Caltech puts the tourney’s ball to a test
The aeronautical engineers at Caltech hold a distinct advantage over all the soccer players and fans who have been complaining about the fancy new ball at the World Cup.
They have a wind tunnel in the basement.
So on Wednesday morning, Beverley McKeon and her colleagues headed downstairs, cranked up the rushing air and turned on the smoke, just like one of those car commercials on television.
In some ways, the assistant professor was asking the same questions voiced by critics who have called the new ball “tricky” and “unpredictable,” but in a very different language.
Surface roughness. Planes of symmetry. Turbulence versus laminar flow.
Speaking over the rumble of a wind generator the size of a school bus, she explained that “it’s quite a complex fluid mechanics phenomenon.”
But what does that mean for the simple act of kicking a ball?
Adidas produces a new ball for each World Cup and players invariably gripe. Four years ago, the German sporting goods giant switched from the traditional 32 stitched panels to 14. The current Jabulani model -- its name means “to celebrate” in Zulu -- is down to eight.
This configuration piqued the curiosity of Caltech professors and graduate students, a multinational group that includes a few soccer fans. They took the Jabulani and a traditional ball to their lab even as the U.S. scrambled to victory over Algeria.
The Lucas Wind Tunnel can generate monstrous gusts, but in this case they set the dial to about 30 meters per second, simulating the average speed of a ball kicked by an elite player.
Though the testing fell short of comprehensive, McKeon -- an Englishwoman who specializes in the aerodynamics of the sphere -- sensed an important difference in the Jabulani.
In addition to fewer panels, it has comparatively shallow seams for a more perfectly round shape, which might seem like a good thing. But the science can be counterintuitive.
Consider the history of the golf ball, which was smooth back in the mid-1800s.
“The Scots learned the hard way,” McKeon said.
The addition of dimples made for a rougher surface but a narrower wake and less drag, which contributed to straighter, longer trajectories.
To some degree, the Jabulani represents a shift in the opposite direction, even with tiny ridges covering its skin. Caltech’s study suggests that it starts with a smooth -- or laminar -- airflow, shifts to something more turbulent, then shifts back again.
These variations can have a big effect.
“So as the goalkeeper sees the ball coming, it suddenly seems to change its trajectory,” McKeon said. “It’s like putting the brakes on, but putting them on unevenly.”
Players will acclimate soon enough, McKeon suspects, but they had only a month to practice with the Jabulani before competition began. And the new ball sparked immediate debate after the U.S. earned a 1-1 tie with England on a seemingly routine shot that somehow squirmed past the keeper.
Was he fooled by laminar flow? Were fluid mechanics to blame?
McKeon the scientist said: “That’s a very political question.” Then, after a brief pause, McKeon the English fan could not help adding: “I’m sure it’s entirely down to the ball and had nothing to do with our goalkeeper.”