[SOLVED] Elmer Two-body heat transferrence

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kamocat
Posts: 17
Joined: Mon Jul 11, 2022 10:39 pm

[SOLVED] Elmer Two-body heat transferrence

Post by kamocat »

Hi folks,
I've been thinking I have a problem with multi-material thermal simulations. But recently I've been getting weird results with just two bodies of the same material.
Here I have two 10mm cubes of FR4. Applied to the right side is a 1mW heat source. The left side dissipates heat at 10W/k/m^2 into an ambient room of 300K. I was expecting an even heat gradient from the right to the left, but instead I get heating in the lower center and cooling in the upper right?!
two bodies 2022-07-14_13-51.png
two bodies 2022-07-14_13-51.png (34.04 KiB) Viewed 873 times
In contrast, Calculix says both bodies are at 301K (which makes sense, since the left face will dissipate 1mW for every degree above 300K)
What am I doing wrong??

I'm using FreeCAD 0.21 due to a few bugs in the Elmer plugin.

Code: Select all

OS: Arch Linux (Budgie:GNOME/Budgie Desktop)
Word size of FreeCAD: 64-bit
Version: 0.21.29529 (Git)
Build type: Unknown
Branch: makepkg
Hash: 1bb9699c9a174acfdf3e854a953913b159f3e8ef
Python 3.10.5, Qt 5.15.5, Coin 4.0.1, Vtk 9.1.0, OCC 7.5.3
Locale: English/United States (en_US)
Installed mods: 
  * Manipulator 1.4.9
  * kicadStepUpMod 10.13.3
Attachments
two_material_fem.FCStd
(182.31 KiB) Downloaded 21 times
Last edited by kamocat on Tue Aug 02, 2022 5:54 pm, edited 1 time in total.
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uwestoehr
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Re: Elmer Two-body heat transferrence

Post by uwestoehr »

kamocat wrote: Thu Jul 14, 2022 8:58 pm I've been thinking I have a problem with multi-material thermal simulations. But recently I've been getting weird results with just two bodies of the same material.
I miss in your setup the definition of the initial temperature of the two bodies. It seems that the initial constraint can only be applied to one body and there is no way to select which one. This is then a bug in FC in my opinion.
To overcome this, make the 2 parts a Boolean Fragment, like in the bimetal example that is part of FreeCAD.

Here is your file where I did this:
two_material_fem.FCStd
(303.46 KiB) Downloaded 22 times
Note however that the solver complains about:

Code: Select all

WARNING:: ListFind:  Requested property: [Heat Conductivity], not found
WARNING:: ListFind:  Requested property: [Density], not found
So there seems to be a bug in the writing of the Elmer input file.

In general, why do you have 2 parts with exactly the same properties? You can then also just have one big block.

kamocat wrote: Thu Jul 14, 2022 8:58 pm I'm using FreeCAD 0.21 due to a few bugs in the Elmer plugin.
Cool. Can you please give the latest FreeCAD master or the next weekly build a try? This comes with multi-threading support for Elmer and needs to be tested:
https://forum.freecadweb.org/viewtopic.php?f=18&t=70307
kamocat
Posts: 17
Joined: Mon Jul 11, 2022 10:39 pm

Re: Elmer Two-body heat transferrence

Post by kamocat »

uwestoehr wrote: Sat Jul 16, 2022 1:56 pm In general, why do you have 2 parts with exactly the same properties? You can then also just have one big block.
I initially had two materials, but seeing as two bodies with the same material had the same result, I thought it was a simpler example.
uwestoehr wrote: Sat Jul 16, 2022 1:56 pm Cool. Can you please give the latest FreeCAD master or the next weekly build a try? This comes with multi-threading support for Elmer and needs to be tested:
https://forum.freecadweb.org/viewtopic.php?f=18&t=70307
Yes, I just saw that. I'll give it a try.
kamocat
Posts: 17
Joined: Mon Jul 11, 2022 10:39 pm

Re: Elmer Two-body heat transferrence

Post by kamocat »

Yes, the boolean fragments fixed the issue, thank you.
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