![]() ![]() However, the fog particles being disturbed by the baseball are moving, and it can be fairly easy to find the location where you can see the particles are moving vs. The two raw images were taken microseconds apart from each other, so the fog particles in most of the image are not moving. There have been times where the processing or post-processing isn’t done quite right, so it’s important to analyze the raw data in order to confirm the separation location. After determining what I think is the separation location, I look at the raw data in the same location to refine the answer.The PIV sees the ball moving but all we care about is the air the ball is disturbing.) (Note: Ignore the answers inside the ball. The typical place to start looking is where the vorticity (the blue or red air) is especially dark. The separation point is typically where a velocity vector is shown normal to the surface of the ball, or where two vectors are pointing in opposite directions. Look at the processed PIV image of the baseball.If you are new to our measurements and how to interpret them, we have a primer here. Note that all images shown in this post were acquired using Particle Image Velocimetry. So here’s the process I use to analyze the data. We are seeing some interesting trends in the data that I look forward to sharing in the future. ![]() ![]() Rather than automatically claiming a pitch has a shifted wake solely based on a seam being positioned within the active range of our old separation map, I’m now analyzing approximately 900 images to determine first if there is a shifted wake, and then the location of all of the seams that are playing a role in the creation of that shifted wake. Additionally, not all shifted wakes are shifted to the same extent, meaning the strength of the SSW force on a pitch varies depending on the seam orientation. In determining if a pitch has a shifted wake, it is important to not only analyze the location of the seam near the hemisphere plane, but also seams upstream, and the relative position of that seam. The wake on every pitch is determined by the location of all of the seams on the baseball. Here is my new claim: The seams further upstream play a vital role in where the flow separates from the ball. In the image on the right, the separation occurs on the seam, but there is a symmetric wake and therefore no sideways force. That is clearly demonstrated by these two images acquired this summer.Īs you can see in the image on the left, the flow on top is not separating on the seam, but there is a shifted wake. There is still much to learn about the complex influence baseball seams have on flow separation. As we have collected more data, it is clear this was over-simplified. It was also assumed, and built into our pitch trajectory simulator UMBA2, that if a seam is present in the green regions of the separation map, there will be a SSW force on the ball. After analyzing many data images, we made the following “separation map.”Īt the time of this map, we assumed seams located anywhere but the green area of the map didn’t have any effect on the location of the flow separation. The hemisphere plane slices through the center of the ball and is 90 degrees to the trajectory of the ball. In our early work, we noticed seams tended to cause this early separation near what we call the hemisphere plane of the ball. This sideways force in addition to Magnus is the reason some systems cannot accurately predict the movement of pitches based on their spin axis and rpm, since they only consider Magnus, gravitational and drag forces. This asymmetric wake is what we refer to as the “shifted” wake (see figure), and causes an unbalanced pressure distribution and a corresponding sideways force on the ball creating movement. The main feature is that a baseball’s seams can cause earlier flow separation on one side of the ball relative to the other. Twitter: time to define a Seam Shifted Wake (SSW) pitch more carefully. ![]()
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