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PUTTING THE SWERVE INTO SOCCER
Some of the world's greatest goalkeepers have been beaten by unusual swerving balls which move to the left and the right before hitting the back of the net, even though they have little or no spin applied to them. The new research has found that the shape and surface of the ball, as well as its initial orientation, is critical in terms of its trajectory through the air. A team of researchers, led by Dr. Matt Carré at the Department of Mechanical Engineering at the University of Sheffield, used the most advanced software, known as Computational Fluid Dynamics, for simulating the physics of airflows in and around objects. They studied and compared airflows around four balls, all with different panel designs, each having been used at different periods over the past 36 years. University PhD student and Sheffield FC player Sarah Barber, alongside Dave Mann, Principal Engineer at Fluent, used a 3D laser scanner, similar to those used in Formula 1 motor racing, 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, has a profound effect on how the ball moves through the air after it is kicked. The side force varies according to the orientation of the ball relative to its flight, meaning that for a kick where the ball is slowly rotating, the side force could fluctuate causing it to swerve. Ultimately the nature of the swerve is affected by the initial orientation of the ball before it is kicked. In collaboration with Dr. 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 one to be used in the World Cup this summer, 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. Commenting on these new findings, Dr. Carré said: "Our work clearly points to the fact that any non-uniformity of design of soccer balls, or asymmetry of manufacture, will have a dramatic effect on the side forces of the ball when there's little or no spin applied to it, and hence its swerve through the air." "We believe that our findings go a long way to explain the phenomenon observed when some players kick the ball with little or no spin, yet get it to swerve in a seemingly erratic manner, possibly producing an 'S' shape trajectory." Sarah Barber added: "As a soccer player I feel this research is invaluable in order for players to be able to optimise their kicking strategies. This knowledge could further be utilised by manufacturers to design future balls which will ultimately enhance the overall experience for players and spectators at all levels of the game." Commenting on all these studies from around the world, Dr. H. Ferit Boysan, Vice President and General Manager at ANSYS, suggests that: "It is becoming more and more obvious that the aerodynamic performance of a soccer ball is very closely linked to its design and manufacture as witnessed by these initial Sheffield studies. Dr Carré and Dr Vogel's work is clearly pointing towards modern computer based simulation techniques permitting us to model any kick, of any ball, in any stadium, such that we will be able to get a prediction of what will happen before a ball is even kicked! This will open up whole new directions for the design and development of soccer balls as well as stadia."
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