Coupling between a flag and a spring-mass oscillator

We address the coupling between a flexible flag and a spring-mass oscillator in the context of energy harvesting. We report a coupling by frequency lock-in leading to resonance conditions over a large range of wind velocities. Large amplitudes of rotation of the flagpole are reported, up to 75 degrees peak-to-peak. We propose to characterize the strength of this coupling with a dimensionless rigidity, which can be considered as the ratio of the flag bending rigidity to the stiffness of the oscillator.