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，采用高速视频图像和产烟剂对非定常尾迹结构特性进行了实验研究。实验数据表明，升力和侧力波动频率与尾迹振荡频率密切相关，具有较高的统计学相关性（r=0.94, p<0.01）。根据这些结果，本文认为尾迹中大尺度涡旋波动现象是由作用于指节球的升力和侧力的非定常特性引起的，这可能与边界层分离时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.