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In FFT analysis, it is often seen that there are several peak frequencies with integer times of frequencies. For example, in the figure below, there are peaks at $0.2, 0.4, 0.6, 0.8$.

The FFT figure below is transformed from the time history of force coefficient $C_{Y2}=F_y/(\rho D U_m^2/2)$ and displacement $Y_2$ on $Y$ direction of a cylinder immersed in a disturbed fluid and attached to a spring.

Fig. 1 FFT spectra of vibration displacement and force

  • How to understand this pattern?

  • If you are familiar with vibration, how to explain these vibration data?

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  • $\begingroup$ Where does $Y_2$ come from, and what does $C_{Y_2}$ mean? Could you give more context around the "vibration" experiment? $\endgroup$ – Laurent Duval Jan 14 '17 at 11:53
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You are looking at harmonics of the fundamental. There's a lot of literature available to read on the topic.

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Depending on the system you are investigating, there can only be (standing) waves of a certain length. Mostly the wavelength is determined by the physical size of your system. The term resonance is often associated with harmonics. Where did you get your data from?

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  • $\begingroup$ Then the harmonics are dependent on the mass and the spring constant. The system can be described with a differential equation which then can be transformed into the frequency domain. With that you should be able compare the measured harmonics with the calculated ones. $\endgroup$ – UpSampler Jan 20 '17 at 11:58

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