Experiment of the Flapping Coupling Modes between Two Parallel Flags in a Aniform Flow
均匀来流条件下并行排列旗帜耦合运动模式的实验

Experiment of two parallel flags in a uniform flow was conducted in a low speed wind tunnel, in which a high speed camera was used to record the kinematic of the two flags with different distances and flow speeds. Coupling modes of the flags, the flapping frequency and amplitude, and the corresponding non-dimensional frequency \%St\% were studied in terms of a self-developed space-time evolution (STE) method. Experimental results show that there are four possible coupling modes: stable, in-phase flapping, out-of-phase flapping and transitional modes for different flow speeds and distances. When the two flags flap in phase, the flapping frequency is much lower than that of a single flag in the same flow; while in the out-of-phase mode, the flapping frequency is much higher. Farther more, in the transitional mode the amplitudes of the two flags change alternately and there are two peaks of frequency in the FFT(Fast Fourier Transform) analysis, while either the in-phase or the out-of-phase modes only contains one peak value.