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jelizondo [soylentnews.org] writes:

I found a very interesting article [theguardian.com] published by The Guardian about the physical characteristics of the new FIFA ball and how it is surprising goalkeepers:

Poor old Luca Zidane. The Algeria goalkeeper has had a turbulent time. In two matches he has conceded five goals, and a pair of them – first from Lionel Messi, then, more embarrassing, from Jordan’s Nizar al-Rashdan – have gone through his fingers.

But Zidane is not alone. Senegal’s Édouard Mendy and Iraq’s Ahmed Basil have got their hands to shots, but been unable to stop them. Is something going on?

Certainly Joe Hart seems to think so. He has frequently been pointing out on BBC that goalkeepers are having trouble reading the speed of the World Cup ball, the Adidas Trionda. “The ball is coming into the keepers a lot faster than it feels when it comes off the foot,” he said. “Zidane is more than capable of saving that ball [from Messi]. When goalkeepers get up to speed with these World Cup balls we’re going to see these shots saved.”

Hart issued his assessment before Zidane had played his second match, when the 28-year-old’s inability to stop al-Rashdan’s outside-of-the-boot effort suggest the problem may continue for a while yet. But there is help at hand and it comes in the form of an 18-page paper produced by South Korean and Japanese academics.

It has the title Orientation-Dependent Drag Crisis and Flight Response of the Fifa World Cup Match Ball Trionda [mdpi.com] and its contents do not deviate from the outline. Researchers took the ball and fired it through a wind tunnel to measure the effect of aerodynamic forces upon it. They did so from six angles and found a consistent outcome.

Regardless of where the ball was struck, if the ball reached a certain velocity it would fly faster. This, the researchers from Seoul Women’s University and the University of Tsukuba found, was down to an effect called “drag crisis”. This occurs when an object flying through the air reaches the point where the air flow around it shifts from a smooth state (known as a laminal flow) to a turbulent one. When the flow is turbulent, it disrupts the drag behind a moving object, allowing it to move faster.

Researchers noted that the “upstream seam and groove arrangements” in the Trionda’s design made drag crisis possible at lower speeds.

If a ball does not slow down as expected, because of the drag crisis effect, you can understand how goalkeepers may be caught unawares. The researchers found further complicating factors. They observed that while there was a drag-crisis effect regardless of where the ball was hit, the level of the crisis would shift depending on whether the ball was struck on a seam or on a panel (hitting on the seam seemed to create the lower drag). Drag crisis was also variable according to altitude, with the higher the game, the less likely the occurrence.

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