Numerical Study on the Unsteady-Force-Generation Mechanism of Insect Flapping Motion

Detailed numerical simulations are conducted to investigate aerodynamic characteristics of unsteady-force generation by a two-dimensional flapping motion under a forward-flight condition. A realistic wing trajectory called the figure-eight motion is extracted from a blowfly's tethered flight under freestream. Computed results show complex vortical flowfields that exhibit very interesting and distinctive unsteady characteristics. Lift is mainly generated during downstroke motion by a high effective angle of attack due to translational and lagging motion. On the other hand, a large amount of thrust is abruptly generated at the end of upstroke motion. Vortical structure in the wake and the pressure field shows that vortex-pairing and vortex-staying mechanisms can be presented as strong evidence for the abrupt large thrust generation, which is fundamentally different from the inverse Karman vortex that is used to explain thrust generation by sinusoidal oscillating airfoil. Additional numerical simulations are conducted to examine the effects of motion components of figure-eight motion. From numerical results and comparisons, it is observed that wing rotational motion at the end of upstroke is crucial in generating the pairing and staying of vortices, which eventually leads to the abrupt thrust generation.