My (now former) student, John Garrett (@jwillyg20) has been working on new measurements of Seam Shifted Wake (SSW) for a couple of years. He just wrapped them up around Christmas and will soon finalize his MS thesis. This and the next few posts will be based on that work. You can download it here.
I’ll try to be more polite in the rest of this post but I thought I needed to make an impression.
If you are new to all this, it might benefit you to read our cliff notes here before plunging in. That said, I’ve tried to make this a bit more self contained that most of my posts.
The main difference between John’s work and the earlier work of Andrew Smith is sample size. Two orders of magnitude more samples. John turned the crank. A lot. And he proved something I’ve suspected for a long time. The baseball will not be known. Under the same conditions, it doesn’t always behave the same way.
Effects that appeared in Andrew’s data as outliers (and were thus ignored) have been found to happen, well, not often, but enough that we need to pay attention to them. To put it succinctly, the same baseball under the same conditions can often behave two (or even three) different ways. This complicates our understanding considerably. It may also explain why pitchers experience such large variations in their pitch movement.
John coined some new terms that we should define. And since it’s been a while, let me back up and mention the entire basis of the SSW is that separation (or the point where the wake behind the ball forms) is farther forward on one side of the ball compared to the other due to the seams.
We study this by launching the ball using a cannon developed by and on loan from the Sports Science Lab at Washington State University. The cannon can spin the ball or not.
Once in flight, we measure the air flow over the ball using a method called Particle Image Velocimetry. The acquisition sequence is demonstrated in the video below (from inside the box).
Through some technical magic, we can color regions with the air velocity is changing suddenly red or blue (depending on sign, not important here). This makes it easy to see where the wake begins forming.
Back to the new terms. “Natural Separation” means the location when the separation occurs on the back of the ball and is not caused by a seam, while “Advanced Separation” is cause by a seam near the “hemisphere plane”, or a slice through the middle of the ball, perpendicular to its direction of travel (shown by the blue line).
Our previous notions on SSW came from a simple map based on less that 100 shots of non-spinning balls. We’ve known all along that things change when spin is added, but we did not know exactly how. Furthermore, the non-spinning stuff is more complex that we previously thought. Let’s begin with that.
The results are for two fundamental configurations, 4-seam (similar to the one above) or 2S, which is the 4S case rotated by 90º on an axis parallel to the direction of travel. I’ll start with 4S results for 90 mph at Logan altitude.
The symbols represent different baseballs with varying seam heights (see post from long ago about how baseball seam height varies). Separation from a seam happens inside the two black diagonal lines, and this is very common. The baseball photo above shows an example on the bottom of the ball. Additionally, the separation may happen after the seam or before the seam. These are the cases that were previously unidentified. And, as is obvious from the plot, the state of the flow is not deterministic. For these carefully controlled conditions, a ball with a seam in the same position (A vertical line on the plot) may have one of 3 major outcomes. For instance, a seam at -20º (after the hemisphere plane) can result in separation before the seam, on the seam, or far after the seam. Only the smallest seamed balls had a separation point well after the seam.
This plot doesn’t give a great sense of how often these different outcomes occur. But John counted them up, and…
Remember that positive angles are in front of the hemisphere plane (i.e. the front of the ball). From the hemisphere plane back to -10º, the separation is nearly always on the seam. That is the easy stuff. We will call that a well-behaved baseball.
On the front of the ball (11 – 6º in front) it happens about 1/3rd of the time. The chance gets better from there to the center of the ball (the hemisphere plane).
On the rear of the ball, the seam does the trick about half the time.
Notice that the seam and advanced separation numbers don’t add up to 100%. The % Advanced Separation is how often the flow separates in front of where it is “supposed to”, leading to Seam Shifted Wake if the other side of the ball is not advanced.
I’ll get to 2S orientations, different speeds and rotating balls in future posts (they are all in the thesis). But I want to show a quick example of the more extreme advanced separation, one that happens in front of the seam. If you want SSW, this is as good as you can do.
OK, so we’d still like a map of what to expect and here it is:
Obviously, there is a lot going on here. The map makes it clear it is all happening near the hemisphere plane. The natural separation point is shown in red. The bands (± 9º) mark the standard deviation, so the natural separation occurs within those bands 68% of the time when no seam is causing an effect. This means no SSW.
The green shows locations where the seam can cause separation in advance of the natural location and thus SSW. The darker green indicates a higher likelihood.
The blue is the new weird stuff. Seams in those locations may cause separation at the blue dot and a range around it of about 4º. This is the point of minimum pressure on the ball and it is prone to separation. While this phenomena appears unreliable at 90 mph (it becomes more reliable at lower speeds, watch for a future post on that), this will also lead to SSW. You can read more about this phenomenon in post 72b.
In a nutshell, the behavior is non-deterministic, meaning we have to treat it statistically. At this point we do not know if what we see for a given shot is constant or if it changes in time. Neither would surprise me. Wakes are very unsteady.
In the near future, I will present another similar post for the 90 mph non-spinning 2S case. That will be followed by posts on 60 mph and 110 mph. Finally I will cover spinning baseballs. Hang on tight.