Wind tunnel experiments have been conducted to investigate the knuckle effect of a soccer ball. In the present experiment, we measured the surface pressure of a ball as well as aerodynamic forces, and made tuft grid visualization to investigate wake structure behind a ball. We are in particular interested in the seemingly random nature of the knuckle effect resulting from the movement of vortical structure behind a ball in the super critical Re range and in the detailed mechanism causing the knuckle effect. Discussion will also refers to knuckle balls of baseball and volleyball.

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