In the 2010 World Cup, players complained the ball was wonky, so this year has an all-new ball. NASA used smoke, lasers, and fluid boxes to test out its aerodynamics. Aside from producing some awesome photographs, the actual science is pretty cool, too.
Scale model of a standard soccer ball in a water channel. Green dye marks flow lines; pink dye is injected behind the ball to highlight the low-pressure zone in the lee of the ball.
A normal ball has 32 panels, but why would the World Cup use an off-the-shelf stock ball? Instead, they use custom designs, each with a unique name. The previous event's ball was Jabulani, an eight-panelled ball, while this year's ball is Brazuca, an six-panelled ball.
Jabulani was prone to knuckling — unpredictable wobbling produced by airflow over the seams when the ball was kicked without spin. The consequence was that strikers loved it and goalkeepers hated it, as a spin-free kick sent the ball on a trajectory with just enough wobble to make it difficult for the goalkeepers to anticipate accurately.
Brazuca ball in a wind tunnel with smoke and lasers highlighting the flow patterns.
To test it out, soccer fans at NASA's Ames Research Center immersed scale models of a traditional ball, the Jabulani ball, and the Brazuca balls in a water channel with dye, and tucked the full-sized balls in a wind tunnel with smoke and lasers.
Although it has fewer panels than Jabulani, Brazuca has a longer total seam length, with seams that run deeper. The seams, and tiny bumps on the panels, impact the aerodynamics of the ball. The function of the bumps is analogous to dimples on a golf ball: the surface roughness impacts the boundary layer.
Dr. Rabi Mehta to inspects flow around the Brazuca ball in a wind tunnel.
A traditional ball experiences maximum knuckling at about 30 miles per hour, which is far lower than the typical speed produced by a professional player. Jabulani, a much smoother ball, has maximum knuckling at 50 to 55 miles per hour, which is exactly the expected kicking speed, so it's totally expected that World Cup players noticed the problem back in 2010. The increased roughness of the new ball, Brazuca, drops the critical speed for increased knuckling back down to 30 miles per hour again, so it shouldn't come into play for this year's World Cup.
You can watch the tests here:
Why is our space agency suddenly getting into testing the aerodynamics of balls? Well, they already have the laboratory setup and wind tunnels for testing new vehicles, but mostly because soccer fans are everywhere, even amongst rocket scientists.
Image credits: NASA Ames Research Center
Still want more World Cup action? Check out this friendly pickup game on the International Space Station, the mind-controlled exoskeleton that made the opening kick, and how the security is taking us one step closer to a dystopian future of perpetual surveillance.