Including a fluid around spark gap

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Gary R
Posts: 162
Joined: 12 Apr 2012, 07:23
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Location: Long Beach CA, USA

Including a fluid around spark gap

Post by Gary R »

Hi all,
Operating System: Debian GNU/Linux 11
KDE Plasma Version: 5.20.5
KDE Frameworks Version: 5.78.0
Qt Version: 5.15.2
Kernel Version: 5.10.0-13-amd64
OS Type: 64-bit
Processors: 4 × AMD FX(tm)-4350 Quad-Core Processor
Memory: 15.6 GiB of RAM
Graphics Processor: AMD CAICOS

Many of my analyses require a fluid (air) around the actual devices of interest. An example being a spark gap surrounded by a gas. The analysis requires the determination of the current through the electrodes so I can't just turn the electrodes into a void and mesh their surfaces. I use FreeCad to build my models, export them as .brep files and mesh them with Gmsh. It seem that Gmsh will only mesh solids. Even if a void area is completely enclosed ( a sphere surrounding a void for example) Gmsh will not mesh the void. I could avoid the problem in 2D by covering the sketches with surfaces. Obviously this doesn't work with 3D. I have had trouble modeling speaker systems and plasma systems because of this problem. Does anyone have a solution to this problem. I have tried the FreeCad forum and have not gotten a viable answer.

Gary R
kevinarden
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Re: Including a fluid around spark gap

Post by kevinarden »

gmsh will mesh 2D on surfaces and solids on volumes. However it works best if you create physical groups for each one. Have you seen Chapter 9 of this manual? I have been more successful and used to step files instead of brep files, not sure why.

https://www.nic.funet.fi/pub/sci/physic ... dElmer.pdf
kevinarden
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Re: Including a fluid around spark gap

Post by kevinarden »

Perhaps you could post a specific FREECAD model that did not work out, and what the issues were? I have found Salome to be the most powerful model creator for Elmer, however, also the hardest to become proficient.
Gary R
Posts: 162
Joined: 12 Apr 2012, 07:23
Antispam: Yes
Location: Long Beach CA, USA

Re: Including a fluid around spark gap

Post by Gary R »

Thanks for the reply Kevin.
I am downloading Salome and will try it. As far as an example goes, I've had difficulty producing one. The only option I have been given is to use a solid sphere, embed the parts, do a boolean subtraction to produce voids and then replace the physical parts. This is messy beyond belief. I'm hoping for something simpler.

Gary R.
kevinarden
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Joined: 25 Jan 2019, 01:28
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Re: Including a fluid around spark gap

Post by kevinarden »

I have found this to work well in salome using partition, not boolean. You create the inner parts, then the volume (air) around the parts. Then use the inner parts to partition the big air volume. In geometry you then create groups of volumes and surfaces. Once you mesh, you can create groups 'group on geometry'. Each solid group becomes an elmer volume, and each face group becomes a boundary.

The mesh is exported as a unv file and can be converted with ElmerGrid or ElmerGUI

I loaded an example here, the salome file is the Study1.hdf

https://github.com/mrkearden/heat_exchanger

In this case the flow is in the pipe, and in the tank around the pipe.
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