Muller, U.K.*; Stamhuis, E.J.; Ellington, C.P.: Quantifying the leading-edge vortex of a hovering robotic insect during the downstroke

A hovering hawkmoth Manduca sexta generates a stable leading-edge vortex during the downstroke. To study the flow in greater detail, a scaled-up robotic model that mimics the wing movements was built (the flapper). The flow around its wings was visualised with neutrally buoyant soap bubbles and mapped using three-dimensional particle tracking velocimetry. Our recordings confirm earlier findings obtained from qualitative smoke flow visualisation (van den Berg and Ellington, 1997): a conical leading-edge vortexdevelops during the downstroke, with a strong spanwise component from the wing base to the wing tip. At 75% wing length, the leading-edge vortex bends towards the trailing edge and feeds into the tip vortex. While the vortex is still attached and close to the leading edge its diameter increases five-fold and the spanwise flow accelerates from 1 to 5 ms^-1. Computational flow fields around a hovering hawkmoth (Liu and Kawachi, 1998) suggest that late in the downstroke a second leading-edge vortex develops with a spanwise flow towards the base rather than the tip of the wing. This vortex was not observed in the previous flow visualisation study (van den Berg and Ellington, 1997) or in the present experiments, which employed a smoother wing surface, leading edge, and camber. References: Van den Berg C. and Ellington C.P. (1997) Phil. Trans. R. Soc. Lond. B 352, 329-340. Liu H. And Kawachi K. (1998) Journal of Computational Physics 146, 124-156.