Numerical Study on the Unsteady Aerodynamic Force Generation Mechanisms of Two- and Three-dimensional Insect Wing Motion

Abstract

This paper investigates aerodynamic characteristics of unsteady force generation around two- and three-dimensional flapping motions under forward flight condition. A realistic wing trajectory, called the "figure-of-eight" motion, is extracted from a blowfly's tethered flight under a free stream condition. In order to find out the tendency of vortex generation and flow pattern, two-dimensional simulations are firstly conducted. Computed results show vortical flow fields which exhibit very interesting and distinctive unsteady flowfields characteristics. Lift is mainly generated during downstroke motion by high effective angle of attack. On the other hand, a large amount of thrust is generated at the end of upstroke motion. Furthermore, analyses on three-dimensional flapping motion are performed to examine vortex formation and unsteady force generation mechanisms. Three levels of grid density are employed to simulate accurate flapping motion. Three-dimensional numerical results show the existence of span-wise vortical flow that prevents the buildup of vorticity separationg from the leading edge of the wing, and safely deposits it into a tip vortex. Vortical structure in the wake and the pressure field reveals that vortex pairing, which has been observed in two-dimensional counterpart, can be presented as a strong evidence for the abrupt large thrust generation.