method about getting the acoustic natural frequency.

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 April 14, 2017, 21:23 method about getting the acoustic natural frequency. #1 New Member   Sungmyung, Lee Join Date: Apr 2017 Posts: 2 Rep Power: 0 I want to check the acoustic resonance for tube bundle. Basic principle of acoustic resonance make effect if vortex shedding frequency is coincide with acoustic natural frequency. By fluent tool, I obtained vortex shedding frequency value at the center of tube bundle. it seemed that Acoustic natural frequency is something in the condition of only single duct. Compred to vortex shedding frequency, I can not find acoustic natural frequency in tube bundle with fluent simulation. please help me to proceed the acoustic resonance checking. thank you

 April 14, 2017, 22:35 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,244 Rep Power: 63 It's very hard to do in Fluent because the available schemes are very dissipative. It's also expensive to do in time domain because you have to check the system response to an impulse. You can do it with flow or without flow depending on how you define acoustic natural frequency. Obvious you do a transient simulation. Give a pressure pertubation (dirac delta or Gaussian, white noise, or even a mode) and then monitor the result. Do modal analysis on the resulting field. But because of numerics, the waves will decay. Depending on the perturbation, you may observe one mode and not another.

 April 17, 2017, 04:30 Acoustic natural frequency by fluent #3 New Member   Sungmyung, Lee Join Date: Apr 2017 Posts: 2 Rep Power: 0 thank you, I agree with your opinion truely. but I have to analyze the acoustic natural frequency for below duct design. Duct width is 1.45 meter, inlet velocity is 13 m/s with air. Duct length is 10 meter. how to set up the frequency condition? in my case, I recored the point's vortex variation along the center line. with files, I calculate FFT process by fluent option with x axis is Hz, y axis is pressure density. how about you review the above acoustic natural frequency calculaton procedure if you do not mind? There were not enough ANSYS manual and book and exports. please help me

 April 17, 2017, 09:12 #4 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,244 Rep Power: 63 That is the general idea yes. Run the simulation, get data out (pressure is most important, you can also do velocity), and do FFT everywhere. I do all my postprocessing in matlab, but you can use whatever tools you have. The most important thing is to make sure your simulation is setup properly and that there is acoustics in it. I've never done it with URANS, only unsteady laminar or LES. But it's very important to put the acoustics in there using a pressure impulse, or externally forced from one of the boundaries. The problem with external forcing is you cannot check for natural frequencies of the domain because you are forcing it. Vortex shedding is an acoustic damper, it absorbs acoustic energy. Only if the vortex impinges on a wall or something does it produce sound. It's possible that you have a simulation with no acoustics and that is why you are not able to find it.

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