PIV experimental study on vortex structures induced by free autorotation fall of a samaras

Taking the typical maple samara blade as the research object, the flow field of the free rotation and falling process is measured by the particle image velocimetry, to study the evolution and the law of the spin flow structure, which has certain guiding significance for the design of the single-wing aircraft. By comparing the effects of different blade lengths, thicknesses, widths, falling angles, spin angular velocities, falling velocities, and different falling attitudes on the transition period and stability period, the shape and spatial characteristics of the induced blade spin are obtained. Combined with the results of characteristic parameter analysis, PIV flow field measurement experiments are carried out for the period of blade spin stability, and the rules of vortex generation and evolution in the process of falling are obtained. The experimental results show that the leading-edge vortices (conical structure along the spanwise direction) are generated at the front tip during the stable period, the trailing edge vortices in the reverse direction are generated at the back-edge, and the two vortices are coupled with each other. The strength of the leading-edge vortex is greater than that of the trailing-edge vortex, which leads to the angle of attack. In front of the leading edge and the tip of the leaf, a higher upward velocity region was observed, and a higher downward velocity region was observed near the rear edge and the root of the leaf, which resulted in an upward lifting force on the seed and a stable falling of the blade.ade.