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Aeroacoustics, my setup

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Old   January 10, 2012, 06:36
Default Aeroacoustics, my setup
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By sharing my setup for this case I hope to get some input and hopefully help others.

The problem: Simulate generated sound in corrugated pipes and find the frequency at specific velocities by looking at pressure fluctuations. The pipe is 20 cm long and roughly 1 cm in diameter.

The setup:
First off I need a mesh. I chose to mesh my corrugated pipe between a smooth pipe and an "atmosphere". The smooth pipe is there so to develop the flow before entering the corrugated pipe. Upon exiting the corrugated pipe my flow enters a large box, which is the atmosphere in order to reduce the impact of my boundary conditions. For the 2D mesh I used a wedge type as this is axisymmetric (this may be a problem as my computational domain is included in the axis. if anyone has a comment on this I would appreciate it).

domain.jpg

I think my mesh is sufficiently dense

mesh.jpg

Next I need some boundary conditions. The most important ones are U and p for inlet, outlet and walls I would imagine:
Inlet: U = 6 m/s p = zeroGradient
Outlet: U = zeroGradient p = atmospheric pressure 1e5
Pipewall: U = 0 p = zeroGradient

There are of course more conditions for the turbulence which are included in the files.

For turbulence I chose originally LES, but that was when I ran 3D simulations. Now I am focusing on 2D because of time restrictions and is therefore using RAS kEpsilon.

The solver used is the compressible rhoPimpleFoam since I need to capture the compressible effects of the pressure generated inside the pipe. Also my flow is transient and I wish to observe a standing wave within the corrugated pipe. I have only used this solver as it seemed to be the most fitting for my problem. If anyone has any reason otherwise I would gladly hear your opinion. (Maybe the rhoCentralFoam is more suited for my case?)

I am running with a Courant number of 0.1 with adjustable time step.

The result: In general I have yet to solve my corrugated pipe problem. My first attempt with 3D flow gave me some kind of fluctuations in the pressure, but the calculation of the frequency gave values much greater than anticipated. I expected frequencies in the 600-1200 Hz area, but got around 3-5000 Hz.

After some time using 3D simulation, I went down to 2D because of time restrictions. here I chose a wedge type mesh with the above boundary conditions. Now I do not get any sensible pressure fluctuations, instead I get only a pressure drop from the inlet to the outlet. However the flow inside a corrugation seems to be correct according to different papers on this subject. I did not get this result with my 3D flow.

pressure.jpgU.jpg

So if I can reproduce the standing wave in my current 2D simulation I would be a big step closer to a correct simulation, but for now I do not know how to obtain this. I would greatly appreciate any input on this matter!
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aerocacoustic, pipe, rhopimplefoam

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