Velocity fields of pitched baseballs using Particle Image Velocimetry

Baseballs are normally pitched with spin. The spin axis and ball orientation vary from one type of pitch to the next. Most pitches (4-seam fastballs, curveballs, sliders and others) move as they do because of the “Magnus Effect,” similar to other sports balls. Other pitches take advantage of the baseball’s unique asymmetric “figure 8” stitching pattern, most notably the 2-seam fastball and the knuckleball. These are the focus of this study. A unique feature of our work is that we make in-flight PIV measurements of a pitched ball—without a wind tunnel. A 3-wheeled pitching machine throws the ball with the desired speed, rotation axis and rate. Two-component velocity data are obtained in a plane that either cuts the ball vertically or horizontally and is aligned with the ball direction. The velocity data are of sufficient resolution to reveal boundary layer separation, which is found to be a rich function of the ball speed, boundary layer state, rotation rate, and location of the stitches. For the knuckleball pitches (during which the ball does not rotate significantly), dozens of snapshots are acquired and analyzed allowing statistics to be obtained on separation locations on both sides of the ball.