The role of seams in the aerodynamics of baseballs

Spherical sports balls can generate forces due to asymmetrical transition to turbulence of the boundary layer. These forces, in turn, alter the movement of the ball. This is commonly seen in football (soccer) as well as in cricket. This effect has often incorrectly been used to explain baseball movement. We will show that in addition to causing transition to turbulence, baseball seams can act as spoilers, causing boundary layer separation closer to the front of the ball than normal. This is unique to baseballs. In fact, this effect has much more to do with baseball movement because baseball seams are arranged such that the boundary layer is nearly always turbulent everywhere on the ball at separation. In this presentation, we will show that in addition to these effects, seams can also cause turbulence transition of the boundary layer, or fail to cause transition in a narrow range of orientations. These conclusions are based upon measurements of the velocity field around a baseball in flight using particle image velocimetry. Lastly, our recent measurements on spheres roughened with macroscopic elements reveal that spin-sensitive drag is due to the wake of individual roughness elements. This may explain the reason that baseball drag is sensitive to spin. It is also possible that the seams contribute to lift due to spin.