example: E = 210 X 10^9 N/m^2
Since this value is in "m", if I change cube dimensions in "mm" should it be
E = 210 X 10^3 N/mm^2 ?

Does anyone have any idea about it?

Best regards,
Sangeeta

tomf November 2, 2012 03:47

Hello Sangeeta,

OpenFOAM will use meters for all it's computations (if SI units are specified in $HOME/OpenFOAM/OpenFOAM-2.1.x/etc/controlDict), so all your material properties have to be specified in the usual SI unit. So I would expect you just keep E = 210 X 10^9 N/m^2, regardless of the size of your problem.

Regards,
Tom

Sargam05 November 2, 2012 12:41

Quote:

Originally Posted by tomf (Post 389802)
Hello Sangeeta,

OpenFOAM will use meters for all it's computations (if SI units are specified in $HOME/OpenFOAM/OpenFOAM-2.1.x/etc/controlDict), so all your material properties have to be specified in the usual SI unit. So I would expect you just keep E = 210 X 10^9 N/m^2, regardless of the size of your problem.

Regards,
Tom



Hi Tom,

Thank you for the reply. Do you think that it will not going to affect final results if I do not change mechanical and thermal properties in mm?

Best regards,
Sangeeta

tomf November 3, 2012 07:19

Quote:

Originally Posted by Sargam05 (Post 389921)
Hi Tom,

Thank you for the reply. Do you think that it will not going to affect final results if I do not change mechanical and thermal properties in mm?

Best regards,
Sangeeta

Hi Sangeeta,

This section in the userguide basically explains the role of dimensions:

http://www.openfoam.org/docs/user/ba...hp#dx18-100002

So if you plan to use SI units, use default SI units for properties i.e. kg, m, s, K, kgmol, A, cd or combinations thereof. For your problem make sure that the dimensions are specified in meters as well (0.005 for 5 mm). OpenFOAM will not know about millimeters, it only knows meters.

Regards,
Tom

Sargam05 November 8, 2012 12:35

Hi Tom,
Thank you for the reply. I appreciate it!

Sangeeta


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