Flow past a transversely rotating sphere

Rotating bluff bodies are known to experience a side force due to a well-known phenomenon called the Magnus effect. The Magnus effect is evident in many sports (cricket, soccer, baseball, and football), where spinning balls are known to change their trajectories dramatically. In this study, we have experimentally investigated the effect of imposed transverse rotation on the drag and lift forces experienced by a sphere in the intermediate Reynolds number range (103 < Re < 104). The results show the considerable effects of Reynolds number and the spin parameter α (the ratio of the equatorial velocity of a sphere to the free stream velocity) on the force coefficients. A sudden drop was observed in both lift and drag coefficients at a ‘critical’ α value, before a recovery at higher alpha. This is unlike the behaviour for low Reynolds number flow (Re < 1000), where the drag and lift coefficients monotonically increase. Flow visualisations reveal that the boundary layer on the advancing side undergoes transition to turbulence close to the critical α, which could be related to the sudden drop in the force coefficients.