The Aerodynamic Characteristics and Flow of Soccer Balls

In recent years, it has become well known that performance in sports involving balls or projectiles is significantly influenced not only by body movements but also by the aerodynamic and flight characteristics of these objects. This review paper examines recent developments in the drag coefficients of soccer balls. It demonstrates that the critical Reynolds number of the Al Rihla (used in the 2020 FIFA Qatar World Cup) is smaller than that of the Jabulani (used in the 2010 South Africa World Cup). Additionally, considering the unsteady nature of free flight in actual ball kicks, the relationship between the side force coefficient and the spin parameter is explored. Findings suggest that as flow velocity decreases, the spin parameter increases while the side force coefficient decreases. Furthermore, in scenarios where the ball is in a non-spin (low-spin) state during flight, such as in a knuckle kick, changes in the vortex structures in the ball’s wake are observed. These changes in vortex structures are believed to cause fluctuations in aerodynamic forces, resulting in the ball wobbling or dropping. Moreover, using an optical 3D motion capture system, the inclination angles of the foot relative to the frontal plane during ball impact in instep kicks, curve kicks, and knuckle kicks are analyzed. The results show that the inclination angle is smallest for instep kicks, followed by knuckle kicks, and largest for curve kicks.