Effect of Panel Shape of Soccer Ball on Its Flight Characteristics
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OPEN Effect of panel shape of soccer ball on its SUBJECT AREAS: flight characteristics MECHANICAL Sungchan Hong & Takeshi Asai ENGINEERING FLUID DYNAMICS Institute of Health and Sports Science, University of Tsukuba, Tsukuba-city, 305-8574, Japan. Received 19 February 2014 Soccer balls are typically constructed from 32 pentagonal and hexagonal panels. Recently, however, newer balls named Cafusa, Teamgeist 2, and Jabulani were respectively produced from 32, 14, and 8 panels with Accepted shapes and designs dramatically different from those of conventional balls. The newest type of ball, named 15 April 2014 Brazuca, was produced from six panels and will be used in the 2014 FIFA World Cup in Brazil. There have, however, been few studies on the aerodynamic properties of balls constructed from different numbers and Published shapes of panels. Hence, we used wind tunnel tests and a kick-robot to examine the relationship between the 29 May 2014 panel shape and orientation of modern soccer balls and their aerodynamic and flight characteristics. We observed a correlation between the wind tunnel test results and the actual ball trajectories, and also clarified how the panel characteristics affected the flight of the ball, which enabled prediction of the trajectory. Correspondence and requests for materials he uniqueness of the shape and design of a ball to a particular sport has meant that there were very little should be addressed to changes over time. However, in more recent years, there have been dramatic changes in the shape and design S.H. (sr7931@hotmail. T of soccer balls. Particularly, there have been substantial changes in the shape and design of the panels used to com) construct the official balls of FIFA World Cup tournaments. The Teamgeist 2, which was the official ball of the 2008 EURO Cup in Austria and Swiss, comprised 14 panels and was significantly different from a conventional soccer ball, which is typically constructed from 32 pentagonal and hexagonal panels. The revolutionary shape of the 14-panel Teamgeist 2 has attracted much attention. At the 2010 FIFA World Cup in South Africa, an eight- panel ball produced by Adidas, named Jabulani, was introduced, which incorporated further modification of the shapes of the panels. The 2013 FIFA Confederations Cup in Brazil adopted another 32-panel ball produced by Adidas as the official ball. The latter ball, named Cafusa, is presently used by many professional soccer leagues and for international matches. Similar to a conventional ball, a Cafusa ball comprises 32 panels. However, whereas the panels of a conventional ball are pentagonal and hexagonal and are arranged in a simple manner, those of a Cafusa ball significantly differ in shape and orientation, although they can be roughly classified into two orientations with eight panels. At the upcoming 2014 FIFA World Cup, which will be held in Brazil, the Brazuca, a six-panel ball also produced by Adidas, will be the official ball. Previous aerodynamics studies on soccer balls can be broadly divided into those that examined the basic aerodynamic properties by wind tunnel tests1,2, those that analytically determined the flight characteristics using various techniques including simulations3,4,5, and those that measured the actual flight paths6,7. There have also been studies in recent years to examine the flight characteristics of soccer balls using both the wind tunnel and actual flight paths8,9,10. Furthermore, there was a recent report on the effect of the surface geometry on the flight trajectory, which was determined by trajectory analysis and high-speed cameras11. In a more recent study, trajectory analysis and wind tunnel experiments were used to glean information for comparing the non-spin aerodynamics of Jabulani and Brazuca12. However, there has been no study on the correlation between the results of wind tunnel tests and the actual flight paths of soccer balls with respect to the shape, number, and orientation of their panels. Thus, in the present study, we examined this correlation using modern soccer balls, including the Brazuca, and also investigated the extent to which wind tunnel test results could explain the actual flight path of a soccer ball. Based on the observed correlation between the wind tunnel results and the actual ball trajectories, we clarified how the panel character- istics affect the flight of a soccer ball, which enables prediction of the trajectory. Results Drag force in the wind tunnel. The wind tunnel tests were conducted using different new soccer balls, namely, Brazuca (Adidas, six-panel), Cafusa (Adidas, 32-panel), Jabulani (Adidas, eight-panel), Teamgeist 2 (Adidas, 14- panel), and conventional (Vantaggio, Molten, 32-panel). The