Unsteady Flowfields Characteristics Around Two- and Three-dimensional Flapping Flight

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 blowflys tethered flight under freestream. 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 due to translation and lagging motion. On the other hand, a large amount of thrust is generated at the end of upstroke motion. Furthermore, analyses on three-dimensional flapping motions are performed to examine the three-dimensional counterparts of vortex formation and unsteady force generation mechanism. Three–dimensional numerical results show the existence of span-wise vortical flow that prevents the buildup of vorticity separating from the leading edge of the wing, and safely deposits it into a tip vortex. Vortical structure in the wake and the pressure field shows that the vortex pairing and vortex staying mechanism, which are firstly observed in two-dimensional flapping motion, are also observed in three-dimensional flapping motion. Consequently, they can be presented as a strong evidence for the abrupt large thrust generation.