Abstract
As configurations of soccer balls and volleyball balls evolves in recent years, techniques to produce a non-rotating or low-rotating shot/serve with an unexpected orbit have been established. This so-called knuckle-ball effect of the shot/serve is assumed to be caused by instability of the fluid force of the vortex which occurs behind the moving ball. The This study explains the correlation between the fluctuation of fluid forces and that of a ball wake. Moreover the flow visualization on a ball wake was performed to confirm the affection against the fluid force of the ball.
