Meaning of harmonics attained through PSD

Hi Guys

Please refer to the attched images.

The PSD has been developed using initial conditions on the "theta" coordinate for a coupled spring pendulum

the natural frequency of the spring system is 2.72Hz and of the pendulum motion is 1.36Hz. The parameters are sized such that parametric resonance exists.

I am struggling to interpret the PSD results. The natural frequencies of the springs and pendulum have extremly small amounts of enerygy. There seems to be harmonics arising in all the results. What would be the cause of such results?

I assume that the results are hamronics of the natural frequencies, which occur due to the unstable behaviour of the system. However how would one interpret the results further? and why do they have more energy compared to the natural frequencies?

[Bjorn Gustavsson]

As far as I recall this type of system is nonlinear - which means that it is not straightforward to analyse it with this type of method.