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News

Engineers discover secrets of soccer free kicks

Fluent : 20 May, 2002  (New Product)
Three collaborating groups of researchers have unravelled some of the underlying mysteries of 'bending' a soccer ball during kicking which will be a feature of the upcoming Soccer World Cup in Japan and Korea this year. They were inspired to understand this technically very difficult 'art' of scoring soccer goals from dead ball "free kick" situations, perfected by such world class soccer players as Brazil's Roberto Carlos, Germany's Michael Ballack and England's David Beckham. Engineers at the University of Sheffield's Sports Engineering Research Group, Yamagata University's Sports Science Laboratory and Fluent Benelux have carried out a fundamental scientific and engineering analysis of this exciting part of the 'beautiful game'.
Three collaborating groups of researchers have unravelled some of the underlying mysteries of 'bending' a soccer ball during kicking which will be a feature of the upcoming Soccer World Cup in Japan and Korea this year. They were inspired to understand this technically very difficult 'art' of scoring soccer goals from dead ball "free kick" situations, perfected by such world class soccer players as Brazil's Roberto Carlos, Germany's Michael Ballack and England's David Beckham. Engineers at the University of Sheffield's Sports Engineering Research Group, Yamagata University's Sports Science Laboratory and Fluent Benelux have carried out a fundamental scientific and engineering analysis of this exciting part of the 'beautiful game'.

Wind tunnel smoke test of a non-spinning soccer ball
FLUENT CFD stimulatuion showing wake flow pathlines of a non-spinning soccer ball, air speed of 12 m/s ( 27 mph )

'We believe that our research into the underlying physics of soccer balls is crucial to helping us explain more about soccer free kicks than ever before," said Dr. Matt Carré from the University of Sheffield Sports Engineering Research Group. He went on, "a combination of wind tunnel experiments, high-speed video camera analysis, trajectory simulations and computer modelling techniques like Computational Fluid Dynamics is a very potent way of explaining what is happening. We think that the work we are doing is giving us a deeper understanding of what makes for ideal soccer ball designs especially the ones that lead to more exciting free kicks. Indeed, we believe that our fundamental approach to the engineering aspects of soccer will lead to insights that can be applied to the training field and in improving the techniques of young soccer players.'

Dr. Keith Hanna, Marketing Communications Director at Fluent, echoed these sentiments and commented that, 'during every Soccer World Cup, talk inevitably touches on the fabulous free kicks, that fool both defenders and goalkeepers alike, because of the way that a soccer ball bends during its flight. In the 1.0 to 1.5 seconds it takes for a soccer free kick to happen, it is clear that a soccer ball experiences some very complex physics. The simulation work we have done with Sheffield and Yamagata Universities has been absolutely fundamental and I believe that it will lead to a range of further studies. It still amazes me that elite soccer players like Beckham and Carlos do what they do in a free kick instantaneously and under immense pressure in critical games. Their brains must be computing some very detailed trajectory calculations in a few seconds purely from instinct and practice. Our computers take a few hours to do the same thing and although we can now better explain the science of what they do, it is still magical to watch!'

Wind Tunnel Study and Trajectory Modelling
At the University of Sheffield, work on a ¼ scale generic soccer ball in a wind tunnel has shown that the air around the ball transitions from laminar to turbulent flow at speeds between 18 and 23 mph (8 and 10m/s) although this is very dependent on the ball's surface structure and texture. This is important because the drag experienced by a ball as it flies through the air during a free kick strongly influences its trajectory especially if the ball is spinning. It has long been known that a spinning ball will move sideways as it travels through the air because of a phenomenon known as the Magnus Force. This force is caused by the fact that on the side of a spinning ball moving through the air where its rotation and airflow are in the same direction, the air speed increases and pressure decreases; on the side where the movement of the ball's surface opposes flow, air speed decreases and pressure increases. This imbalance of pressures produces the Magnus side force that is so pronounced at the end of a ball's flight when it slows down, especially when considerable spin is applied to the ball at the same time. This balance of sideways force and drag force stays roughly the same for most of the ball's trajectory but alters considerably near the goal as the flow around the ball transitions.

