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Old   July 17, 2012, 05:30
Default Fast Fourier Transform
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Faisal Durrani
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Hello,

I am using LES to look at buoyancy driven flows. I wanted to plot the energy cascade as shown in the attached figure 1. Using CFX post I plotted the power spectral density of the vertical velocity (figure 2). I do get the same -5/3 and -3/1 laws in the curve. But the vertical axis is in dB??? I dont understand why its in dB while I want it in the units of energy. Can you guys help me plot something like figure 1?

Thank you very much,

Faisal Durrani.
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Old   July 17, 2012, 19:26
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Glenn Horrocks
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dB is simply the log of energy in this case so it should be fine.

If you do not get the -5/3 slope then consider this: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

If none of the actions there help then I suspect for this case you need to look at the dissipation inherent in CFX. It may be too high or low and is causing the turbulence spectrum to have the wrong slope.
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Old   August 3, 2012, 05:38
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Jordi
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a little bit more precisely, a magnitude measured in dB's is:

x(dB) = 10 * log ( x / xreference)

Where xreference is some base value for x (it corresponds to 0 dB).

sometimes it's written dBm when reference is a mV, mA, mW, etc. It's also usual to see dBu with micro-unit as reference.
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Old   August 3, 2012, 05:56
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Hi

so isn't that just a different range setting of the monitor-axis?
-> right klick on middle of monitor -> Monitor Properties -> Range Settings -> ...
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