Effect of seam characteristics on critical Reynolds number in footballs
Because of its direct relationship to the ball flight trajectory, the aerodynamics of sports balls attracts a high level of interest in terms of both science and engineering (Mehta et al. 1983, Thompson, 1910, Watts and Ferrer, 1987, Zayas, 1986) An important known characteristic in the aerodynamics of sports balls is that the air drag of a ball in flight dramatically drops when the boundary layer surrounding a football transitions from a laminar to a turbulent one (Mehta, 1985) When the boundary layer transitions and the drag dramatically drops (non dimensional velocity of flow) the Reynolds number is termed as the critical Reynolds number (Bearman and Harvey, 1976) and forms the most important aerodynamic indicator that expresses the drag characteristics of a ball. To date, the critical Reynolds number has often been reported in studies of football aerodynamics (Asai et al. 2007, Goff and Carre, 2009, Passmore et al. 2012) but in recent years, new demand has arisen to cast light on the drag characteristics and critical Reynolds number of footballs with varying panel shapes and surface design used in official matches (Hong and Asai, 2014, Hong et al. 2015) Moreover, the critical Reynolds number is an aerodynamic indicator directly linked to the ball velocity and flight trajectory, and clarification of the critical Reynolds number of footballs in recent years has been considered very important from the perspective of sports science and sports engineering as well as from the perspective of competitive arena. On the other hand, the effect of roughness of a spherical surface on the critical Reynolds number has been reported in studies based on smooth spheres (Achenbach, 1974) However, the surface of a football is shaped using a complex combination of panels and grooves of varying forms, and almost nothing is known about the effect of the various surface shape parameters of footballs on the ball critical Reynolds number or roughness. Accordingly, studying the ball surface shape parameters that affect the critical Reynolds number Abstract In recent years, the design of footballs, including the number and shape of the panels forming the surface of footballs, has undergone a significant change. However, panels of varied shapes and seams are combined in complex ways to form the surface shape of footballs, and almost nothing is known about the effect of these surface shapes on the football drag characteristics. The present study used a wind tunnel to study the relationship between the critical Reynolds number and the length, depth, and width of the panel joints (seams) of 10 most recent football types used in matches in recent years. The results show the tendency that the drag coefficient in the super critical regime (30 m/s) of a football increases as the panel joint length increases. Moreover, the depth of the panel joint indicates the highest correlation with the critical Reynolds number (r 0.71, p 0.01) and is considered to be a strong and convenient indicator that expresses the roughness (large scale) of the football surface. This study reveals the drag characteristics of the latest footballs and enables some degrees of prediction of the critical Reynolds number for the latest football types and those that will be developed in the future.