Dr. Rabi Mehta uses smoke and lasers to inspect the flow pattern around an adidas Brazuca football.NASA Ames Research Center
Knuckle is a word without many positive connotations. Thugs use brass knuckles; people knuckle under to pressure; knuckle-dragging has never been a cool-looking locomotion technique; knuckle balls drive baseball hitters crazy; and soccer balls can "knuckle," flying through the air with an unpredictability that can earn the enmity of players.
The 2010 World Cup ball, the Jabulani, had a knuckling problem. If it was kicked with none or very little spin it would fly unpredictably. Simply put, it took some pretty crazy flight paths through the air.
To address those concerns, adidas for the 2014 Cup created the Brazuca, which, the company hoped, would handle better, and provide a truer flight path, than the Jabulani.
The Brazuca has only six panels, where the Jabulani had eight. But the seams are longer and deeper. The panels are also covered with small bumps. The overall result is a rougher ball, one built to decease the incidence of knuckling at regular kicking speeds.
Did adidas succeed? Is the Brazuca an improved soccer ball, truer in flight? The folks at NASA -- no slouches when it comes to testing objects in flight -- decided to find out.
Rabi Mehta, chief of the Experimental Aero-Physics Branch at NASA's Ames Research Center, set out to study the aerodynamics of the Brazuca. (Aerodynamics, in simple terms for we ordinary civilians, looks at how air and fluid flow around something. That something could be an airplane, a satellite or a soccer ball.)
Mehta put the Brazuca through its paces in a 2x2 wind tunnel, which blew air past the ball at varying speeds. To increase visibility of the air path around the ball, the team used a controlled flow of laser-lighted smoke over the ball's surface (see photo below).
A close up of the Brazuca ball in Ames' Fluid Mechanics Laboratory's two-foot by two-foot wind tunnel. Smoke highlighted by lasers visualizes air flow around the ball.NASA Ames Research Center
What Mehta wanted to find was the speed at which the smoke pattern made an abrupt change, which would show the speed at which knuckling occurred.
The fanciful Jabulani, from 2010, knuckled at about 50 miles per hour, which is right in the typical kicking speed zone of a skilled World Cup striker. So the "ordinary" speed of play in 2010 saw some quite extraordinary ball behavior.
By contrast, Mehta found the new Brazuca to maximum-knuckle at a much slower speed of around 30 miles per hour, well below regular ball-striking speeds. (The traditional 32-panel soccer ball -- the kind we're all used to seeing -- also knuckles at around 30 miles per hour, Mehta observed.)
"The materials used, the ball's surface roughness and its distribution determines its aerodynamics," said Mehta. The rougher surface of the Brazuca had decreased the knuckling effect seen in the smoother Jabulani.
The net result? The Brazuca should offer a more predictable flight path. "The players should be happier with the new ball," said Mehta. "It is more stable in flight and will handle more like a traditional 32-panel ball."