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Jabulani: myth and fact
As the second round of games in South Africa 2010 draws to a close, the one thing that has dominated headlines more than any team performance or player has been the labulani ball. Players, coaches, experts and viwers have raised questions about the ball. Adidas have claimed that this is the best ball they have produced. With so much contrasting opinion, the Sports Aerodynamic Research Centre led by Dr Firoz Alam at RMIT University, Melbourne, Australia has undertaken a series of experimental testing in RMIT Industrial Wind Tunnel using six component force sensor. The tests have revealed some interestingfindings:
(1) The 8 panel Jabulani and the 14 panel Teamgeist balls have the same magnitude of aerodynamic drag coefficient at speeds above 40 km/h. Therefore,
both balls will have the same aerodynamic resistance in flight
(2) The drag coefficient variation between the two sides of the 8 panel Jabulani ball is around 9% in the range of 30 to 120 km/h speeds. This variation is enough to create unpredictable flight path (flight trajectory)
(3) In comparison, the drag coefficient variation between the two sides of the 14 panel Teamgeist ball is approximately 4% in the range of 30 to 120 km/h speeds, which is 5% less than the Jabulani ball. Naturally, theTeamgeist ball will have more stable or predictable flight trajectory compared to Jabulani ball
(4) Overall, the aerodynamic performance of the Teamgeist ball is better compared to the Jabulani ball.
Also, important to note is that the Jabulani ball is not perfectly smooth as it has special ridges on its surface along with seams between the panels. As the airflow passes over the ball, the ridges and seams generate asymmetric airflow creating pressure variation around the ball. This pressure variation can cause deviation from the expected flight trajectory. This deviation is what is commonly referred to as swerve, swoop or knuckle etc.
But this also raises the question. What role does
altitude play in all of this?
The average altitude of six cities of the total ten stadium cities of South Africa is 1350 m from the sea level. The study indicates that high altitude will have significant effect on the ball's aerodynamic drag and speed.
For example, at 90 km/h ball speed in calm wind condition at high altitude stadiums in South Africa, the Jabulani ball's air resistance can be around 13% less compared to the sea level stadium cities such as Cape Town or Durban. As a result, the ball can travel at 7% higher speed on average in high altitude cities. However, in Johannesburg, which is 1750 m above sea level, the Jabulani ball can travel almost 10% higher compared to sea level stadium cities in South Africa. Therefore, the players can overshoot the ball.