[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]
Anyone who studies fluid dynamics (flow of liquids AND gasses) probably has a fetish for vortices, or little whirls of fluid. Walk into any Fluid Dynamics PhD’s office and there is probably a photo or poster somewhere in there involving at least one vortex.
It turns out that there is a really cool vortex phenomenon that we have discovered in baseball flight. It’s pretty common in nature if you know where to look. It’s called the Kelvin-Helmholtz instability, and it is basically what happens when you have two layers of fluid near each other that are moving at different speeds. We call that a “shear layer”. A baseball example is shown in Figure 1.
The boundary layer produces vorticity, which is colored blue. The boundary layer remains attached to the ball until about the middle of the picture, where it suddenly separates to form a shear layer. It’s clear there is shear there since the velocity on the right side of the blue layer is large (see the arrows) and on the left is nearly zero. Shear layers are unstable and like to roll into vortices, and two are labeled in the picture. We see this quite often in our data, but the vortices are small and at the edge of our resolution, so you have to know what you are looking for to spot them. In some airfoils (wings), this phenomenon can lead to re-attachment of the boundary layer and a so-called “laminar separation bubble.” We have seen claims that these happen on baseballs but have not seen an example.
Such a flow pattern is common in nature. A “jet” of air, such as a hair dryer, is an example. Imagine you had a long thin hair dryer. It makes 2 shear layers, one above and one below. The vortex pattern is easy to see in our high-speed schlieren video in Figure 2. Unfortunately, this flow is moving right to left, and the shear layers are backwards from the baseball.
Figure 3 is raw PIV data of a similar jet of air, this time flowing left to right, to keep you on your toes. Again, this is high-speed video, so you may be surprised that this is air and not water (they look the same in slow motion). This is taken very near the point where the vortices begin to form.
Both of these jets are about 1/2″ tall, so those vortices are about that size too. But the cool thing about fluid dynamics is how it scales. The same stuff happens in the atmosphere, and Figure 4 shows some clouds at the edge of an atmospheric shear layer. If you look closely, you may notice that it looks like half of what you see in Figure 2 (although flowing the opposite direction). The cloud is moving to the right faster than the air above it.
I remember a children’s book that asked “What’s bigger than the sky?” Well, Jupiter is. And it’s easy to see shear in Jupiter’s atmosphere in Figure 5.
In fact, a google search of “kelvin helmholtz jupiter” will land you a lot of pretty pictures, including many from right here on Earth.
In closing, let me caution you that what you are looking at in this post is NOT the famous von Karman Vortex Street. These are the vortices that form on the back of bluff bodies, including baseballs. They are much larger, in general, than KH vortices.