different results Freecad and Solidworks

[Eduardo1]

It's not the fault of CalculiX because I solved your model using another preprocessor for this solver and the results agree with SolidWorks Simulation very well. Be extremely careful when applying heat fluxes (concentrated or surface fluxes) to multiple faces unless you are sure that you know how particular software handles it. In SolidWorks you have the "per item" option so you can be sure that the specified value is applied to each face. Otherwise, it's common in FEA software that the specified value is uniformly distributed on all the selected faces (so that each face is loaded with a fraction of total specified flux). The same applies to nodes within a face - sometimes the specified concentrated heat flux may be applied to each node while sometimes it will be distributed over the face so that each node receives a fraction of the specified flux. I'm not sure how it works in FreeCAD since it's not mentioned in the documentation and there might be some bugs. I will try to take a closer look at this if I have time, some simple tests and examination of the exported input file will be needed to verify how it works here.

Hi NewJoker, Finally I obtained the same results in PrePoMax, I was doing simulation using ''Coupled temperature-displacement step'' when it should be ''Heat transfer step'', but I have the same question, Why Freecad do bad calculations?, in freecad I changed 6 W/m2 instead 3409 W/m2 and the results are similar. even in the video shared I didn't make an error (I hope).

beforehand thanks a lot.

[Eduardo1]

It's not the fault of CalculiX because I solved your model using another preprocessor for this solver and the results agree with SolidWorks Simulation very well. Be extremely careful when applying heat fluxes (concentrated or surface fluxes) to multiple faces unless you are sure that you know how particular software handles it. In SolidWorks you have the "per item" option so you can be sure that the specified value is applied to each face. Otherwise, it's common in FEA software that the specified value is uniformly distributed on all the selected faces (so that each face is loaded with a fraction of total specified flux). The same applies to nodes within a face - sometimes the specified concentrated heat flux may be applied to each node while sometimes it will be distributed over the face so that each node receives a fraction of the specified flux. I'm not sure how it works in FreeCAD since it's not mentioned in the documentation and there might be some bugs. I will try to take a closer look at this if I have time, some simple tests and examination of the exported input file will be needed to verify how it works here.

I found the error but it is weird, in Freecad all the temperatures, heat fluxes are correct but the problem is in "Constraint Initial Temperature", in the simulation the temperature starts 353.15 K , then the "Constraint Initial Temperature" = 353.15 K, and is the same temperature in "ConstraintTemperature", then "ConstraintTemperature"=353.15 K, but the results are illogical, but if I change in constraint temperature to 353.16 K all the simulation is correct. I attach the link to the video.

https://drive.google.com/file/d/1BDT-xw ... sp=sharing

is it a bug right?

[NewJoker]

I was doing simulation using ''Coupled temperature-displacement step'' when it should be ''Heat transfer step''

In FreeCAD the *Coupled temperature-displacement step is used for all thermal and thermo-mechanical analyses, there's no support for the *Heat transfer step as of now.

Interestingly, it seems to be the source of meaningless results in this case. I exported the input file from your FreeCAD FEM model and submitted it twice - once without any changes and once with mechanical properties commented out and step changed to *Heat transfer, steady state. The results are correct only in the latter case:

corr and incorr.png (101.02 KiB) Viewed 1400 times

I don't know yet why it happens but it seems that FreeCAD would benefit from the implementation of pure heat transfer analysis. Also, I will try to take a closer look at this and report the problem on CalculiX forum where it might reach the developer.

I found the error but it is weird, in Freecad all the temperatures, heat fluxes are correct but the problem is in "Constraint Initial Temperature", in the simulation the temperature starts 353.15 K , then the "Constraint Initial Temperature" = 353.15 K, and is the same temperature in "ConstraintTemperature", then "ConstraintTemperature"=353.15 K, but the results are illogical, but if I change in constraint temperature to 353.16 K all the simulation is correct. I attach the link to the video.

https://drive.google.com/file/d/1BDT-xw ... sp=sharing

is it a bug right?

Initial temperature is especially important for transient analyses (this one is steady-state). But still, I haven't noticed that your value is too high. It should be equal to room temperature in most cases (unless the structure is in some hotter or colder environment).

[Kunda1]

@NewJoker would be good to track this on the tracker?

[NewJoker]

@NewJoker would be good to track this on the tracker?

For now, it seems to be a bug in the CalculiX solver itself so not a fault of the FreeCAD code. I will try to dig deeper into it and discuss it on the CalculiX forum. Maybe I missed out on something and it’s not a bug but I’m afraid that it is.

[Kunda1]

For now, it seems to be a bug in the CalculiX solver itself so not a fault of the FreeCAD code. I will try to dig deeper into it and discuss it on the CalculiX forum. Maybe I missed out on something and it’s not a bug but I’m afraid that it is.

Do you mind x-posting the ccx forum url here so we can track it?

[NewJoker]

Do you mind x-posting the ccx forum url here so we can track it?

Sure, here's the thread on CalculiX forum: https://calculix.discourse.group/t/wron ... ansfer/998