Experimental study of two-dimensional flapping wings in tandem configuration

Abstract

This paper reports a fundamental investigation on the effect of phase difference (φ) between the flapping motion of forewing and hindwing on the lift and thrust generation of two-dimensional (2D) tandem wings in a forward flight condition at a Reynolds number of 5,000. Force sensor was used to measure time-dependent aerodynamic forces acting on the two wings, and digital particle image velocimetry (DPIV) technique was employed to obtain the associated vorticity field and flow structures. Three cases of phase difference were studied, namely 0° (in-phase), 90° and 180° (anti-phase). The results reveal that: (a) the cycleaveraged lift and thrust coefficient of the forewing are higher than the corresponding values of a single flapping wing for all the three cases; (b) the cycle-averaged lift coefficient of the hindwing is approximately the same as that of the single wing and is relatively independent of the phase difference; (c) the thrust coefficient of the hindwing decreases with increasing phase difference, and except for the case of φ = 180°, they are higher than that of the single wing; (d) the maximum thrust coefficient that occurs on the hindwing during in-phase stroking is 85.4% higher than that of a single wing case. Also, it is found that when the leading edge of the hindwing interacted with the wake structures of the forewing, the thrust increased rapidly. In the absence of such interaction, especially for the case of φ = 180°, the leading edge vortex (LEV) on the hindwing interacted destructively with the wake structures of the forewing, leading to a reduction in force generation compared to that of a single flapping wing. This finding supports previous computational modeling studies that the timing of vortex-vortex interaction plays a crucial role in the overall force generation of the hindwing.