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Fast Fourier Transform on Lift Coefficient

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Old   November 26, 2016, 07:22
Default Fast Fourier Transform on Lift Coefficient
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Hello,
I have conducted a simulation of a hydrofoil in deep water head waves and monitored the lift coefficient in time. As expected the coefficient fluctuates around a mean value as in the figure below (click link).
https://drive.google.com/file/d/0Bz2...ew?usp=sharing

Is it possible to analyse this spectrum by doing Fast Fourier Transform in excel? I am starting to think that the fact that the "signal" doesn't fluctuate around zero imposes complications..

Any input on this would be much appreciated!
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Old   November 26, 2016, 11:28
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You can take a Fourier transform of the signal. The mean value should be returned as the coefficient of the term for f=0. If that concerns you, then compute the mean and subtract it from your signal. Then transform the unsteady part. But the real question is in the interpretation of your spectrum. You need to make sure that your solution is both timestep and grid independent in order to have some assurance that your spectrum has physical relevance.
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Old   November 26, 2016, 11:56
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The only problem I see is that you must extract the periodicity time from your sequence. Then you should see your spectrum characteristic of a laminar flow with the main frequency ad one or two peaks at higher frquencies.
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Old   November 26, 2016, 14:37
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Thank you both for your quick replies. Is it easy for one of you to give me some guidance on how to do the FFT? I should probably clarify that I have no background at all in signal processing. Ideally I would like to do the FFT in Excel, or could possibly do it in Matlab. Unfortunately I didn't manage to find any instructions to match my case elsewhere..
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Old   November 26, 2016, 14:45
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https://it.mathworks.com/help/matlab....mathworks.com
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Old   November 26, 2016, 20:50
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If you have no background in signal processing I would strongly encourage you to spend some time reviewing the basics of the Fournier transform. It is a powerful tool that can be (and often is) misapplied.
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