When the boundary layer of a sports ball undergoes the transition from laminar to turbulent
flow, a drag crisis occurs whereby the drag coefficient (Cd) rapidly decreases. However, the aerodynamic
properties and boundary-layer dynamics of a soccer ball are not yet well understood. In
this study we showed that the critical Reynolds number (Recrit) of soccer balls ranged from
2.2 × 105 to 3.0 × 105. Wind-tunnel testing, along with visualisation of the dynamics of the
boundary layer and the trailing vortex of a ball in flight, demonstrated that both non-spinning
and spinning (curved) balls had low Cd values in the super-critical region. In addition, the Recrit
values of the soccer balls were lower than those of smooth spheres, ranging from ~3.5 × 105 to
4.0 × 105, due to the effects of their panels. This indicated that the aerodynamic properties of a
soccer ball were intermediate between those of a smooth ball and a golf ball. In a flow visualisation
experiment, the separation point retreated and the Cd decreased in a super-critical regime
compared with those in a sub-critical regime, suggesting a phenomenon similar to that observed
in other sports balls. With some non-spinning and spinning soccer balls, the wake varied over
time. In general, the high-frequency component of an eddy dissipated, while the low-frequency
component increased as the downstream vortex increased. The causes of the large-scale fluctuations
in the vortex observed in the present study were unclear; however, it is possible that a
‘knuckle-ball effect’ of the non-rotating ball played a role in this phenomenon
When the boundary layer of a sports ball undergoes the transition from laminar to turbulentflow, a drag crisis occurs whereby the drag coefficient (Cd) rapidly decreases. However, the aerodynamicproperties and boundary-layer dynamics of a soccer ball are not yet well understood. Inthis study we showed that the critical Reynolds number (Recrit) of soccer balls ranged from2.2 × 105 to 3.0 × 105. Wind-tunnel testing, along with visualisation of the dynamics of theboundary layer and the trailing vortex of a ball in flight, demonstrated that both non-spinningand spinning (curved) balls had low Cd values in the super-critical region. In addition, the Recritvalues of the soccer balls were lower than those of smooth spheres, ranging from ~3.5 × 105 to4.0 × 105, due to the effects of their panels. This indicated that the aerodynamic properties of asoccer ball were intermediate between those of a smooth ball and a golf ball. In a flow visualisationexperiment, the separation point retreated and the Cd decreased in a super-critical regimecompared with those in a sub-critical regime, suggesting a phenomenon similar to that observedin other sports balls. With some non-spinning and spinning soccer balls, the wake varied overtime. In general, the high-frequency component of an eddy dissipated, while the low-frequencycomponent increased as the downstream vortex increased. The causes of the large-scale fluctuationsin the vortex observed in the present study were unclear; however, it is possible that a‘knuckle-ball effect’ of the non-rotating ball played a role in this phenomenon
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