Effects of wings number on the flight performance of autorotating seeds

Some natural wind-borne plants spread their seeds in a mode of autorotation, for example, maple, pterocarya stenoptera, and tristellateia. However, these wind-borne seeds have different numbers of wings. The current work focuses on the effects of wings number on the flight performance of autorotating seeds. Experiment and numerical simulation are used to analyze the aerodynamic performance of tristellateia seeds with different numbers of wings. In the free fall experiments, it is found that reducing the wings number leads to larger wing loadings, larger stable descending velocities, and larger spinning rates. The growth rate of descending velocity and spinning rate are related to the wings number. However, as the number of clipped wings increases, the seeds are more likely to fall into unsteady rotation even free fall. Numerical simulations are used to analyze the flow field around the rotating tristellateia seeds. It finds that seeds with clipped wings have larger pressure difference between the upper and lower wing surfaces, which contributes to larger lift. Three kinds of vortex systems occur on the rotating seeds: the leading-edge vortex, the wing tip vortex, and the separated vortex. Seeds with different clipped wings present various vortex morphology and structure. A stable separated vortex ring dominates the stable autorotation of tristellateia seeds. However, an unstable separated vortex emerges on the seeds with less wings, which makes these seeds difficult to achieve stability.