Understanding the Effect of Seams on the Aerodynamics of an Association Football

Abstract

* Corresponding author ABSTRACTThe aerodynamic properties of an association football were measured using a wind tunnel arrangement. A third scale model of a generic football (with seams) was used as well as a 'mini-football'. As the wind speed was increased, the drag coefficient decreased from 0.5 to 0.2, suggesting a transition from laminar to turbulent behaviour in the boundary layer. For spinning footballs, the Magnus effect was observed and it was found that reverse Magnus effects were possible at low Reynolds numbers. Measurements on spinning smooth spheres found that laminar behaviour led to a high drag coefficient for a large range of Reynolds numbers and Magnus effects were inconsistent, but generally showed reverse Magnus behaviour at high Reynolds number and spin parameter. Trajectory simulations of free kicks demonstrated that a football that is struck in the centre will follow a near straight trajectory, dipping slightly before reaching the goal, whereas a football that is struck off centre will bend before reaching the goal, but will have a significantly longer flight time. The curving kick simulation was repeated for a smooth ball, which resulted in a longer flight time, due to increased drag, and the ball curving in the opposite direction, due to reverse Magnus effects. The presence of seams was found to encourage turbulent behaviour, resulting in reduced drag and more predictable Magnus behaviour for a conventional football, compared to a smooth ball.

Experimental studies of the aerodynamics of spinning and stationary footballs

“…Such results have not been reported before for this type of ball. Carre et al [13], in experiments with footballs, concluded that there are no effects on C D due to ball orientation, but in that work the rotation of the ball was about the streamwise axis and could not therefore be expected to have an effect. Figure 7 shows the effect of orientation for the four real footballs.…”

The application of simulation to the understanding of football flight

“…The tangential velocity at the ball surface, U T , is estimated by equation 9 with the radius equated at height, h, on the primary spin axis with -denoting the high pressure plate (B), and + denoting the low pressure plate (A): [9] The skin friction coefficient C F is also calculated separately for the high and the low pressure plate using the standard flat plate model and is given by: [10] Substituting into equation 8, the total force is given in equation 9: [11] Consequently the angular deceleration of the ball due to surface friction, with the total torque estimated to act at the centre of the plate, is given by: …”

“…The wake structures were confirmed with flow visualisation. Earlier work by Carre 9 demonstrated that reverse Magnus effects are possible at low Reynolds numbers. The work was completed with a 1 / 3 scale rapid prototype football (66mm diameter) and a 'mini' football (140mm diameter adidas Fevernova™ Replica).…”

“…Consequently the range of spin parameters reported cannot be used to represent flight conditions. Where aerodynamic data is obtained from wind tunnels a great deal of the published work 5,6,7,8,9 , begins with discussions of the non-rotating case; given that a ball is nearly always rotating during the game it must be questioned why an emphasis is placed upon the behaviour for the static case. It is true that this can be indicative of others responses, but cannot accurately describe any flight.…”

CFD Study of Drag and Lift of Sepak Takraw Ball at Different Face Orientations

“…For baseballs, the relationship between rotation and lateral forces was evaluated [3], as well as the effects of seams on their aerodynamic properties [4]. Several similar studies have investigated tennis balls [5], soccer balls [6][7][8], and cricket ball [9].…”