Contact behavior of a fluttering flag with an adjacent plate

Abstract

A flag fluttering near a plate exhibits diverse contact behaviors depending on physical conditions such as material and dimensions of the flag and incident air velocity. Here we experimentally investigate the dynamic interactions of the fluttering flag and the plate, combining high-speed imaging and triboelectric sensing. We find that the stability criteria of the flag are identical to those of the isolated flag for an insignificant boundary layer thickness compared with the flag-plate distance. The flag-plate contact modes are classified as tapping, regular clapping, weakly chaotic clapping, and fully chaotic contact modes. We build a regime map to predict the contact modes based on the mass ratio of the fluid to the flag and the velocity ratio of the fluid to the bending wave. Despite contact with the plate, key parameters identifying the characteristics of fluttering of the flutter, such as the reduced frequency (the ratio of the time scale of fluid flow passing through the flag to the period of oscillation) and the Strouhal number (the speed ratio of the flag edge to the wind), are found to be hardly altered compared to those of isolated flags. This indicates that the flag-fluid interaction still plays a dominant role in the fluttering dynamics of the flag adjacent to a plate. Results of this study can serve as a guide in the design of flutter-induced energy harvesting systems and help the biomechanical understanding of the vocal organs of mammals and birds.