If you are still scratching your head about the thoroughly unimaginative headline, I can only assume that you have renounced the world and found solace under a giant rock in the middle of nowhere. Welcome back to civilization! In case you missed it, the collective eyeballs of the world are currently fixated on the FIFA World Cup. My involvement in the World Cup is a little more personal since my first project at CD-adapco involved the aerodynamics of soccer balls. The 2010 World Cup ball, the Adidas Jabulani, had its fair share of criticism due to its shallower seams and 8-panel design leading to erratic flight paths. The ball was difficult to control for the players and rumors have it that some goalkeepers still have recurring nightmares of the ball. In 2010, CD-adapco teamed up with Wilson Sporting Goods to study the aerodynamic behavior of soccer balls and the impact of the newer panel designs.
The ball for the 2014 FIFA World Cup, Brazuca, has already provided maximum entertainment with 136 goals scored in just the first round, just one shy of the 137 goals scored in the entire 2010 World Cup. While it is highly certain that the high number of goals can be explained away by Lady Luck’s propensity to smile on teams this year, it is equally possible that the Brazuca is just a better ball aerodynamically than the Jabulani. Before we delve into the why and how, here’s a possible reason for the Jabulani’s bizarre behavior: The Knuckling Effect.
In baseball, a knuckle ball is often used where the pitcher doesn’t impart any spin on the ball and the Gods of Aerodynamics will take care of the rest. So what is knuckling exactly? When a soccer ball is kicked, the air flowing over the ball ‘hugs’ the ball tightly forming what is called a ‘boundary layer’ over the ball. The roughness of the ball and the depth of the seams will then affect this boundary layer which determines the motion of the ball. When a ball is kicked with little or no spin, the air flowing over the ball is ‘tripped’ by the seams and separates from the ball, forming a region of wake behind it. The unsteadiness of the wake behind the ball is what pushes it into an unsteady flight path. The ball may oscillate from left to right or up and down when kicked and the trajectory of such a kick is hard to predict for the kickers and goalkeepers alike. During knuckling, the unsteady wake behind the ball will oscillate in direction leading to lift and side forces on the ball moving from one direction to the other. Recently, researchers have shown that the Brazuca knuckles less at 50 mph, the average speed with which free kicks are taken and hence is better than the Jabulani.
Is this really true? Based on our earlier research on the Wilson and Adidas soccer balls, I used STAR-CCM+ to study the behavior of Brazuca at 50 mph. The model is courtesy of www.grabcad.com and is a representative model of the actual ball. I meshed the Brazuca in STAR-CCM+ using the Overset Mesh technology and ran a Detached Eddy Simulation (DES) of the flow over the stationary ball at 50mph. A DES simulation is necessary to accurately capture the turbulent eddies in the wake region. In the animation below, you can clearly see that while there is still some unsteadiness and asymmetry in the wake region behind the ball, the wake doesn’t wildly fluctuate and oscillate from one side to the other. To confirm this, I looked at the behavior of the lift and side forces acting on the ball.
A free kick is usually taken from around 30m from the goal. Considering the 50mph speed of the ball, it will take approximately 1.3 seconds before it reaches the goal. The drag force in this simulation stabilized around 0.2 seconds after which time the knuckling effect will kick in. Looking at the behavior of the forces after 0.2 seconds, there is clear oscillation from one side to the other suggesting knuckling but the magnitude of the lift force is well under 1N and that of the side force is even smaller. This suggests that while there is still knuckling at 50 mph, the effects are not as pronounced as on the Jabulani. Of course, most often, the free kicks have a certain spin to them. Time permitting, I may also have a video showing the wake behavior on the Brazuca with spin.
Happy World Cup watching!