A study of the knuckle effect in football

To date, miniature soccer balls (Carre et al., 2004; Carre and Asai, 2004) and standard-size soccer balls (Asai et al., 2006) have been studied in terms of their basic aerodynamic properties, and observed for surrounding flow using both wind tunnel experiments and computer-simulated computational fluid dynamics (CFD) (Asai et al., 2000; Barber et al., 2006). However, all of these studies have focused on stationary analyses; to understand the knuckle effect, which is fundamentally a non-stationary phenomenon, it is essential to use a non-stationary analysis that incorporates a time component. In the present study, we therefore analyzed the dynamics of the wake of non-rotating soccer balls by non-stationary CFD using a combination of large eddy simulation (LES) and a fluid visualization method using titanium tetrachloride. We also examined the fundamental mechanism of the knuckle effect.