Abstract
The purpose of this study was to discuss the fluid mechanics and frequency of forces acting on a non-spinning and low-spinning flight soccer ball, which called "Knuckleball", using high-speed VTR image of a real place kick. Moreover, the fluid around the knuckleball in flight was visualized using a smoke agent (titanium tetrachloride), and an attempt was made to clarify the fundamental fluid mechanics expressed by the knuckleball as well as to analyze the vortex dynamics. Therefore, in the case of the knuckleball, the peak value of the vortex lift reached approximately 2.0 N, which was larger than that of approximately 0.5 N in the case of an instep kick. Furthermore, the knuckleball was observed to have an average vortex lift force frequency of approximately 3.5 Hz. A comparison of this vortex lift frequency and vortex oscillation frequency indicated that these frequencies tended to act in unison with a high statistical correlation (r = 0.81, p < 0.001). Thus, it is considered that the oscillation of the large-scale vortex structures of the knuckleball was the fundamental mechanism that produced the fluctuating forces acting on the knuckleball in soccer.