Dr. Carré added that "the wind tunnel test proves a long-held suspicion by researchers that a non-spinning soccer ball has similar drag characteristics to that of a golf ball and is significantly different to that of a smooth sphere. This point of transition from turbulent to laminar flow around the ball is critical in soccer free kicks because the drag experienced by the ball increases by 150% in a split second when it happens. It is this phenomenon coupled with the almost constant spinning Magnus force that produces the exciting sudden dips and sideways motions of the best free kicks as the ball approaches the goal. This turbulent to laminar boundary layer transition also seems to move according to the spinning rate of the ball and the surface seam pattern of the ball used. At high spin rates transition occurs at faster ball speeds."

Dr. Carré explained that the technique they have developed in Sheffield has allowed them to analyse in detail the spectacular goal by David Beckham of England versus Greece during the World Cup Qualifiers in 2001. Beckham's shot left his foot at about 36 m/s (80 mph) from about 27m out with considerable spin and he lifted it half a metre over the defensive wall. The ball rose over the height of the crossbar during its flight as it moved laterally about 3m due to the high spin employed, before finally suddenly slowing down to about 19m/s (42 mph) and dipping into the corner of the goal. "Almost certainly, the flow around the ball changed from turbulent to laminar several metres from the goal" he noted" because otherwise our calculations suggest that it would have gone over the crossbar. Beckham was applying some very sophisticated physics to his kick!"

Computational Fluid Dynamics Study
CFD simulations to complement the wind tunnel study were carried out by Mr Joeri Wilms of Fluent Benelux who used the same model as that used in the experimental analysis. His work showed good agreement with the experimental results and extended the analysis to areas the tunnel could not handle, together with providing detailed explanations of underlying flow phenomena. For instance, he discovered that at low non-spinning soccer ball velocities a large flow separation was visible behind the ball. As the air velocity increased the separation got smaller. This separated wake also became skewed to the side as increasing spin was imparted to the ball in the CFD simulations. Detailed force balances were easily derived from the CFD study that could then be fed into a free kick trajectory visualisation model. He also confirmed that the ball's seam caused the air boundary layer over the ball to "trip" and dependent on the orientation of the ball to the oncoming flow (and the pattern of surface patches on the ball) the flow separations in the wake behind the ball were very different and very complex.

Ball/Foot Kicking Simulations
Dr. Takeshi Asai and his group in Japan have developed techniques for analysing high-speed video footage of soccer players kicking a ball, and in particular they are examining how both the ball and the kicker's foot deforms at the point of impact. This is critical for understanding and predicting the subsequent movement of a ball through the air. Computer simulation of the structural deformations involved has allowed them to predict the amount of spin a player can transmit to the ball for a given impact speed, angle and point of contact. This in turn has led them to the deduce the exact "sweet spot" on a ball where it can be kicked to impart the most optimum spin, a desirable effect especially at free kicks. The Yamagata work shows that if a soccer ball is struck off-centre by 80 mm, then roughly twice the spin (8 rev/sec) is imparted to the ball when compared to a strike 40mm from the centre (4 rev/sec) of the ball. Moreover, on a wet day when the coefficient of friction between the boot and the ball is lower, the amount of spin induced on the ball can fall by as much as a third of that seen on a dry day.

Dr. Asai comments "I think that the computer modelling techniques my group has developed should help us design better soccer boots in the near term and explain how a soccer player's foot deforms as it interacts with the ball. This has important implications for kicking techniques and preventing injuries to the foot. Collaborative work with Fluent and the University of Sheffield means that we can complement each other with our individual expertise feeding into the overall understanding of the science of soccer. We are presenting a joint paper on this subject at the forthcoming 4th International Conference on Engineering in Sport in Kyoto later this year."

Dr Hanna concluded, "I believe that the collaborative approach being developed could be applied to optimising a particular free kick type for a certain point outside the penalty area. Young players could then be trained to reproduce these optimum kicks. Indeed, Dr. Asai's work may even point to the most optimum footballer's foot size and shape for a given free kick type. There is clearly some exciting ongoing work ahead.".
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