balls were mounted as shown in Figure 1. Two panel SCIENTIFIC REPORTS | 4 : 5068 | DOI: 10.1038/srep05068 1 www.nature.com/scientificreports Figure 1 | Photograph of the wind tunnel test setup. Figure 2 | Soccer balls used for the test and their panel orientations. (a, b) Adidas Brazuca: small dimple and six panels, (c, d) Adidas Cafusa: small orientations of the soccer balls identified as orientations A and B (see grip texture and 32 modified panels, (e, f) Adidas Jabulani: small ridges or Figure 2) were used for the study, and the corresponding aerody- protrusions and eight panels, (g, h) Adidas Teamgeist 2: small namic properties were measured. protuberances and 14 panels; (i, j) Molten Vantaggio (conventional soccer It was observed that the drag varied substantially with the ball type ball): smooth surface and 32 pentagonal and hexagonal panels. (Photo by (Figure 3). The variation of the drag coefficient with the panel ori- S.H.). entation was also significant for Cafusa and Jabulani, whereas it was relatively small for Brazuca, Teamgeist 2, and the conventional ball. respectively. It was further observed that the variation of the drag The drag crisis regime, which indicates a sudden change in the drag on Jabulani with the panel orientation was relatively substantial for Reynolds numbers in the range of 3.0 3 105–5.0 3 105. coefficient Cd, was lowest for Brazuca, followed by the conventional ball, Cafusa, Teamgeist 2, and Jabulani, in increasing order. In the Side and lift forces in the wind tunnel. Figure 4 shows the scatter case of Cafusa, Cd decreased from ,0.5 to ,0.2 or less at a Reynolds number Re of 1.7 3 105 for panel orientation A, and at Re of 1.5 3 105 diagrams of the lift and side forces that acted on the soccer balls. The for panel orientation B (Figure 3b). The critical Reynolds numbers diagrams indicate that the irregular fluctuations increased as the flow 5 5 21 for Cafusa were ,2.9 3 10 (Cd < 0.14) and ,2.4 3 10 (Cd < 0.16) velocity was increased from 20 to 30 m?s . The same trend was for panel orientations A and B, respectively. The critical Reynolds observed when the panel orientations were changed. The change in 5 number for Jabulani for panel orientation B was ,3.6 3 10 (Cd < the irregular fluctuation with increasing speed was least for 5 0.12), which was less than the value of ,3.3 3 10 (Cd < 0.16) for Teamgeist 2 (Figures 4g-1 and 4h-1) and greatest for panel panel orientation A. These values were less than those for the other orientation A of Jabulani (Figure 4f-1). The irregular fluctuation balls (Figure 3c). The variation of the drag coefficient with the panel was more prominent for the conventional ball when the flow orientation was observed to be small for Brazuca, Teamgeist 2, and velocity increased. The SD of the side and lift forces also increased the conventional ball (Figures 3a, 3d, and 3e). The critical Reynolds with increasing flow velocity (Figures 4k and 4l). This trend was also 5 numbers for Brazuca were determined to be ,2.5 3 10 (Cd < 0.15) observed when the panel orientation was changed. The SD of the 5 21 and ,2.2 3 10 (Cd < 0.16) for panel orientations A and B, respect- forces was highest for Jabulani for a flow velocity of 20 m?s , and the 5 ively. The corresponding values for Teamgeist 2 were ,3.0 3 10 (Cd irregular fluctuations were observed at the intermediate velocity. The 5 < 0.17) and ,2.8 3 10 (Cd < 0.15), and those for the conventional SD of the side forces for panel orientation A of Jabulani did not 5 5 ball were ,2.5 3 10 (Cd < 0.16) and ,2.8 3 10 (Cd < 0.17), increase with increasing flow velocity. Furthermore, the SD of the Figure 3 | Variation of the drag coefficient with the type of ball and panel orientation: (a) Brazuca, (b) Cafusa, (c) Jabulani, (d) Teamgeist 2, (e) conventional ball. SCIENTIFIC REPORTS | 4 : 5068 | DOI: 10.1038/srep05068 2 www.nature.com/scientificreports Figure 4 | Scatter plots of the side and lift forces of the balls and SDs of the respective forces for each flow velocity (after 9 s). As the flow velocity increased from 20 m?s21 (a–j) to 30 m?s21 (a-1– j-1), the irregular fluctuations of the side and lift forces increased. The SD of the side (k) and lift (l) forces increased with increasing flow velocity. side and lift forces for panel orientation B of Jabulani decreased with the side and lift forces. The extended total distance of the panel bonds increasing flow velocity, which was different from the cases of the and the number of panels were as follows: 3.32 m and six panels for other balls. Brazuca, 4.47 m and 32 panels for Cafusa, 1.98 m and eight panels The correlation between the growth rates of the SD of the side and for Jabulani, 3.47 m and 14 panels for Teamgeist 2, and 3.84 m and lift forces when the flow velocity was increased from 20 to 30 m?s21 32 panels for the conventional ball.