Post 72b: Separation upstream of a seam.

In post 72, I presented the map below (this post won’t make a lot of sense if you haven’t read that). This post will be about the blue region. A seam in this region (roughly from -10º to -28º) can sometimes cause laminar separation farther forward. The average location is the blue dot.

This is a strange phenomena and a bit hard to explain, although I think I understand mostly what is happening. In these cases, the separation always happens while the flow is laminar. I’d like to show what that looks like, because it’s pretty dang cool. This movie below is the raw data for Particle Image Velocimetry. The ball is moving toward the left at 90 mph and is not spinning. It’s two images of theater fog acquired a couple dozen milliseconds apart. What is interesting here is the black streak that emanates from the ball starting near the very top and radiating backward at about 15º. That is a line of very strong shear that is generated by a laminar separation from the ball. Notice how the particles behind that line are moving with the ball (at about 90 mph) while in front of it they are much slower. The shear spins the air and whips the particles out of that region, forming the black streak.

Before you start saying Bauer was right about this, let me note that the flow on the front of the ball is always laminar over at least part of the ball and maybe all the way up to the hemisphere plane. What is special here is not that the flow is laminar, but that the separation occurred on the laminar part of the ball. As a starting point, I assume the flow on the front of the ball is always laminar and on the back it’s always turbulent at this speed. I have not seen anything yet to make me think otherwise. And this is not a coincidence since pressure is decreasing on the front and increasing on the back. Increasing pressure promotes turbulent flow and visa versa.

It is also VERY important to note that we have never seen a separation like this unless a seam was nearby on the back of the ball. I am not sure how it triggers the upstream laminar separation, but I am confident that this configuration is necessary. I have a couple of friends who have suggested it could be due to what the seams are doing in the other plan (nearest and farthest from the camera). It’s a very good theory.

Many have spoken of seams as roughness and their presence leading to turbulent flow. In my opinion, no part of a baseball is “smooth”. They do not remotely behave like smooth balls (as show in John’s thesis). The situation on the front of the ball is very insensitive to a bump like a seam and on the back it is so touchy that a seam nearly always causes separation.

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