Vortex Lift Contribution in Flapping Flight

Abstract

Using steady state aerodynamic theories, it has been claimed that insects and birds cannot fly. To make matters worse, insects and birds fly at low Reynolds numbers. Therefore, a recurring theme in the literature is the importance of understanding unsteady aerodynamic effect and how the vortices behave when they separate from the moving surface that created them. In flapping flight, birds and insects can modify wing beat amplitude, stroke angle, wing planform area, angle of attack, and to a lesser extent flapping frequency to optimize the generation of lift force. An important point to solving the mystery of insect flight is understanding vortex shedding and how the vortices behave when they separate from the moving surface that created them. Insects depend on vortices to keep them aloft, especially when they are hovering. Vorticity is highly dependent on animal size, wing form, and flight speed and kinematics. To minimize energy expenditure during flight the birds should optimize the generation of the vortex wake. Some birds are thought to employ two different gaits(a vortex-ring gait and a continuous-vortex gait) and unsteady aerodynamic effect(Clap and fling, Delayed stall, Wake capture and Rotational Circulation) in flapping flight. Leading edge vortices may produce an increase in lift. The trailing edge vortex could be an important component in gliding flight. Using a mechanical ornithopter with wings fabricated in-house, vortices were identified at several different locations along the span of the wing, and at numerous different points throughout the flapping cycle under a variety of operating conditions. The lift generated by these spanwise planar oriented vortices was quantified experimentally using Digital Particle Image Velocimetry. Thus, this study investigated how vortices generate lift in flapping motion.