Flow Visualisation Experiments on Sports Balls

AbstractFluid dynamics plays a significant role in many sports, principally affecting the trajectory of the associated ball. Boundary layer theory can be used to explain why some of these effects take place, demonstrated here for the games of cricket and golf.The asymmetric nature of a cricket ball, due to the presence of a seam, causes the boundary layer to be tripped into turbulence on one side. On the other hemisphere, the smooth surface promotes laminar flow which separates at a smaller angle relative to the stagnation point. This results in a net pressure force and lateral movement known as swing. In golf inverted dimples are applied to the ball to reduce drag by promoting transition to turbulent flow, this in turn increases the maximum achievable range.In this study, scaled versions of a smooth sphere, a cricket ball and a golf ball were used to perform wind tunnel experiments in which these fluid dynamic effects were demonstrated. A novel infrared flow visualisation technique, in conjunction with measurements of pressure, highlighted the fluid mechanics at the representative conditions found in each sport. The results underlined the dependence on surface roughness, and provided qualitative visual evidence of the state of the boundary layer at a Reynolds number of 1 x 105