Flow-structure interactions of two tandem inverted flags in a water tunnel

Abstract

The dynamic behaviors and vortex interaction modes of two tandem inverted flags at different gap distances and flow velocities are investigated experimentally in this paper. The results show that the straight, flapping, and deflected modes can still be observed with a successive increase in the flow velocity. The fixed-phase, different-frequency, changing-phase, alternating-phase, and aperiodic flapping modes are further found in the flapping mode. Considering the fixed phase difference and predominance of the fixed-phase mode, we mainly focus on this mode. The amplitude of the front flag is almost similar to that of an isolated flag due to the unaffected leading-edge vortex when they are both in the flapping mode, while it is smaller for the rear flag, indicating the great significance of the rear flag amplitude for studying the dynamic behaviors of two tandem inverted flags. First, the relation between the amplitude of the rear flag and the phase difference contains two linear segments with almost the same slope and a constant segment, fully demonstrating their strong correlation rather than the irrelevance. Second, the rear flag and the trailing-edge vortex (TEV) of the front flag are found to interact via the strong and weak interaction modes. Only in the strong interaction mode can the rear flag extract energy from the TEV of the front flag at lower critical flow velocity, resulting in a larger amplitude than that of the front flag. In general, a small gap distance is more conducive to energy harvesting at low flow velocity, while a large gap distance is more beneficial at high flow velocity.