system with moving part- acoustic

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Gary R
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system with moving part- acoustic

Post by Gary R »

Hi all;
I am trying to analyze an acoustic waveguide. I have the hyperbolic waveguide, the piston and an air cylender. The three parts are assembled with FreeCAD Assembly 4 workbench. There seems to be a problem with the .step format ( doesn't like the hyperbolic curves) that doesn't allow transfering the assembly file to a step file. See the assembly below.
Assembly.png
Assembly.png (36.27 KiB) Viewed 697 times
https://forum.fre

I need to be able to use FreeCAD's internal netgen and then use Elmer to solve the Navier Stokes equation for the system. I am trying to get the radiation pattern for the waveguide. I have used Elmer some for beam deflections and fluid flow but never for a 3 piece system like this. I don't see how to assign bc's to each part nor how to mesh the system. Could someone layout a workflow for this project.

Gary R.
kevinarden
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Re: system with moving part- acoustic

Post by kevinarden »

The hyperlink does not work for me, not sure what information was trying to be sent. When there are multiple bodies in Elmer that need to interact at their faces they have to share nodes at those faces. Salome has a nice feature called partition where you create the cylinder, then the guide, and use the guide to partition the cylinder, resulting in a new single part that is a cylinder with a wave guide in it. In FreeCAD, I think you would have to cut the cylinder with the guide, then control the mesh at the boundary of the two parts, probably ending up with coincident nodes that could be merged,
Another option is mortar boundary conditions at the boundaries, which would allow independent meshes at the interface, the mortar boundary conditions handle the interface between the two independent meshes.
Is it your intent to move the piston during the simulation? I would start with just the wave guide in a cylinder before adding a piston.
raback
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Re: system with moving part- acoustic

Post by raback »

Hi Gary,

If you're interested in acoustics perhaps it would be easiest to start with the Helmholtz equation. You could use some eigenmode as a wake for the wave.

To capture the waves nicely you need 10-20 elements for each wavelength in Helmholtz equation. This may limit your capabilities in higher frequencies.

If you would use Navier-Stokes equation you need additionally a boundary layer mesh than can capture the viscous and thermal boundary layers accurately. If you're just starting your career in meshing this is probably not the route you would want to take. Also, the Navier-Stokes could easily lead to need of supercomputing due to huge difference in scales.

-Peter
kevinarden
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Re: system with moving part- acoustic

Post by kevinarden »

If I were to start developing this simulation, I would start with 2D using a plane at the center of the model. If I could get that working, I would move on to 3D and then the piston.
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