NASA Turns World Cup into Lesson in Aerodynamics
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The previous World Cup ball, the Jabulani, was described as sometimes demonstrating "supernatural" movements. It was beloved by strikers but hated by goalkeepers because, when kicked with little or no spin, the ball "knuckled," giving strikers a greater chance of scoring. Knuckling occurs when, at zero or near-zero spin, the seams of the ball channel airflow in an unusual and erratic manner making its trajectory unpredictable.
Taking full advantage of a ball’s flight characteristics to gain an advantage is nothing new in sports. In baseball, the only difference between a curveball, a fastball, a slider or a knuckle ball is how a pitcher manipulates the spin of the baseball with respect to its stiches. On a football, there are no external stiches piecing the outer covering of the ball together but it does have seams, many of them.
To address the unpredictability of the Jabulani ball, Adidas worked with hundreds of players to develop the Brazuca football. A traditional football has 32 panels, the Jabulani has eight panels and the Brazuca has only six.
Despite having fewer panels, the finger-like panels on the Brazuca increase the seam length, compared to previous World Cup balls. The seams are also deeper than those of the Jabulani and the panels are covered with tiny bumps; all of these factors influence the ball’s aerodynamics.
What seems like common sense about air moving around a simple sphere does not, in fact, bear true. The airflow around a sphere is not smooth; a great amount of drag is created behind the object. An example of this can be seen on a golf course, where a smooth golf ball travels much shorter distances than a regular, dimpled golf ball. The dimples on the ball’s surface agitate the air creating a smaller low-pressure wake behind the ball and decreasing drag, therefore increasing its distance.
“There is a thin layer of air that forms near the ball’s surface called the boundary layer and it is the state and behavior of that layer that is critical to the performance of the ball,” said Mehta. “The materials used, the ball’s surface roughness and its distribution determines its aerodynamics.”
The overall increased roughness of the Brazuca football will help to decrease the ball’s knuckling tendencies at kicking speeds typically encountered in the World Cup.