Flow structure of knuckling effect in footballs

Abstract

a b s t r a c tThe flight trajectory of a non-spinning or slow-spinning soccer ball might fluctuate in unpredictable ways, as for example, in the many free kicks of C. Ronaldo. Such anomalous horizontal shaking or rapid falling is termed the 'knuckling effect'. However, the aerodynamic properties and boundary-layer dynamics affecting a ball during the knuckling effect are not well understood. In this study, we analyse the characteristics of the vortex structure of a soccer ball subject to the knuckling effect (knuckleball), using high-speed video images and smoke-generating agents. Two high-speed video cameras were set at one side and in front of the ball trajectory between the ball position and the goal; further, photographs were taken at 1000 fps and a resolution of 1024 Â 512 pixels. Although in a previous study (Taneda, 1978), shedding of horseshoe vortices was observed for smooth spheres in the Reynolds number (Re) range of 3.8 Â 10 5 o Re o 10 6 , in the case of the soccer ball, the vortex structure, which consisted of distorted loop vortices, appeared in the wake behind the ball in the supercritical Re number region. Moreover, after the knuckleballs were airborne, large-scale undulations were observed in the vortex trail visualised with a smoke technique. On the other hand, aerodynamic forces acting on the ball were estimated from the data of the ball's flight trajectory, and a statistically high correlation (r¼ 0.94, p o 0.01) between the fluctuation frequency of the lift and side forces and the undulation frequency of the vortex trail was shown to exist. This fact suggests that the phenomenon of large-scale undulations of the vortex trail is closely related to the cause of the unsteady aerodynamic forces acting on the knuckle ball.

Aerodynamic drag of modern soccer balls

“…The occurrence of irregular and unsteady Asai and Kamemoto 2011 flying with no spin or a low-speed spin (Asai and Kamemoto 2011). However, because this study focused on the relationship between the constant resistance of the ball and its flight trajectory, knuckle effects were ignored in the trajectory simulation.…”

“…For example, the Adidas Teamgeist II, the official match ball of the 2008 Beijing Olympic Games, has 14 panels, and the Adidas Jabulani, the official match ball of the South Africa 2010 FIFA World Cup, has only 8 panels. Despite this trend toward fewer panels (Asai et al 2007), few studies (Asai and Kamemoto 2011) have focused on the aerodynamic characteristics of these balls. Having said that, the Adidas Tango 12, the new soccer ball used at the UEFA Euro 2012 and the 2012 London Olympics, has 32 panels having a new shape and a curved design.…”

Effect of seam characteristics on critical Reynolds number in footballs

“…The occurrence of irregular and unsteady lift and side forces known as the 'knuckle effects', has been reported for soccer balls flying with no spin or with a low-speed spin (Asai and Kamemoto, 2011). However, because this study focused on the relationship between the constant resistance of the ball and its flight trajectory (Passmore et al, 2008, Tuplin et al, 2012, the knuckle effects were ignored in the trajectory simulation.…”

Flow Visualisation around Spinning and Non-Spinning Soccer Balls Using the Lattice Boltzmann Method

“…The vortex pairs in the wake of the spinning ball (curve) tended to be stable, whereas those in the wake of the non-spinning or slowly spinning ball tended to be unstable, exhibiting large fluctuations such as rotation about the axis in the motion direction and breakdown. These instabilities are considered to be one of the causes of the knuckling effect observed in nonspinning and slowly spinning balls [17][18][19]. In addition, the deviation of side force coefficient of the spinning ball determined by CFD tended to be smaller than that of the non-spinning ball.…”