Particle image velocimetry measurements of the flow structures induced by the free-falling spin flight of maple seeds

Abstract

Most measurements of flow field caused by a rotating maple seed were performed in a vertical wind tunnel limited to fixing the vertical axis to simulate the stable falling spin period. In the nature, however, the free-falling motion of maple seeds induces airflow, which makes the seeds spin and generates lift to reduce falling speed. In this study, the flow structures around free-falling Japanese maple seeds in spin are measured using particle image velocimetry. In general, the free-falling spin flight of a seed is divided into three stages: transition period I, transition period II, and the stable period. In transition period II, it can be observed that the sizes of wingtip and root vortices increase with decreasing coning angle of the seed wing and result in an increase in seed rotation speed and decrease in free-falling speed. We also find that the flow structures induced by the free-falling motion of the maple seed in the stable period are different from those that suspend the rotation seed in a vertical wind tunnel. The leading-edge vortex (LEV) and trailing-edge vortex (TEV) are observed above the front of the leading edge and at the top back of the trailing edge, respectively. The position of the LEV core is located above the front of the leading edge, and it gradually approaches the leading edge of the seed along the spanwise direction, toward the wing tip. As the seed enters the stable period, the root vortex shrinks, and the tip vortex grows.