Why fluid dynamics matters to football

Ahead of the Euro 2021 semi-final, we take a look back at football controversy back in 2010...

“The football is horrible,” said Julio Cesar, the Brazil goalkeeper. “It is like one of those you buy in the supermarket.” Fluid dynamics and football may seem to have little in common but in the 2010 World Cup goalkeepers from around the world united to criticise the flight of the Jabulani football. David James, playing for England said: “The ball is dreadful, It’s horrible.”

The goalkeepers were up in arms over the unpredictable flight of the new ball introduced by Adidas for that world cup. Rather than following a predictable path like previous balls, the Jabulani was liable to dip and swerve erratically, making it a nightmare for goalkeepers to stop and for outfield players to pass accurately.

This unpredictable flight was due to the new design of the Jabulani. Designed with extensive aerodynamic testing, the ball was a further step forward from the 2006 world cup ball, which instead of having the traditional 32 hexagonal panels was made from only 14 curved panels, in order to smooth the surface and reduce drag. The Jabulani ball pushed this further, having only 8 spherically moulded panels, the smoothest football ever, and was designed in collaboration with Loughborough University.

However, the designers had been caught out by the tricky nature of aerodynamics. While the new ball had reduced drag, at the same time it had become susceptible to the “knuckling effect” at key moments. This effect, seen in other sports such as baseball, occurs when the ball is not spinning, and the seams interact with the airflow to produce different flow on each side of the ball. This results in a force to one side or the other, leading the ball to move around unpredictably.

While previous ball designs experienced this knuckling effect around 30 miles per hour, the new Jabulani ball tended to knuckle around 45-50 miles per hour: about the speed of a world cup free kick. This led to an increase in errors from both goalkeeper and striker, as neither could predict whether the ball would dip left or right, up or down.

The outrage and criticism led Adidas to make the next ball they designed, the Bracuza, rougher by adding dimples to the surface. These work by encouraging the flow near the surface of the ball to become turbulent at a lower speed, meaning the ball no longer swerved unpredictably from a free-kick or a long pass.

Since this change, the players have been much happier and it seems like Adidas have managed to get a handle on the aerodynamics of a football. However, the controversy caused by the Jabulani ball in 2010 serves as a reminder that the complex, chaotic nature of fluid dynamics can cause headaches even in unlikely areas.

Written by CDT student Daniel Richards.