Diaphram displacement on Helmholtz Resonator

Numerical methods and mathematical models of Elmer
Gary R
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Diaphram displacement on Helmholtz Resonator

Post by Gary R »

Hi all,
I have been trying to record the deflection of a diaphragm vs frequency in the side of a Helmholtz Resonator. The scan seems to work but I keep getting zero output to the SaveScalar solver. I have included a .png of the device and zip file of the whole project. Could someone look over the code and tell me what is wrong.
Screenshot_20220830_143529.png
Screenshot_20220830_143529.png (46.14 KiB) Viewed 1101 times
If you wish to run the project you will have to re-mesh using the .brep file to Gmsh and run the .msh file through ElmerGrid 14 2 -autoclean -merge 1. The .mesh files were too big to fit the 1 meg file limit.

Gary R.
Attachments
CabinetProject.tar.gz
(40.12 KiB) Downloaded 61 times
kevinarden
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Re: Diaphram displacement on Helmholtz Resonator

Post by kevinarden »

Bodies 2 and 3 have no material
Body 3 is solid elements, the SMITC solver is for elastic plate elements, stresssolver is for solid elements
Body 3 is not attached to the box and has no displacement constraints, which means if any load is applied it will move to infinity
Pressure Wave 1 = 1000 is not a recognized load for the linear elastic solvers

https://github.com/ElmerCSC/elmerfem/tr ... zStructure
kevinarden
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Re: Diaphram displacement on Helmholtz Resonator

Post by kevinarden »

If you can do the Helmholtz solution on its own, I can add the structural displacement part to it.
Rich_B
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Joined: 24 Aug 2009, 20:18

Re: Diaphram displacement on Helmholtz Resonator

Post by Rich_B »

Hello,

Attached is a gmsh model, based on your dimensions, of what looks like a bass reflex speaker. The attached sif file can be run after generating a mesh with gmsh, and converting the mesh using elmergrid.

Maybe someone with more experience with acoustics simulation can evaluate whether the results are sensible.

Rich.
pwave1.png
pwave1.png (79.3 KiB) Viewed 1052 times
Attachments
Speaker.geo
(2.03 KiB) Downloaded 51 times
case.sif
(2.4 KiB) Downloaded 61 times
kevinarden
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Re: Diaphram displacement on Helmholtz Resonator

Post by kevinarden »

I added the linear elastic part, assuming the box is rigid, no displacements. The diaphragm can deflect. Using the
Structural Interface = Logical True
results in a solution trivially zero since no pressure load is applied.
If you apply a pressure load using Normal Force then a solution is obtained, but this is not the pressure load from the acoustic model.

So the question is how is the pressure from the acoustic model applied to the diaphragm as a load?
case.sif
(3.42 KiB) Downloaded 62 times
kevinarden
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Re: Diaphram displacement on Helmholtz Resonator

Post by kevinarden »

https://github.com/mrkearden/cabinet

This version models the box around the air as well as the diaphragm. The box is restrained from motion at it's bottom.
Has the same issue, doesn't work with
Structural Interface = Logical True
Rich_B
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Re: Diaphram displacement on Helmholtz Resonator

Post by Rich_B »

Interesting, thanks for putting together another geometry model. From reading more about speaker design on the internet, I think the air in the room outside of the speaker should be part of the model. Think of two boxes connected by a moving diaphragm, where one box is small (the speaker) and the other is larger (the room). Adding the reflex port as a second connection between the two boxes would be close to a model of a bass reflex port speaker sitting in a room.

Maybe some of the shoebox FSI tests will be helpful for the coupling between sound waves and the moving diaphragm.

Rich.
Rich_B
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Re: Diaphram displacement on Helmholtz Resonator

Post by Rich_B »

Hello Kevin,

Looking at your cabinet model in ElmerGUI, I'd like to report something. If you hide boundary 5, and then select either boundary 3 or 6, the red highlight is mottled. This usually means there are two boundaries right on top of each other, and non-conforming.

Rich.
boundary3.png
boundary3.png (250.25 KiB) Viewed 1029 times
kevinarden
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Re: Diaphram displacement on Helmholtz Resonator

Post by kevinarden »

Thanks Rich.
I did a merge nodes in salome and it fixed the mesh, but no change in the results. I updated the files in the github.
kevinarden
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Re: Diaphram displacement on Helmholtz Resonator

Post by kevinarden »

There are 3 HelmholtzStructure test cases in the elmer fem test case directory. They all have the structure moving creating sound waves in the air. This case is looking for the opposite. Sound waves loading the structure.
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