Understanding the aerodynamics of a spinning soccer ball

The aerodynamic properties of a generic soccer ball (football) under the influence of various velocity and spin conditions was first measured using wind tunnel experiments and the conditions were also simulated using Computational Fluid Dynamics (CFD) on a non-spinning ball of exactly the same geometry. For the wind tunnel experiments, a third scale model of a generic soccer ball (complete with a typical seam pattern) was used as well as a 'mini-football' (roughly half the size of a standard soccer ball). It was found that as the wind speed was increased, the drag coefficient decreased from 0.5 to 0.2, suggesting a transition from laminar to turbulent behaviour in the boundary layer. For spinning balls, the Magnus effect was observed and it was found that reverse Magnus effects were possible at low Reynolds numbers. Measurements on spinning smooth spheres found that laminar behaviour lead to a high drag coefficient for a large range of Reynolds numbers and Magnus effects were unstable, but generally showed reverse Magnus behaviour at high Reynolds number and spin parameter. The CFD simulations of the generic soccer ball revealed details of the near-ball air flow field and complex flow separation zones. Good agreement with experimental drag measurements from the wind tunnel was achieved for both fully turbulent and fully laminar flow conditions for the non-spinning ball. Further CFD studies are suggested to delineate the underlying spinning ball flow features.