CFD Simulation of Soccer Ball Knuckling

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.