NEW SSW RESULTS: Effect of Seams Upstream (PART 2 OF 4). POST 73

Part 1 was focused on non-spinning balls in a 4S configuration at 90 mph. This post will turn the ball 90º to focus on 2S orientations. In the next post, we’ll talk about results from lower and higher speeds followed by spinning baseballs.

For each of these cases, there are many similarities and a few interesting differences. Note that this post does not stand alone; you are going to have to read Part 1 to understand the rest. As with Part 1, this is all based on John Garrett’s thesis.

You may wonder why so much focus on balls that are not spinning? After all, knuckleballs are pretty rare! Here you go:

By placing the ball on a tee in different positions, John mapped out behavior for many 2S orientations. In general, we are interested in what happens when a seam is near the hemisphere plane, or a slice through the middle of the ball perpendicular to its direction of travel. An example is shown below with a seam just ahead of the hemisphere plain on top of the ball, which is causing separation. There is no seam below, and “natural” separation occurs downstream of the hemisphere plane.

Figure 1: Non-spinning 90 mph ball in a 2S orientation. In this case, the seam is causing a separation on top of the ball. There are no seams on the bottom of the ball.

This is a single instant for a single orientation at 90 mph. John made hundreds of shots, and the results for similar orientations and speeds vary from shot to shot. In Part 1, I commented about this and noted that we are unsure whether this is because of unsteady effects or because the behavior is non-deterministic. Chaos is sexy, but, thanks to a conversation with my friend Patric Dufour (@LePhysichien), I’ve become convinced it is just unsteadiness. Take a look at the unsteady flow on the rear of an airplane wing and imagine capturing snapshots of it and random points in time.

We condense these results into a map such as the one in Figure 2 for 90 mph, non spinning, 2S orientations. This map works identically to the ones described in Part 1. The red dot shows the “natural” separation point, or where the flow separates when no seam is nearby. The error bars show one standard deviation of that position based on many samples. In other words, the separation point is within that band (nearly 9ª) 68% of the time. It’s within twice that range 95% of the time.

The green area is where a seam will cause the flow to separate at that location, and the darker green means separation is more likely.

There is also a range of angles (marked blue) that will cause separation of the laminar boundary layer to separate ahead of the seam, and the location indicated with the blue dot. Note that the flow is never laminar past the hemisphere plane. Also note the blue dot shows where separation occurs, but the blue line is the location of the seam causing that separation. The blue and green together make up “advanced separation”, or SSW potential.

Counter to what I would have expected, the results do not depend on which of the 2 seams are near the hemisphere plane (note that they are too far apart for both of them to be in the vicinity at the same time). In other words, if you take the ball in Figure 1 and rotate it clockwise about 48º, you have a different seam near the hemisphere plane, but with no seam upstream, and the results are similar.

While a 4S orientation ensures a seam lies in each quadrant of the ball, the 2S orientation often results in both seams either being on one side or the other of the ball. The purple dot indicates the natural separation point for a case where both seams are on other side of the ball, as is the case in Figure 1. Note that if both seams are on the top side of the ball but not in the region of the hemisphere plane, we expect a natural separation point of 16.8º as indicated by the red point.

Figure 2: Separation map 90 mph non-spinning 2S case. The red dot shows the “natural” separation point, or where the flow separates when no seam is nearby. The error bars show one standard deviation of that position based on many samples.
The green area is where a seam will cause the flow to separate at that location, and the darker green means separation is more likely.
The blue line is the range of seam angles that will cause separation of the laminar boundary layer ahead of the seam, and the location of separation is indicated with the blue dot.

OK, deep breath. Now let’s focus on what all of this means. Let’s compare the 2S map to the 4S map from part 1.

The natural separation point is significantly closer to the hemisphere plane for the 2-seam orientation while the effect of seams is similar on the front of the ball. I will speculate on why this is below, but first, let’s focus on what this will do.

The farther back the flow separates when no seam is present, the more asymmetric the wake becomes and thus the more SSW potential there is. So, does this mean 2S pitches have less SSW potential? No, as I explained in the video at the top, 2S pitches have 4S orientations in the non-spin plane and visa versa. Generally, MLB data show is 2S pitches have much more SSW effects than 4S, and this may be one reason why.

When the closest seam to the hemisphere plane is approximately 35° in front of the hemisphere plane, the other seam is very close to the front of the ball where disturbances such as the seam are damped out. As such, the disturbance from a seam in this region has little effect and transition from laminar to turbulent occurs later. A laminar boundary layer is more likely to separate near the hemisphere plane where the pressure gradient changes from favorable to unfavorable.

Figure 4: A ball with one of the 2S exactly on the front of the ball

Likewise, if both seams are located behind the hemisphere plane, there is no seam upstream to disturb incoming laminar flow, and the flow tends to separate near the hemisphere plane.

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