The flapping of a flag in the wind is not primarily driven by variations in the wind velocity - even in a perfectly steady wind, it will still flap, and that’s what’s happening in the simulation.
What happens is a feedback loop - depending on their angle, some parts of the mesh catch more of the wind, so parts of the mesh move differently. The tension of the fabric makes all the parts interact, and it gets pulled into a new shape, which results in a new wind force distribution, and so on…
This feedback loop can result in periodic oscillations of various frequencies and sometimes chaotic motion.
There isn’t a way to control the oscillation frequency directly - it is the result of complex interaction of multiple factors, including fabric stiffness, wind strength, damping and mesh density.
Even if you did add some controlled variation of the wind strength over time it would not give you any direct control over the oscillation frequency of the flag - it would just be one more variable to adjust in the system.
Here’s a demo
flappingflagdemo.gh (22.5 KB)