Transition Mode of Two Parallel Flags in Uniform Flow

The coupled flapping of two side-by-side identical flags in uniform flow is observed experimentally and numerically. Besides the early reported stable, in-phase, and out-of-phase modes, a transition mode between the in-phase and out-of-phase modes is newly presented. Essentially different from the other modes, the flapping in this transition mode is no longer a single cycle motion. Especially, the FFT analysis of the displacement-time curve indicates that there are multiple peaks of frequency in this mode, of which the smaller value is comparable to the frequency of the in-phase mode and the larger one is close to that of the out-of-phase mode. Changing of the weights of different single cycle motions may explain the mode transition.