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?