Reference : How Cristiano Ronaldo performs his knuckleball?
Scientific congresses and symposiums : Unpublished conference/Abstract
Physical, chemical, mathematical & earth Sciences : Physics
How Cristiano Ronaldo performs his knuckleball?
[en] Comment Cristiano Ronaldo produit ses tirs flottants ?
Cohen, Caroline [LadHyX - Ecole Polytechnique > Département de mécanique > > >]
Darbois-Texier, Baptiste mailto [LadHyX - Ecole Polytechnique > Département de mécanique > > >]
Quéré, David [PMMH - ESPCI > > > >]
Clanet, Christophe [LadHyX - Ecole Polytechnique > Départment de mécanique > > >]
65th Annual Meeting of the APS Division of Fluid Dynamics
18 au 20 novembre 2012
San Diego
[en] knuckleballs ; physics of sports ; aerodynamics, ballistics
[en] A soccer ball kicked at very low spin can exhibit a zigzag trajectory. Along its straight path, the ball deviates laterally from about 0.2 m, that is to say one ball diameter. One zig zag happens as the ball travelled about 15 m. As the deviation direction seems unpredictable, this effect is highly annoying for goalkeepers. That why Cristiano Ronaldo and many soccer players are looking for this phenomenon. Those trajectories called knuckleballs are also observed on volleyball and baseball. We study experimentally indoor knuckleballs for different balls varying from soccer balls to smooth spheres. We show that knuckle effect doesn't derive from ball deformations at foot impact or ball seams. Actually, aerodynamic lift forces on a smooth sphere are fluctuating and are responsible for knuckleballs. From this study, we deduce side force intensity exerted on smooth spheres and sport balls for typical game Reynolds number (Re∼10^4−10^6). Finally we discuss required conditions to observe a knuckleball on the sport field.

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