Research Paper: Investigation of Frequency Spectrum and Poincaré Surfaces in Double Pendulum

In this paper, the behavior of the double pendulums has been studied by considering the effect of initial conditions (angular displacement of the outer pendulum for four cases 12, 30, 90, and 150 degrees) and the influence of system geometry (increasing rod length and mass of the second pendulum), also. After extracting motion equations using the Lagrangian method, in order to deal with the frequency spectrum, traces of bobs, and understanding the system behavior for every case, the Fast Fourier Transform (FFT) technique and Poincare sections have been applied. The obtained results show that the consequence of the rising angular displacement the outer pendulum is to increase the energy level of the system and the change of its behavior from quasi-periodic for angular displacement is less than 90 degrees to chaotic when it is 150 degrees. Therefore, the energy level, in this case, has increased more than twice compared to the first. In addition, it seems a quasi-periodic behavior is forming at the heart of chaotic behavior. On the other hand, the results indicate a very significant effect of geometrics on a system's behavior. According to the calculations, the consequence of increasing the length of the rod of the second pendulum has only led to a different behavior from its similar case (case number 3) completely. however, their energy level is the same. Increasing the mass of the outer by twice not only to lead decrease energy level of the system by 330J but also has shown chaotic behavior (in comparison to the case3).