First PIV Measurements on MLB Balls Post #21

Thanks to Mike Fast of the Atlanta Braves and Professor Lloyd Smith at Washington State University, we have a large collection of brand new MLB balls and a means to launch them at high speed without damaging the ball. And we’ve rebuild our PIV setup. I am confident that this will be the start of something big. For a description of PIV, go here.

PIV Setup. The PIV system resides inside the fabric covered box. The WSU cannon is being fed by Andrew Smith on the right while John Garret runs the PIV (MLB.TV on the screen on left).
The PIV components inside the box

The cannon as pictured incorporates a “flex tip” that imparts spin to the ball. This component is currently damaged. We hope to get back to spinning balls in about a month. For now, we are looking at non-spinning balls, aka knuckleballs. Except we are “throwing” them 90 mph. We are not tracking the “knuckling” action, but rather focusing on the boundary layer behavior near the center of the ball.

The cannon is much more repeatable than a pitching machine. As a result, we can zoom in with our cameras to look at a small portion of the ball. I made a high-speed video of the acquisition with my iPhone;

PIV data acquisition (slowmo). When the ball trips both laser sensors (the red beams), the PIV system fires both laser sheets twice (green) the acquire 2 images of the ball 5 microseconds apart.

Much of what we will see in this study will have been seen earlier in studies using well-used high school balls with large seams. Our aim right now is to see how a pristine MLB ball (which has much lower seams and is smoother) behaves.

[If you are new to our measurements, you may take a minute to read here about vorticity (the colors in our plots), boundary layer separation and wakes]

In all of these results, the ball is moving right to left at 90 mph and is not spinning. We present air velocity vectors on top of the vorticity field. Understanding what vorticity is requires a lot of time, but suffice it to say that regions of high vorticity (red or blue) mark the location where the boundary layer separated from the ball. In the first dataset, there are no seams in the measurement plane on top of the ball. The ball is “smooth”, and the boundary layer separates about 15 degrees beyond the top of the ball. In this case, it appears to me that the boundary layer becomes turbulent (see the dark blue streak) does before it separates to form the wake.

As we have often seen, the boundary layer separates at the seam in for a case where the seam is near the rear of the ball. In this particular picture, the seam lies near the location where the separation occurs with no seam, but, as discussed here for spinning balls, there is a range of locations on the rear of the ball where the seam can cause the boundary layer to separate. Note that in that post, I argue that seems on the front of the ball have no effect because the boundary layer is always turbulent. The MLB balls used for the present post seem to be laminar on the front of the ball and even beyond when seams are not present, as discussed below.

Below is another case with the seam on the rear of the ball, but farther forward that the location where a the no-seam cases separates. Note that the boundary layer still separates from the seam. This supports my hypothesis of “seam controlled wakes.” (Note that this image may look noisier than the others. This is due to the choice of color levels and I will fix it soon)

When the seam is on the front of the ball, I see evidence that the boundary layer is turbulent (notice all the blue marked air near the top surface of the ball) and the boundary layer remains attached much farther around the ball.

I have said in the past (based on rough, high-seam balls) that the boundary layer of a baseball at 90 mph is always turbulent, but I may have to walk that back.

We plan to use results like these to learn why the aerodynamic drag of baseballs varies so much, and why a systematic change in the drag can lead to seasons with strikingly increased home run rates. Stay tuned! Much more to come.

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