[Sigi]

I did a simulation of an airfoil that makes a heave movement and a simulation of an airfoil that makes a pitch movement. Each movement is periodic and oscillates at frequency f1, and I ran both Grid Movement and Surface Movement cases.

After running, I took the data and did an FFT for the movement received in time and I got that in the heave movement (up and down movement) there is no movement in x (as expected), and in the y direction there is only one frequency oscillation which is f1 as expected as well.

However, when I did something similar to the pitch then I got that there are several frequencies that affect the movement (true for both Grid Movement and Surface Movement), and that they are all harmonics of f1, as described in the attached image where there is the spectrum of X and Y at the point x=0 and y=0 which is the point of the LE of the airfoil. Similar results were obtained for all points in domain.
What is the explanation for this? Is it something physical or because of how the code calculates the pitch movement?

[wkernkamp]

1. The motion is not linear because the angle causes a curved trajectory.
2. The n=3,5,7.. displacements are small.

Try with the amplitude reduced by half and see if the higher order peaks diminish.

[Sigi]

Hello Will,

I appreciate your response, and you're absolutely right about the presence of smaller high harmonics for lower amplitudes in the nonlinear motion.

I have a follow-up question: While it's clear why the second harmonic exists in both the x and y motions of certain points, I'm curious about why higher harmonics (e.g., 3rd harmonics and beyond) also come into play. Could this be attributed to interactions between different harmonics in the x and y coordinates?

Thank you for your insights.