Coupled motion characteristic of standard flag and inverted flag in tandem arrangement

In this paper, the dynamics of the flow-induced flutter of standard flag and inverted flag in tandem arrangement were experimentally studied in wind tunnel. The kinematics of the flapping flags was investigated using highspeed camera and the flow field around the flags were examined using particle image velocimetry technique. The experimental results show that the critical velocity of inverted flag and standard flag in tandem were both decreased due to the flow perturbation between two flags. In particular, the hysteresis loop of the tandem standard flag was decreased significantly compared with single standard flag. Hence, the flow velocity range for flag flapping is broadened widely as compared to that of a single flag. By the action of the flapping flags, flow around the tandem flags exhibited severe turbulence. Small-scale vortices and Kelvin-Helmholtz instabilities could be observed. These findings will provide theoretical support for the mechanism design based on flag fluttering and overcoming the inherent limitations of the single flag in practical application.