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Acoustic waves and Courant number

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Old   May 11, 2015, 01:33
Default Acoustic waves and Courant number
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Costin Ruja
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Good morning !

After some fiddling on my own, i would appreciate your comments on the topic.

The problem is quite straightforward.I have a transient Ahmed body simulation, about 5 mil. elements and 1.3 mil nodes.I would like to do some aeroacoustics on it just for personal training and for the fun of it.

The first thing I know is that I want it to run SAS-SST with central differentiating scheme to better capture the buffeting wake.Then i got stuck at choosing the timestep size. CFX being an implicit solver I know I don't have to go with Courant number=1 for the flow, but is this also true with the acoustic wave (sound speed=340m/s) ?

Also i expected something like concentric pressure waves emmanating from the back side of the ahmed body (where flow separation takes place), but I don't see the yet.

I set up a pressure monitor point, and i think it captures quite nicely some quasy-sinusoidal wave.Is it correct to assume this is the acoustic pressure and just do some FFT and dB conversion on it to find the SPL ?

Since I'm interested only in finding the near-field SPL (only a meter a way or so), only with CFX, and not coupled with some third party tool like LMS Sysnoise or MSC Actran what is the best approach to this problem ?

Thanks !
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Old   May 11, 2015, 03:16
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Glenn Horrocks
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Yes, as CFX is an implicit solver the time step size is linked to time resolution accuracy, not Courant number. So do a time step sensitivity study and see how fine the time step needs to be to capture the acoustic waves to an accuracy you are happy with.

Just checking - are you running a compressible fluid simulation? You won't get pressure waves if your fluid is incompressible.

Yes, you can do a SPL calculation from the pressure monitor point if you are only interested int he near field SPL. Keep in mind reflections off the outer boundary - this will distort the result if not managed properly.
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Old   May 11, 2015, 03:23
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Yes, I did model the air as compressible (air ideal gas) and total energy option turned on.

I guess i'll have to wait for some good number of hours to see the flow develop and begin to oscillate.

Thanks a bunch Glenn !
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Old   May 11, 2015, 05:54
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As a starting point I would guess a time step of 1/10 of the highest frequency you will be analysing. It will probably require a finer time step than that, but it is a starting point.
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Old   May 11, 2015, 10:26
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I'm currently running with a timestep of 4e-4s as found by the solver to give a RMS Courant of 1, so according to your 1/10 rule, I should get well resolved waves up to 250Hz.For now, i'm letting it solve at this rate, although.
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