This paper is pretty awesome. The authors take a commonly observed yet mysterious phenomenon and break it down with some hard-core physics. Turns out that the slow spin seen when a soccer ball flies through the air doesn’t depend on how the ball is kicked, because the authors observe the same behavior when the ball is shot out of a machine, or even when dropped straight down. They go on to do a series of experiments, including several involving shooting soccer balls through smoke, and determine that the seams on the surface of the ball are required for the spin, although different sewing patterns don’t seem to matter (pun intended).
The strange flight behaviour of slowly spinning soccer balls.
“The strange three-dimensional flight behaviour of slowly spinning soccer balls is one of the most interesting and unknown phenomenon associated with the trajectories of sports balls. Many spectators have experienced numerous exciting and emotional instances while observing the curious flight behaviour of these balls. We examine the aerodynamic mechanisms of erratic ball behaviours through real flight observations, unsteady force measurements and flow pattern visualisations. The strange behaviour is elucidated by the relationship between the unsteady forces on the ball and the wake flow. The irregular changes in position for twin longitudinal vortices have already been discovered in the supercritical Reynolds number region of a sphere with a smooth surface. This finding is applicable to the strange behaviour of the flight of soccer balls with this supercritical flow. The players, spectators, and television viewers will gain greater insight into the effects of soccer ball flights.”
Bonus quote from the full text:
“The results of the free-fall, ball shooting, and wind tunnel experiments indicated that the seams on the soccer ball surface promoted the transition of boundary layer from laminar to turbulent flow and yielded the supercritical Re number flow, accompanied with the Ω vortex and the twin longitudinal vortices around soccer balls. Thus, the random behaviour of the slowly spinning soccer ball’s flight is mainly caused by the unstable movements of these vortices.
Incidentally, strangely behaving volleyballs that undergo floater serves exhibit the same type of moving behaviour and aerodynamic as that of soccer balls. Although scientific viewpoints regarding similar types of sports ball phenomena have generally been ignored in the literature, this finding may evoke scientific interests in sports science.”
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