2019 in Review: Post 40

With 2020 around the corner, I thought I’d step back and look at what we have learned. Just about a year ago, I wrote the questions shown below in italics. Below each question, I will describe my current understanding of that topic.

What effects baseball drag?

  • Seam height
  • Center of gravity
  • Roundness
  • Leather

This got much more clear due to the results of the 2019 MLB Home Run Committee report. They concluded that the seam height affects drag, although they also concluded that some other unknown element was affecting drag more. I believe we will eventually know that drag is the story. They dismissed ball roundness, and I have in the past also, but Meredith Wills has stayed on me about it and I realize it may deserve more thought/study. I do not know of any measurements that have ever been done.

But here’s why I am interested: For fixed drag coefficient, the drag force increases like the diameter squared. So if I squash a ball and launch it spinning so sometimes its diameter is larger and sometimes smaller, the average of that will be larger than the drag from a constant diameter.

Am I saying that non-round balls have more drag? No, I am not. But I am saying it’s possible and we should look at it. The home run committee concluded that roundness doesn’t matter, but did not mention how they came to that conclusion.

Although I have no evidence, I also argued that center of gravity was unlikely to be important to drag.

We looked into leather smoothness. Since boundary layer transition to turbulence can only reduce drag, a smoother ball (which the 2019 may be) cannot reduce drag.

Can a spinning baseball have an asymmetric transition to turbulent flow resulting in an asymmetric wake and a sideways force on the ball unrelated to the Magnus effect (i.e. the laminar express)?

It can, but I do not believe this happens. See Post 29

Or can the seams on the rear of a 2-seam fastball pin the locations of wake formation and make an asymmetric wake?

Yes! And this happens for exactly the orientations that have a smooth front, thus the confusion about “laminar effect.” We spent quite a lot of time on this, culminating in my APS Presentation and our Gallery of Fluid Motion entry.

For a 4-seam fastball with a horizontal spin axis, where does the flow transition from laminar to turbulent flow and how is that location impacted by passing seams.

The seams on the front side of the ball always cause the flow to become turbulent. There is a lot of evidence in my APS Presentation and my SABER presentation.

For a 4-seam fastball with a horizontal spin axis, does the location where the pressure gradient changes from favorable to unfavorable depend on spin rate?

No idea. The only ways to get at this question would be Computational Fluid Dynamics (which is very unlikely in my opinion) or a wind tunnel study with its associated issues. You can see my reasoning here.

What does the “hiss” of a baseball tell us?

It’s telling us something, but I don’t know what. We made a sad attempt at looking into this, but I have generously left this question open for some other enterprising researcher.

If a knuckleball could be thrown with no spin, will it start spinning as it travels?

I strongly suspect so. We acquire our PIV data just a few feet from where the ball is launched, and the orientation is often changed. How much it changes seems to depend on the initial orientation. Boundary layer separation induces torque, bottom line.

And, the grand question posed by Rob Friedman (@pitchingninja), If God is really all powerful, could he throw a slider so filthy that he couldn’t hit it?

We did not find time to work on this during 2019, but perhaps Rob did?

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