Computational Investigation of Three-dimensional Unsteady Flowfield Characteristics around Insects' Flapping Flight

Abstract

This paper investigates the unsteady flowfield characteristics around three-dimensional insect flapping motion under forward flight conditions. A realistic wing trajectory, called the figure-of-eight motion, is extracted from a blowfly's (phormia regina) tethered flight experiment under a freestream velocity of 2.75 m/s. In the authors' preliminary research [Lee, J., Kim, J., and Kim, C., Numerical Study on the Unsteady-Force-Generation Mechanism of Insect Flapping Motion, AIAA Journal, Vol. 46, No. 7, 2008, pp. 1835―1848. doi:10.2514/1.35646], the two-dimensional blowfly's wing motion was computationally investigated, and the results revealed very interesting and distinctive vortical flowfields, which provide a decisive clue in understanding the rapid maneuverability of insects' flight. On the line of continuous efforts, the present work investigates the role of three-dimensional vortical structure in unsteady aerodynamic force generation. Detailed numerical simulation and analysis on three-dimensional flapping motion are conducted, and interesting flow features of insects' flapping flight are observed, such as the existence of a spanwise flow component, leading-edge vortex, wing tip vortex, trailing-edge vortex, and various forms of vortex tubes and vortex rings. It turns out that vortical structures play a crucial role in determining unsteady characteristics of lift and thrust generation. In particular, the vortex pairing and vortex staying phenomena, which have been observed in two-dimensional flapping motion, are also observed but with a more complicated pattern. On top of that, distinctive lift and thrust generation is observed owing to three-dimensional wing shape, trajectory, and vortex structure. Consequently, the results of the present work provide an important clue in understanding generation of aerodynamic force for rapid maneuverability in insects' flight.