Flow Visualization on a Real Flight Non-spinning and Spinning Soccer Ball

Abstract

Wind tunnel test with a full size non-spinning soccer ball was carried out to measure the aerodynamic forces (drag, lift and side force). The drag coefficient of a non-spinning soccer ball in the subcritical regime was nearly 0.43 and that of in the supercritical regime was nearly 0.15. It is estimated that the critical Reynolds number of a soccer ball is about 2.2×105. In the visualization experiment using titanium tetrachloride, we compared the flow around the ball during a non-spinning, low-speed kick (5 m/s) and a high-speed kick (29 m/s). During the low-speed kick, the boundary layer separation point was about 90 degrees from the front stagnation point, while during the high-speed kick the separation point had receded to about 120 degrees from the front stagnation point.