A new measure of roughness to predict the aerodynamic performance of sports balls

The aerodynamic performance of sports balls in linked to the surface roughness of the ball, the Reynolds number and the spin parameter. The traditional way to characterize the surface roughness of spheres has been to use k/D where k is the typical roughness height and D the diameter of the sphere. Wind tunnel data were compared for roughened spheres, golf balls and a soccer ball, and it was found that the parameter k/D was not enough to make a prediction of either the critical Re or the minimum CD at transition. The surface geometries of the sports balls and spheres were analysed statistically and a measure of skewness calculated for each. A negative skewness indicates a profile in which peaks are dominant, while a positive value indicates that the profile is dominated by valleys. It was found that the skewness value could be used to accurately predict the critical Reynolds number for the range of sports balls studied, but a combination of k/D and shewness was propbably needed to predict the minimum CD for all types of roughness.