2402 ラグビーボールの短軸まわりにおける自励回転運動(S43-1 スポーツ流体工学(1),S43 スポーツ流体工学)  [in Japanese] 2402 Auto-Rotation on Short Axis of Rugby Ball  [in Japanese]

It has been observed in the wind tunnel test that a rugby ball, which can rotate freely in its short axis, starts auto-rotation when the ball is placed with the lace part against the wind tunnel flow ; the rotational speed increases rapidly to reach a constant rpm in a few seconds. The present work is intended to clarify the mechanism of the auto-rotation in the short axis of a rugby ball. In order to investigate the phenomenon, we have carried out wind tunnel tests involving measurement of rotational speed and aerodynamic forces, and flow visualization by oil flow. Experimental results reveal that the seam lines on the ball induce the boundary layer transition, and the resulting asymmetrical distribution of air pressure is attributable to the auto-rotation of the rugby ball.