Visible waves generated by Helmholts Equation.

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Gary R
Posts: 122
Joined: 12 Apr 2012, 07:23
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Location: Long Beach CA, USA

Visible waves generated by Helmholts Equation.

Post by Gary R »

When I started to model a hyperbolic acoustic horn assembly, I wanted to illustrate the expanding wave fronts. My goal was to determine how much directionality would be introduced at higher acoustic frequencies. I had trouble finding examples that did this. Most Helmholtz analyses showed a general energy distribution and not the individual wave fronts. So I hope this helps.

This is a static case illustrated by the following .png.
HyperbolicHorn3Khz.png
(327.97 KiB) Not downloaded yet
The outline was created with FreeCAD and exported as a .brep file.The .brep file was loaded to gmsh (Version 4.1.3) and exported as a .msh file. The .msh file was then passed through ElmerGrid (14 2) to create the .mesh file directory needed by Elmer.

Attached is a tarball containing the case.sif, .brep and .msh files. For higher frequencies, the mesh may be needed to be refined. The recommendation is 1/10 wavelength spacing. The long side of the figure is 4ft (1.219 m). The coordinate scaling is set to 0.001 in the simulation section. This makes the FreeCAD files compatible with the scaling used in Elmer, this assuming you are using English units. Millimeters would be a better choice.

Some notes:
The angular frequency is set in the Simulation section only. That is the Setup section if using ElmerGUI (eg $2*pi*3000 for a 3Khz frequency)

For hard (reflective) surfaces set Wave Flux 1 and2 to 0.0
For Open Boundaries set Plane Wave BC to True
For the Driver set Wave Flux 1 to 1 and Wave Flux 2 to 0.0
Of course the material should be set to air.

Hope this all is helpful

Gary R
Attachments
Helmholtz.tar
(60 KiB) Downloaded 22 times
Gary R
Posts: 122
Joined: 12 Apr 2012, 07:23
Antispam: Yes
Location: Long Beach CA, USA

Re: Visible waves generated by Helmholts Equation.

Post by Gary R »

A couple of corrections.
The driver bc's should be changed to pressure wave1 = 10 and pressure wave2 = 0. The fluxes should be left blank.

Attached is a new .brep file than is a complete slice of the horn. The half slice seemed to be introducing some artifacts. The new file also increases the air volume to 8ft x 8ft which is closer to a normal listening area.

I added 2 .png files for 2Khz runs.
HyperbolicHornThroat2.0Khz.jpg
(25.75 KiB) Not downloaded yet
HyperbolicHorn2.0Khz.png
(186.61 KiB) Not downloaded yet
Hope this helps.

Gary R
Attachments
HyperbolicHornSlice.brep
(13.07 KiB) Downloaded 7 times
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