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How to predict & verify resonance frequency of pipe by flow?

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Old   April 4, 2013, 21:03
Default How to predict & verify resonance frequency of pipe by flow?
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Heebeom.Park
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Dear all
I'm studying the pipe like below dimensions
- Pipe Length = 2m
- Pipe Dia = 0.065m
- V_inlet ~ 24m/s with NRBC (by mass flow rate)
- P_outlet = 0 with NRBC
- Temperature = 25 celcius degree
- Analysis Domain
................|―――――――――――――|
――――――
|(inlet) ...................................... |
――――――
...............|―――――――――――――|
<--------->|<----------------------->
pipe ........... free space(outlet)
- turbulent model : SAS
- cell courant number < 3
- air : ideal gas density
- time step : 2.5e-5 sec
- finished flow time : 0.12 sec

First, I started the analysis with steady analysis result as initial conditions
And then I had enough time to diminish the fluctuation of the solving hydrodynamic pressure as transient period by transient solving for about 0.1sec
Then I did acoustic analysis.
But i got about 740Hz as 1st resonance, please see below pics
First resonance frequency for both opened pipe is about 85Hz.
Why happened these results?
-FFT result-
https://docs.google.com/file/d/0B2VH...it?usp=sharing
-Grid view-
https://docs.google.com/file/d/0B2VH...it?usp=sharing

Last edited by aerodigit; April 4, 2013 at 21:34.
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