Vortex Interactions in Tiny Insect Flight

Tiny insects flying at chordwise Reynolds numbers less than ~30 employ U-shaped wing-tip trajectories and the clap-and-fling mechanism to generate lift, yet the underlying aerodynamic mechanisms are not well understood. Here, we present 2D flow measurements around freely flying tobacco whiteflies (Bemisia tabaci) with body length of ~0.85 mm flying at speeds of up to 400 body lengths s^-1. We used an ultra-high speed brightfield micro-PIV system and 3D photogrammetry system to measure time resolved (10 kHz) flows and wing and body kinematics and analyze the circulation of the vortices formed throughout the wingstroke. During clap, a high-speed jet (500 mm/s) ejected from between the wings forms a viscous vortex ring that rapidly dissipates after t = 5 ms. We model this vortex as an impulsive Stokeslet. During the fling, the leading edge vortex from the previous half-stroke is preserved as it translates to the wing’s trailing edge, which may be an energy-saving mechanism. Finally, wing-tip vortices generated during the forward sweep are intersected and weakened by the wings rowing through them on a subsequent stroke, possibly indicating an energy-saving wing-wake interaction. These observations provide new insight into the energetics and impulsive nature of tiny insect flight.