Flow visualization of dragonfly hovering via an electromechanical model

Here, the first physical images revealing the integral How structures and their evolutions of dragonfly hovering are presented, based on the dye flow visualization conducted on an electromechanical model in water tunnel. Together with the quantitative information derived from the images, several findings were obtained: 1) spanwise flow was conspicuous regardless of the large aspect ratio of the wing; 2) the leading-edge vortex contained dual-vortex structure: the primary vortex did not develop along the leading edge but moved inboard, leaving a space for the formation of a same-sense minor vortex outside the primary vortex; 3) the development of the leading-edge vortex showed a delay with respect to the translational motion of the wing; and 4) in most cases with forewing-hindwing interaction, the interaction was generally detrimental to the leading-edge vortices and was attenuated with the increase of the wing-root spacing.