Effect of Seams on the Boundary Layer Post #5

Our preliminary study using high-seamed Wilson 1030 baseballs and a brutal 3-wheel machine.  These results were presented at the 2018 APS DFD meeting.

These results are based on velocity measurements in the air using PIV. All of these data sets were acquired in a vertical plane.

We start with a smooth sphere the same size and weight as a baseball.  This gives us an idea of where the boundary layer would separate for this speed and rotation rate if the seams where not there.

Moving to baseballs at the same speed and rotation rate, the movie below was formed by taking several sets of data at different ball orientations.  In each shot, the blue dots indicate the location where the boundary layer separated from the ball to form a wake.  What we notice in these data is that the seams cause the point of separation to move toward the seam.

By examining the air velocity near the baseball, we fine that the seams increase the near-wall speed if the seam moves in the same direction of the ball (as on the bottom of a fastball) and visa versa.  In other words, the seams act as a “paddle.”

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