Laminar and Turbulent Boundary Layer Separation on New MLB balls Post #22

We are continuing our measurements on MLB balls and are learning some interesting things. First off, the physics are a bit more complicated than they were on machine-pitched high-seam balls. In that case, we found that the boundary layer was always turbulent when it separated. This finding made me somewhat skeptical of claims about the “laminar express” pitch, which could not happen if the flow was already turbulent.

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 and wakes and on our current setup.

A boundary layer usually flows along a surface, but if the flow decelerates too quickly, it can separate from the surface. On a “bluff body” such as a baseball, boundary layer separation causes a wake to form behind the ball.

This post is about cases where boundary layer separation happens when the boundary layer is laminar as opposed to turbulent. These two circumstances have a huge impact on the wake that is formed. Up until now, we’ve only seen turbulent boundary layer separation on the rear of the ball. But a brand new MLB ball can generate either condition depending on its orientation.

All data presented here is for a non-spinning MLB ball moving 90 mph right to left. Velocity vectors are presented over contours of vorticity (blue meaning clockwise rotation of air and red counter clockwise. A seam upstream of the middle of the ball acts as a strong disturbance to the boundary layer, which becomes turbulent as a result. Separation occurs at the rear of the ball at a location marked with an arrow.

Non spinning ball moving right to left at 90 mph with a seam on the front of the ball. The boundary layer separation point is marked with an arrow. The blue patch over the seam is likely a measurement error.

However, if the front of the ball is smooth, the boundary layer may remain laminar. The laminar boundary layer separates on the front of the ball to form a very strong shear layer (note the drastic change in velocity across the blue line) and a very wide wake.

Laminar boundary layer separation from the front of a smooth ball to form a large wake.

These results appear “noisy” because Particle Image Velocimetry suffers from large random errors in the presence of flow shear. Interestingly, it is actually easier to see the boundary layer separation location in the raw PIV data, as shown below. Laminar boundary layer separation generates so much shear that our particles are flung from the shear layer resulting in a dark streak.

Raw PIV data. The boundary layer separation location is marked with an arrow.

Note that we have seen all of these behaviors in multiple data sets.

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