The aerodynamics of a tennis ball

Abstract

The aerodynamics of a tennis ball are presented and discussed here with the aid of recent wind tunnel measurements. A flow visualization study was conducted on a 28-cm diameter tennis ball model to establish the boundary layer separation locations and Reynolds number effects for both nonspinning and spinning cases. The flow visualization results showed that the separation location on a nonspinning tennis ball occurred relatively early, near the apex, and appeared very similar to a laminar separation in the subcritical Reynolds number regime. Qualitatively, the flow regime (boundary layer separation location) appeared to be independent of Reynolds number in the range, 167 000 < Re < 284 000. Asymmetric boundary layer separation and a deflected wake flow, depicting the Magnus effect, were observed for the spinning ball. In the second phase of the experiments, the drag coefficient of a variety of new and used tennis balls, including prototypes of the recently approved oversized ball, was measured for the case simulating a perfectly flat serve (a serve with zero spin). The measurements were conducted in a Reynolds number range of about 80 000 < Re < 300 000, which corresponds to a velocity range of 19 < U < 70 m s−1 (43 < U < 157 mph). The present data, which indicate relatively high drag coefficients for new tennis balls (CD≅ 0.6–0.7), are compared to existing data for spherical models with varying degrees of surface roughness. The observed (unexpected) behaviour of the tennis ball drag coefficient is explained in terms of a new flow model that includes the drag contribution of the ‘fuzz’ elements.