Aerodynamics of dragonfly flight: impact of wing kinematics and morphology

Xiaohui Liu
Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong Special Administrative Region of China

Date(s) : 14/04/2021   iCal
9 h 00 min - 10 h 00 min

Micro air vehicles (MAVs) are operated in the size region under the same environmental conditions as natural flyers. Insects are a good teacher for bioinspired designs of MAVs, and dragonflies are highly aerobatic insects having high aspect ratio wings in tandem. The forewing and the hindwing of a dragonfly have different geometry that could be an evolutionary specialization for better aerodynamic performance via sophisticated wing pitch control. Firstly, the dragonfly wing surface structure is reconstructed with FTP, and the wing deformation under two different wing speed are measured. It shows that dragonfly wing is corrugated over the whole surface, especially in the root and leading-edge region. Compare with forewing, the hindwing is easier to occur chord deformation due to larger chord length. Secondly, we measured the flow around the flapping wings using time-resolved particle image velocimetry (TR-PIV) to investigate the consequences of shape and the pitching mechanisms of the wings on the aerodynamics of dragonflies. The flow fields and pitching angle variations of the naturally actuated wing of the dragonfly were compared with that of the same wing artificially actuated only by flapping motion. We found that the trailing edge vortex dynamics and the wake were affected by the wing shape only for the in-vivo experiment with muscle induced pitching. Under the in-vivo with muscle induced pitching, the hindwing took more part in generating horizontal momentum with larger pitching magnitude, due to the larger chord length compared with forewing. Meanwhile, when there was only pitching due to the wing membrane deformation of artificially actuated flapping, a slight difference in the surrounding flow structures was found between the hindwing and the forewing, and the net flow in one period was reduced nearly to zero. Thirdly, we measured the kinematic parameters of the wings in two different flight modes (normal flight mode (NFM) and escape flight mode (EFM)). When the specimens switched from normal to escape mode the flapping frequency was invariant, but the stroke plane of the wings was more horizontally inclined. The flapping of both wings was adjusted to be more ventral with a change of the pitching angle that resulted in a larger angle of attack during downstroke and smaller during upstroke to affect the flow directions and the added mass effect. Noticeably, the phasing between the fore and hind pair of wings varies between two flight modes. It is found that the momentum stream in the wake of EFM is qualitatively different from that in NFM. The change of the stroke plane angle and the varied pitching angle of the wings diverts the momentum downwards, while the smaller flapping amplitude and less phase difference between the wings compresses the momentum stream.

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