A fundamental and applied study on fluid science and engineering for sports

为了分析足球在实际飞行中的关节效应，利用高速摄像和烟雾发生剂对非定常尾流结构进行了实验研究。实验数据表明，升力和侧向力的脉动频率与尾流振荡频率之间存在着密切的相关性（r= 0.94，p <0.01）。研究结果表明，尾流中的大尺度涡波动现象是由作用在关节球上的升力和侧向力的非定常特性引起的，这可能与边界层分离的异常不稳定性有关。

In this study, to analyze the knuckling effect of a soccer ball in actual flight, the characteristics of the unsteady wake structure were experimentally investigated by using high-speed video images and a smoke-generating agent. The experimental data indicate that the frequency of fluctuation of lift and side forces and the frequency of wake oscillation are closely related to each other with a statistically high correlation (r=0.94, p<0.01). From these results, the present study concludes that the phenomenon of large-scale vortex undulation in the wake is caused by the unsteady characteristics of the lift and side forces acting on a knuckleball, which might be related with the extraordinary instability in the separation of boundary layers.