FAST question about noise

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Stefano.Cottafavi
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Joined: Wed Jul 09, 2008 2:26 pm
Organization: Ampyx Power
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FAST question about noise

Postby Stefano.Cottafavi » Sat Aug 30, 2008 6:37 pm

Hi,

I'm trying to learn more about noise computation. Basically I'm using the 1.5 MW WTG from the examples with the Aero and CompNoise flags set and the rest as default, so the timestep is 0.005s

Now my question..." is the 1/3 octave spectrum reliable?"

correct me if I'm wrong... how can I "reproduce" a 22000Hz sine wave if I'm time marching with 1/0.005 = 200Hz frequency ... I mean, the 1/3 spectrum is always provided up to 20kHz and more but is that ok also if the step is not as small (so the freq as high) as supposed for the "sampling theorem" ? (audio bandwith is approx 22kHz wide, so 1/(2*22k) = 2.3e-5s )


maybe I've missed something in the source code...

thank you in advance for your help

stefano

Pat.Moriarty
Posts: 40
Joined: Mon Nov 07, 2005 4:59 am
Location: NWTC/NREL

FAST Noise module

Postby Pat.Moriarty » Tue Sep 02, 2008 4:34 pm

Stefano,

The code uses an empirical relationship developed by Brooks, Pope, and Marcolini (1989) to produce the amplitudes of noise levels in third octave bands, but doesn't directly calculate the sound waves. So, the calculation is independent of the time step used in the simulation.

cheers,
Pat
Big whirls have little whirls, That feed on their velocity; And little whirls have lesser whirls, And so on to viscosity - Lewis Richardson.


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