The flight trajectory of a non-spinning or slow-spinning soccer ball may fluctuate in unpredictable ways as similar as C. Ronaldo's FK in Football. Such anomalous horizontal shaking or rapid falling is ascribed to a phenomenon called the 'knuckling effect'. However, the aerodynamic properties and boundary-layer dynamics of a ball during the knuckling effect are not well understood. The purpose of this study is to analyze the characteristics of vortex structure of the knuckling effect ball using high-speed video images and titanium tetrachloride. Two high-speed video cameras were set to the side and in front of the ball trajectory between the location of the ball and the soccer goal, and photographs were taken at 1,000 fps and a resolution of 1024 x 1024 pixels. A horseshoe vortex on smooth sphere was observed in the range 380000 < Re < 1000000 (Taneda, 1978). However, the vortex structure behind the soccer ball in super critical Re number region might be a distorted loop vortex. After balls that were under the influence of the knuckling effect were airborne, large-scale undulations of the vortex trail were observed.
