Why some smooth spinning balls curve backward (reverse Magnus effect)
Why do some smooth, spinning balls at intermediate speeds curve in the direction opposite to what the ordinary Magnus effect predicts?
History and references
Partly settled. Some of it is understood, some is still argued about.
Magnus described the basic sideways deflection of a spinning object in 1852, but ballisticians and later golf/baseball aerodynamicists noticed that smooth spinning spheres could sometimes curve the 'wrong' way, first clearly reported by Maccoll in 1928. The difficulty was that the effect only appears in a narrow window of Reynolds number and spin ratio, and depends sensitively on where the boundary layer transitions from laminar to turbulent on each side of the spinning ball. Kim, Park, Choi and Yoo's 2014 Journal of Fluid Mechanics study mapped out exactly when and why the reversal occurs, tying it to asymmetric laminar-to-turbulent boundary-layer transition on the advancing versus retreating sides. Because the full picture across all roughness, spin, and Reynolds-number combinations (relevant to golf balls, baseballs, etc.) is still being refined, this is best classed as partially rather than fully solved.
A spinning ball can be tricked into curving exactly backward from what everyone expects, just by nudging where its boundary layer decides to become turbulent.
Explanations that circulated
- The reversal is a measurement artifact or wind-tunnel wall effect
- The reversal happens for any spin rate above zero, rather than only within a specific Reynolds-number and spin-parameter window
References
- Inverse Magnus effect on a rotating sphere: when and why, Jooha Kim, Hyungmin Park, Haecheon Choi, Jinsung Yoo, Journal of Fluid Mechanics, 2014
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