Questions about my Helmholtz coil simulation.

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grindhold
Posts: 2
Joined: 14 Sep 2022, 12:10
Antispam: Yes

Questions about my Helmholtz coil simulation.

Post by grindhold »

Hey there, elmer-community!

I have been playing around with elmerfem with the plan to simulate the magnetic field of a helmholtz coil and the goal of being able to tell how i have to construct it in order to reach a magnetic flux density with a specific strength and spatial evenness between the two coils. I merely a hobbyist, so i have a few unknowns. I hope that if I'd state my assumptions about the setup of the simulation here, you could help me by confirming or denying them.

I have read the tutorial Nr. 30 "Asymmetrical Conductor with a Hole" thoroughly and derived most of my assumptions from that.

Assumptions:
1) i want 3 bodies in my simulation. Coil A coil B and a body of air C that both coils are seamlessly embedded in
1a) i cannot just make the two coils one body.
2) i want one MgDyn equation and one MgDynPost equation in my simulation. all three bodies are being assigned that equation
3) i want two materials, copper [assigned to A and B] and air [assigned to C]

I am also having the problem with generating the required multibody mesh. I want to generate my mesh using FreeCAD. I have tried the following stategies all of which failed:

1) Generate 2 Tubes with the dimensions of the desired coils. subtract them from the body of air.
generate two more coils with the exact same dimensions and positions.
a) → make the whole geometry a BooleanFragments
→ send it through a CompoundFilter
→ export as STEP
→ importing in elmer results in a single body
b) → make the whole geometry a Boolean Fragments
→ send it through a Compound Filter
→ make a meshregion for each coil and the air
→ export as UNV
→ i have only two bodies in elmer although the file displays 3 meshgroups
2) a version of 1) without the CompoundFilter → results in bs

i know this topic has been heavily discussed in the past and i have read through many threads without any hint being able to solve my problem or point me in the right direction. i have been reading the code of the elmer import mechanism only to find that the number of elements value gets set in the importer somehow. there have to be people here who can tell me without me having to reverse-engineer all the things.

Maybe it might interest people that in the process of doing all this, i've repaired the broken elmer package on nixOS and made elmer 9.0 available and usable again.

Thank you in advance for your time.
grindhold
Rich_B
Posts: 421
Joined: 24 Aug 2009, 20:18

Re: Questions about my Helmholtz coil simulation.

Post by Rich_B »

Hello,

Your case looks similar to this tutorial:

https://wiki.freecadweb.org/FEM_Example ... _Two_Balls

with two smaller spheres being cut from a large sphere, creating 3 bodies.

If you have already tried this tutorial, what happened?

Rich.
grindhold
Posts: 2
Joined: 14 Sep 2022, 12:10
Antispam: Yes

Re: Questions about my Helmholtz coil simulation.

Post by grindhold »

I've seen this, but the tutorial is carried out in the Elmer-FreeCAD-integration which is not my goal. I am trying to use ElmerGUI standalone. I want to get sif-files out of that which enables me to run ElmerSolver headless and automate processing further down the road. Then i want to create different multibody geometries that i can run with that sif-file over and over on a headless server.
I've investigated my .unv-file with gmsh. it tells me that it contains 1 Volume and 3 Physical Groups. When load in elmer i see only two bodies.

EDIT: In addition i don't seem to have any possibility to describe the amperage/mm² through my coil, leave a lone simulate a coil in the first place. This i only find in ElmerGUI.
Rich_B
Posts: 421
Joined: 24 Aug 2009, 20:18

Re: Questions about my Helmholtz coil simulation.

Post by Rich_B »

Hello,

That Freecad tutorial may be useful just for the geometry. What if you only go through it as far as creating the geometry and then exporting the
geometry?

EDIT: the Freecad tutorial is not useful for our purposes. Exporting the geometry as step, brep, or stl doesn't produce a useful output. exporting the mesh as brep or stl doesn't produce any output. Or, which is more likely, simply exporting a selected object isn't the right way. Anyone out there know if/how this can be done, from the tutorial? Thanks.

Also, have you looked through this repository?

https://github.com/ElmerCSC/elmer-elmag

Rich.
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