A time-resolved PIV study on the force dynamics of flexible tandem wings in hovering flight

To uncover the mechanisms how the flexibility and phase difference affect the lift dynamics of flexible tandem wings in hovering flight, aerodynamic forces, wing deformation and time-resolved particle image velocimetry (PIV) measurements were carried out. Three sets of dragonfly-like wing models, with a set of rigid wings as the reference, were investigated in single configuration and tandem configurations with phase differences Ψ = 0° , 90 ° and 180°. In both single and tandem cases, significant bending deformation was found to cause the lags of wing locations in plunging direction between the flexible and rigid wings. In single configuration, the Wing III model with appropriate flexibility obtained stronger LEVs compared with the rigid wing due to the lags. Moreover, the shedding of LEV on the Wing III was also restrained slightly by the lags. These effects resulted in different lift dynamics of Wing III and rigid wing. In tandem configurations, the flexibility not only caused the lags but also the modifications in the relative positions of forewing and hindwing. The lags and modified relative positions affected the interactions of the tandem wings, and thus the lift dynamics on the hindwings mainly in two ways: (1) The flows induced by fore-Wing III and rigid wing showed different effects on the strength and location of LEVs and TEVs on the hindwings. (2) The modifications in the relative positions resulted in quite different interactions of LEV on the hindwing and TEV on the forewing for the tandem Wing III and rigid wings, especially in 90° and 180°-phased tandem configurations. Finally, remarkably different lift dynamics were obtained by the tandem Wing III and rigid wings at different phase differences.