why does flag flutter when strong winds are blowing on a certain day?
Answers
Flag Flutter is a flow instability which occurs due to a coupled fluid structure interaction between the flow and the flag. This can be a highly non-linear phenomena and flutter speed signifies the dynamic point at which the flag can extract energy from the mean flow and transfer it back to the small scale motions in the flow.
The exact mechanism of flow flutter instability has been a widely studied primarily because of its wide range of applications from energy harvesting to air-cooled condensers to bio-mimicry. At the onset of flutter, any perturbations on the surface of the flag ( which can be influenced by structural inhomogeniety, body forces such as gravity or fluid perturbations such as turbulence) get accentuated because changes in the shape of the flag lead to changing static pressure differential above and below the flag. This creates a net force on the flag which together with the flags structural forces cause flutter.