Inherent aspects of root flapping tandem wing arrangements in nature: forewing – hindwing interactions in dragonfly flight

Speaker :
Department of Mechanical and Aerospace Engineering, HKUST
Date : 15 Jan 2019 (Tue)
Time : 9:00 am
Venue : Room 1511 , HKUST (1/F., Lift #27/28)


Flying in nature has inspired artists and engineers since ancient times. Scientific interest toward the characterization and exploration of how birds, bats and insects fly gained significant technical relevance with the appearance of micro air vehicles (MAVs) that operate in the same size region under the same environmental conditions as natural flyers. Insects are a prime target for bioinspired designs of MAVs, as their wingroot fixed musculature is simpler than the flight muscles in the wings and shoulders of birds and bats. Dragonflies are among the most aerobatic insects possessing unique features, such as the long aspect ratio wings in tandem, streamlined body with a highly mobile abdomen, and a concentrated body mass at the insect’s center of gravity. Dragonflies control individual wings, giving them the ability to utilize wake elements of the forewings by their hindwings to generate additional aerodynamic forces. This thesis contains my effort to characterize the unique interactions between the wings in live dragonfly specimens. I use a spanwise resolved approach to address the gradual change of wing geometric relations in the root fixed flapping wing system. With the use of in-vivo flow measurements two characteristic regions with distinct flow features, and two transient regions delimiting these, were identified. Additionally, the dimensionless arc length was introduced to describe the effect of interaction in the wings spanwise direction. This parameter includes not only the phasing relations of the wings, but the ventral or dorsal shift in a wing’s flapping, thus it is more generally applicable. The modulation of the circulation of the hindwing leading edge vortex (LEV), caused by the interaction, reasonably correlates with the dimensionless arc length, that proves the applicability of this parameter. Additionally, the flight direction was found to affect the inter-wing interactions.

Secondly, a bioinspired multilayer wing is proposed and tested to generate lift with simple flapping motion in MAVs. The wing takes up a different shape during the downstroke and during the upstroke; that put simply, mimics the load direction dependent flexible deformation of dragonfly wings. The wing also generates an additional trailing edge vortex between its separating layers during upstroke that can boost thrust generation. The characteristic parameter defining the performance of the double layer wing is the difference between the dynamic shape deformation during the upstroke and the downstroke. It relates to the ratio of the chord length of the wing’s layers. A chord ratio of 0.5 resulted in the best performance.

The results of both the characterization of inter-wing interactions in dragonflies, and the multilayer wing concept could contribute to the design and development of agile and efficient MAVs. I hope that the findings summed up in this thesis will inspire further scientific research on dragonflies as well as MAV engineering.

(Supervisor: Prof. Huihe Qiu and Prof. Wei Shyy)