How seams alter boundary layer separation points on baseballs

Many sports balls develop forces that change their motion due to spin, or the Magnus effect. Spin causes the boundary layer separation points to change leading to an asymmetric pressure distribution on the ball, and therefore a force. Seams on baseballs can play a similar role without spin. This paper reports the results of instantaneous velocity field measurements of non-spinning baseballs in flight. Two distinct effects of seams are reported. The first is the effect of seams on the front half of the ball where they may promote boundary layer transition. This is common in other sports balls. A second, more important, effect is to promote boundary layer separation over the seams. This only occurs near the center of the ball. Three Reynolds number values that straddle the drag crisis are measured. Various orientations of the seams are studied mapping out a full rotation of the ball. It is reported that the transition and separation points vary from one shot to the next. This is possibly due to subtle differences in baseball construction and orientation, but is more likely because the data represent random instances in a time-varying process. The average differences in separation points on opposite sides of the ball correlate well to previous force measurements on baseballs.