How to understand multiple peaks in FFT analysis? 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. 




How to understand this pattern?
If you are familiar with vibration, how to explain these vibration data?
You are looking at harmonics of the fundamental. There's a lot of literature available to read on the topic.