Fluttering flags: An experimental study of fluid forces

Unsteady fluid forces are measured at the onset of flutter and during the post-critical flutter of flags placed in a wind tunnel, focusing on the drag force and the moment around the flagpole. The evolution of these forces during flutter mode switches, induced by varying either the mass ratio or the wind velocity, is discussed by using additional high-speed imaging. For the highest wind velocities, losses of periodicity and snapping events are reported. Time-averaged and unsteady drag coefficients are reported and compared to previous works.