CFD puts a spin on free kicks

Last month saw the results of pioneering work taking place at the University of Sheffield in collaboration with CFD software developer Fluent Europe – into swerving footballs. A team of researchers, led by Matt Carré at the Department of Mechanical Engineering at the University of Sheffield, used Computational Fluid Dynamics (CFD) software to compare airflows around four different types of football, including the Adidas ‘Teamgeist’ ball being used in this tournament. University PhD student and Sheffield FC player Sarah Barber – alongside Dave Mann, principal engineer at Fluent – used a 3D laser scanner to obtain accurate surface detail of each individual ball, including their stitches and seam patterns. They demonstrated that the shape, surface and asymmetry of the ball, as well as its initial orientation, had a profound effect on how the ball moves through the air after it has been kicked. The side force varies according to the orientation of the ball relative to its flight. For a kick where the ball is slowly rotating, the side force could fluctuate – causing it to swerve. In collaboration with Takeshi Asai at the University of Tsukuba in Japan, the team used wind tunnel measurements to verify their CFD studies and demonstrated that in match conditions the drag of non-spinning soccer balls has fallen by as much as 30% over the last 36 years. Newer balls, like the ones being used in the World Cup, which manufacturers claim to be rounder and which have more uniform seam geometry, have been found to be more consistent in high speed kicks with little or no spin. Click on the weblink for an airflow pathline animation.