PIV experimental study on vortex structures induced by free autorotation fall of a samaras
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摘要: 本文利用粒子图像测速技术研究种子叶片自由旋转下落过程中不同涡系的相互作用机理。以典型枫树种子叶片为研究对象，通过对比不同叶片长度、叶面厚度、叶面宽度、下落锥角、自旋角速度、下落速度和下落初始角度等参数对过渡期和稳定期的影响，分析了诱发叶片自旋的外形特征和空间特性。结合特征参数分析，对叶片自旋稳定期开展了PIV流场测量实验，解析了无干扰下种子叶片下落过程中涡系的产生和演化机理。实验结果表明：稳定期气流在叶尖正面位置产生前缘涡（沿展向呈圆锥状结构），后缘位置产生反方向的后缘涡；两个涡发生相互耦合运动，前缘涡的强度大于后缘涡，从而导致叶面产生锥角。在前缘和叶尖前方观测到较高的速度向上的区域，而在后缘和叶根附近则出现较高的速度向下的区域，从而对种子产生向上的升力，使叶片实现自旋稳定下落。通过枫叶种子自由下落的无干扰PIV测量，初步获得了贴近叶片表面前缘涡的运动性状，验证了后缘涡的存在，结论对单翼型旋转叶片的设计有一定指导意义。Abstract: 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.
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Keywords:
- maple samara /
- free autorotation fall /
- vortex /
- LEV /
- PIV
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图 1 带有丰富叶脉结构的果叶及自旋下落俯视状态
Fig. 1 Maple samaras with vein structure and the top view of auto-rotation fall
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图 2 测量种子自旋下落的实验装置和参数
Fig. 2 Experimental setup and measurement methods for free auto-rotation fall of maple samaras and parameters
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图 4 枫树种子横向断面的瞬时流场结构
Fig. 4 Instantaneous flow structures on the cross section of maple samara
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图 5 枫树种子展向断面的瞬时流场结构
Fig. 5 Instantaneous flow structures on the span section of maple samara
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图 6 种子横向断面的瞬时流场结构
Fig. 6 Instantaneous flow structures on the cross sections of maple samara
表 1 枫树种子平均外形特征参数统计
Table 1 Average shape characteristic parameters of maple samaras
类型 叶弦长
L/mm叶面最大宽度
b/mm叶面平均厚度
k/mm果实厚度
t/mm种子质量
M/mg小型 17.3 5.7 0.2 3.8 47.0 中型 20.0 6.7 0.2 4.0 49.3 大型 22.7 8.7 0.2 4.0 52.0 
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表 2 典型中型枫树种子表面平均特征参数
Table 2 Average parameters of typical surface characteristics
叶弦长
L/mm平均叶脉
数量叶面平均
厚度k/mm叶凹凸幅度/
mm叶凹凸高度/
mm前缘厚度/
mm20.0 70 0.2 0.15 0.18 0.53
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表 3 枫树种子在稳定期中自旋运动的平均特性参数
Table 3 Average parameters of maple samaras in falling
类型 雷诺数
Re下落锥角
β/（°）下落速度
V/（m·s–1）自旋角速度
ω/（rad·s–1）过渡期距离/
cm小型 19 300 16.20 1.12 45.00 32.00 中型 22 510 22.61 1.13 34.14 33.75 大型 21 480 26.45 0.95 19.00 33.00
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