FreeCAD 0.20.1 vtk result export may have units conversion problem

About the development of the FEM module/workbench.

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HoSoon-Choi
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FreeCAD 0.20.1 vtk result export may have units conversion problem

Post by HoSoon-Choi »

FreeCAD 0.20.1 vtk result export may have units conversion problem.

Next image is displacement result from 0.20.1 vtk result export.
Model size unit is in mm, but displacement unit is in m.
1.png
1.png (93.44 KiB) Viewed 1233 times
Displacement result from calculix vtk converter.
1-1.png
1-1.png (80.31 KiB) Viewed 1233 times
Von Mises stress result from 0.20.1 vtk result export unit is in Pa.
2.png
2.png (109.11 KiB) Viewed 1233 times
Von Mises stress result from calculix vtk converter unit is in MPa.
2-1.png
2-1.png (109.31 KiB) Viewed 1233 times
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bernd
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Re: FreeCAD 0.20.1 vtk result export may have units conversion problem

Post by bernd »

AFAIK There have been changes taken place in this regard to only support SI units in some places of FEM. Thus no matter what units are used in FreeCAD FEM, the units in vtk result will always be SI-units.

@uwestoehr should know about this. He has been the one who has forced this implementation.
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Re: FreeCAD 0.20.1 vtk result export may have units conversion problem

Post by bernd »

another example ... open the Start FEM ccx cantilever example. The really old one. This has vtk results included. ReRun the analysis create a VTK result and see they are different from the ones saved.

IMHO there has been added a regression to force SI units even if there are no SI-units used.
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Re: FreeCAD 0.20.1 vtk result export may have units conversion problem

Post by bernd »

means AFAIKT not the exporter is wrong but the vtk result creator is wrong because the results are wrong (SI-units, not FreeCAD-CCX units) in FC already.
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Re: FreeCAD 0.20.1 vtk result export may have units conversion problem

Post by uwestoehr »

HoSoon-Choi wrote: Sat Sep 24, 2022 2:37 am FreeCAD 0.20.1 vtk result export may have units conversion problem.

Model size unit is in mm, but displacement unit is in m.
This is by design. Since Elmer is designed as multi-physics solver, it only works with a consistent unit system. The ISO-standardized unit system is the SI therefore since FC 0.20 the Elmer analyses use consistently SI units, no matter what equation you use, see also the release announcement:
https://wiki.freecadweb.org/Release_not ... provements
(another nice benefit is that you can share your case.sif file with the Elmer community or take their files are parts of it since they are using SI units too.)

Regarding the display of units, the plan is to make it possible to change the appearance between scientific notation and engineering notation. At the moment only scientific notation is possible.
There is much work on the FEM user interface and this is on my ToDo-list.

Since you are a user of Elmer, checkout this collection of bugfixes on top of FreeCAD 0.20.1:
https://forum.freecadweb.org/viewtopic.php?f=18&t=71058
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