[snappyHexMesh] Dimension or scale is incorrect

Anis Hanani · June 7, 2022, 01:44

Greetings everyone!

I am currently doing a project that needs me to mesh a 0.4mx0.1mx0.4m model. The stl file I created was correct with the dimensions but after conducting snappyHexMesh, the model was much bigger than intended.

Is there any settings that I may have messed up in snappyHexMeshDict perhaps?

Thank you in advance!

Yann · June 7, 2022, 04:10

Hi Anis,

CAD software usually works with millimeters while OpenFOAM uses meters.
As a result, the STL file often has coordinates written in mm but it is interpreted as m by OpenFOAM, which leads to a geometry being 1000 times larger than intended.

You can fix this in OpenFOAM using the surfaceTransformPoints utility to scale down your STL file by a factor 0.001.
The other way around is to do the scaling in the CAD software, or if possible, to change the units from mm to m when exporting the STL file.

Cheers,
Yann

Anis Hanani · June 8, 2022, 02:20

Dear Yann,

Thank you for the reply.But it seems like the problem here is not the conversion of units.
The lenght of the model in the stl file is 1.46 inches but when meshed using sHM, the length became 1.46 metres.

Is there a way I can make it to be 0.4 metres?

Thank you in advance!

Yann · June 8, 2022, 04:09

Sounds like a units conversion problem to me. Your STL is 1.46 inches but OpenFOAM does not know 1.46 is supposed to be inches. It uses meters by default, this is why you get 1.46m.

You can still use surfaceTransformPoints to fix this by using the proper scaling factor to convert inches to meters.

Cheers,
Yann

June 7, 2022, 01:44	Dimension or scale is incorrect	#1
Anis Hanani New Member Anis Hanani Join Date: May 2022 Posts: 6 Rep Power: 4	Greetings everyone! I am currently doing a project that needs me to mesh a 0.4mx0.1mx0.4m model. The stl file I created was correct with the dimensions but after conducting snappyHexMesh, the model was much bigger than intended. Is there any settings that I may have messed up in snappyHexMeshDict perhaps? Thank you in advance!

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June 7, 2022, 04:10		#2
Yann Senior Member Yann Join Date: Apr 2012 Location: France Posts: 1,208 Rep Power: 28	Hi Anis, CAD software usually works with millimeters while OpenFOAM uses meters. As a result, the STL file often has coordinates written in mm but it is interpreted as m by OpenFOAM, which leads to a geometry being 1000 times larger than intended. You can fix this in OpenFOAM using the surfaceTransformPoints utility to scale down your STL file by a factor 0.001. The other way around is to do the scaling in the CAD software, or if possible, to change the units from mm to m when exporting the STL file. Cheers, Yann

June 8, 2022, 02:20		#3
Anis Hanani New Member Anis Hanani Join Date: May 2022 Posts: 6 Rep Power: 4	Dear Yann, Thank you for the reply.But it seems like the problem here is not the conversion of units. The lenght of the model in the stl file is 1.46 inches but when meshed using sHM, the length became 1.46 metres. Is there a way I can make it to be 0.4 metres? Thank you in advance!

June 8, 2022, 04:09		#4
Yann Senior Member Yann Join Date: Apr 2012 Location: France Posts: 1,208 Rep Power: 28	Sounds like a units conversion problem to me. Your STL is 1.46 inches but OpenFOAM does not know 1.46 is supposed to be inches. It uses meters by default, this is why you get 1.46m. You can still use surfaceTransformPoints to fix this by using the proper scaling factor to convert inches to meters. Cheers, Yann