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One of the main differences among soccer balls is how much the air flow follows the ball's surface. Turbulent air flow would hug the curvature of the ball, reducing drag. In this image of the Jabulani ball, however, Caltech scientists demonstrate that the air flow actually separates after about halfway across the ball.
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The World Cup is in full swing, complete with an official new soccer ball
named Jabulani, meaning "to celebrate" in Zulu. The players, however,
aren't exactly celebrating. Instead, many of them are complaining that the ball's
trajectory is too hard to predict, and that the ball itself is too hard to
control, resulting in wayward passes, shots flying off target, and goalkeepers
looking silly. So what exactly is it about the new ball that's provoking all
the controversy? To find out, Caltech engineers in the lab of Assistant
Professor of Aeronautics Beverley McKeon put an official Jabulani through its
paces in the Lucas Adaptive Wall Wind Tunnel. And what they found there may
explain the seemingly unpredictable nature of the Jabulani.
To start with, the classic black and white soccer ball, stitched together
from 32 panels of pentagons and hexagons, has deeper grooves. The Jabulani,
which is made from only eight panels thermally bonded together, has more
shallow grooves, as well as tiny raised patterns along its surface. Heavily
textured surfaces can result in more turbulent air flow around a ball, which,
McKeon explains, reduces drag at the speeds typical of a soccer ball kick and
allows a ball to travel farther. (That's the reason golf balls have dimples,
for all you putting aficionados out there.) As a soccer ball slows down after
it's kicked in the air, the air flow around it changes from turbulent to
smooth. It is likely that the details of this transition are different for the
traditional soccer ball than for the Jabulani, which from the point of view of
a soccer player translates into a ball that's behaving unpredictably.
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Members of the media gather to cover the test. [Credit: Jenny Somerville]
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It's also possible that
wind, spin, or other influences can have a stronger effect on the Jabulani's
trajectory. As the ball hurtles through the air, the varying air flow around it
can send it on unexpected paths, to the consternation of many soccer players.
Still, the Jabulani isn't so
unpredictable that players can't learn to control it. "It seems like
anytime the ball is changed, it takes a while for people to adapt," McKeon
says. And from a fluid mechanics standpoint, the Jabulani's also pretty
interesting. Studying the air flow around soccer balls is a natural extension
of McKeon's research, in which she and her colleagues study how surface
roughness affects air flow, using spheres as test models. They're looking at
how they can control air flow just by changing the texture of a surface. Their
work has many applications in aircraft design, for example.
The team that conducted the
test includes postdoc Michele Guala, graduate student Ian Jacobi, and Melissa
Christensen, an undergraduate from UC San Diego working in McKeon's lab this
summer.
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