Starting in June, 2019, we use 2019 MLB balls in our testing unless otherwise stated.
We measure the airflow over a moving ball using a measurement technique called Particle Image Velocimetry (PIV). The data we get back is the air velocity and direction everywhere in our image. In the picture below, the colors represent velocity with red meaning large velocity moving to the left and blue meaning zero velocity.
Notice how the front of the ball (left side) has a red region. In other words, there is a narrow layer of air moving fast, and just outside of that, the air is not moving at all. We call this the “boundary layer“. The boundary layer on a spherical object like a baseball will remain near the surface of the ball when the flow is stable (the front side of the ball) but will detach when the flow is unstable (usually some place on the back).
There is high pressure on the front of the ball, and low pressure on the back of the ball. This difference in pressure is drag.
The reason that the boundary layer separation location is important is that the wake begins forming at this location. How low the pressure is on the back depends on when the boundary layer separates. The longer it stays attached, the higher the pressure on the back of the ball gets, and the lower the drag.
As important to our work, if the boundary layer separates farther back on one side of the ball than the other, there is a sideways pressure force on the ball that can cause movement.
So, what we care about is how wide the wake is when it starts forming (the red arrow above) and the angle of the wake (the green line).
In most of my presentations, I use color to indicate vorticity rather than velocity. Vorticity is a tough quantity to understand, but it is basically how much the air is rotating. Blue means rotating clockwise and red means counter-clockwise. What I like about vorticity is that it makes it easy to see where the wake is and where the boundary layers separate. An example is shown below.
So, when you look at a picture like this,
- Find the location on the back of the ball where a blue or red band makes a line off the ball and you’ve found the boundary layer separation location.
- Ignore blue or red on the front of the ball. This is usually a measurement error. There is little vorticity on the front of the ball and nothing interesting happens until at least near the middle of the ball.
- Blue or red? Doesn’t matter.
- The wake is the colored region.