Fem: Load heat flux from CalculiX results - fixes #12117

[marioalexis84]

Fixes #12117.
The heat flux values are saved in the HeatFlux property of the FemResult object.
The visualization is accesible from the pipelines.

@FEA-eng

[FEA-eng]

Thanks, it works as expected. But it seems that one test needs to be updated:

AssertionError: 'Comparing /usr/local/Mod/Fem/femtest/data/calculix/thermomech_bimetall.inp to /tmp/FEM_unittests/ccxtools_thermomech_bimetall_13ac396c1aca/Mesh.inp failed!\n@@ -7114 +7114 @@\n-S, E\n+S, E, HFL\n' is not false : ccxtools write_inp_file test failed.

[marioalexis84]

Note that heat flux unit is $\frac{kW}{m^2}$.

[FEA-eng]

Note that heat flux unit is kW/m2.

With the input deck always in SI(mm) I would expect CalculiX to output it in mW/mm^2 and FreeCAD to convert it to SI(m) so W/m^2. Does it have to use kW instead of W?

[marioalexis84]

There is no coversion. Internally only displacements are converted from mm to m (the lengths are assumed to be in mm).
But this hardcoded in the function that load the data for the pipeline.
We need to fix this and and units issues in fem in general.

[FEA-eng]

There is no coversion. Internally only displacements are converted from mm to m (the lengths are assumed to be in mm). But this hardcoded in the function that load the data for the pipeline. We need to fix this and and units issues in fem in general.

I don't know how the result handling works internally but the effect is that all outputs in FreeCAD FEM are in SI(m) so displacements are in m, stresses in Pa and so on. This is definitely annoying in mechanical analyses and was introduced only to fix the issues with electromagnetic ones. It would be best to offer an option to change the units in results but using the SI(mm) system by default with a proper handling of all quantities should be possible as well. Currently, it's also confusing that you enter the data in mm and get the results in m without even an annotation next to the legend showing the quantity and unit (such labels themselves would greatly improve user-friendliness).

[marioalexis84]

so displacements are in m, stresses in Pa and so on

Actually only displacement and stresses are rescaled.
By saying that "there is no conversion", I meant that only these variables are rescaled, using a hardcoded factor. Other variables in the .frd file will maintain the unit used in the .inp file.

See:

FreeCAD/src/Mod/Fem/App/FemVTKTools.cpp

Lines 944 to 964 in b72a8c4

	if ((scalar.first.compare("MaxShear") == 0)
	|| (scalar.first.compare("NodeStressXX") == 0)
	|| (scalar.first.compare("NodeStressXY") == 0)
	|| (scalar.first.compare("NodeStressXZ") == 0)
	|| (scalar.first.compare("NodeStressYY") == 0)
	|| (scalar.first.compare("NodeStressYZ") == 0)
	|| (scalar.first.compare("NodeStressZZ") == 0)
	|| (scalar.first.compare("PrincipalMax") == 0)
	|| (scalar.first.compare("PrincipalMed") == 0)
	|| (scalar.first.compare("PrincipalMin") == 0)
	|| (scalar.first.compare("vonMises") == 0)
	|| (scalar.first.compare("NetworkPressure") == 0)) {
	factor = 1e6; // to get Pascal
	}
	else if (scalar.first.compare("DisplacementLengths") == 0) {
	factor = 0.001; // to get meter
	}
	else {
	factor = 1.0;
	}

[FEA-eng]

@marioalexis84 Ok, I see. Then it's even more confusing for the user than I thought. The unit display would definitely improve the situation. I would expect the heat flux to be in W/m^2 but it's actually in mW/mm^2 = kW/m^2.