Auto-Rotation on Short Axis of Rugby Ball

摘要

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.