Aerodynamics of dragonfly flight : impact of wing kinematics and morphology

Abstract

Dragonflies are aerobatic insects with unique physiological features that help them to exploit highly unsteady aerodynamic effects in the low Reynolds number regime. Such aerodynamic effects include added mass effects, flow separation that results in the formation of wing attached vortexes, as well as wake capture and forewing-hindwing interactions. 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. In this work the complex dynamic wing morphology and kinematic control of dragonfly aerodynamics are studied experimentally. First, the dragonfly wing surface is reconstructed, and the wing deformation is measured in steady wind conditions with Fourier transform profilometry (FTP). It shows that a dragonfly wing has a complex surface and the wing deformation caused by the wing flexibility has less effect on gliding flight. Second, the flow fields using time-resolved particle image velocimetry (TR-PIV) and pitching angle variations are measured to study the pitching effect and morphology of wings of a live dragonfly versus motor actuated wings. Results show that pitching employed by the live dragonfly helps thrust generation, and the different wing planforms of the hind and forewing affect local aerodynamic performance along the span. Last, the wing kinematics and resulting flow fields of live dragonflies are studied in two distinct flight modes, normal forward flight, and escape flight. It is found that a dragonfly performs active pitch control, and to generate fluid momentum for an impulsive escape maneuver, the dragonfly does not change its flapping frequency, but controls the angle of attack of its wings via pitch and stroke angle adjustment, while the flapping phasing and stroke amplitude of the wings decreases. The findings presented in this work can be considered for the bio-inspired design of Micro Air Vehicles operating in the low Reynolds regime as natural flyers.

Date of Award	2021
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Huihe QIU (Supervisor) & Wei SHYY (Supervisor)