Coupling between a flag and a spring-mass oscillator

In the context of energy harvesting we address the coupling between a flexible flag and its flagpole equipped so that it constitutes a spring-mass oscillator. An extensive set of experiments is carried out in wind tunnel for various flag and oscillator parameters. Results are analyzed in terms of frequency and amplitude of rotation of the flagpole. We report numerous configurations of coupling by frequency lock-in leading to resonance conditions. In the case of strong coupling, high amplitudes of rotation of the flagpole are reported, up to 75° peak-to-peak, over a large range of wind velocities. We also propose to characterize the strength of the coupling with a dimensionless rigidity B˜, which can be considered as the ratio of the flag bending rigidity to the stiffness of the oscillator.