The Aerodynamics of the Knuckleball page 1

Knuckleball

This is perhaps the hardest pitch to master. Not just for a pitcher, but for an aerodynamicist as well. Some believe that a knuckleball thrown without any spin will be at the mercy of any passing breeze. An thus, dances through the air in an unpredictable fashion. However, the most likely reason for the "dance" of a knuckleball is a very slow spin. Researchers have learned that a slight change in the orientation of the ball with respect to the flow of air results in dramatic changes in the forces acting on the ball. Not only does the magnitude of the force change, but the direction also changes. This is why the ball appears to "dance".

The mechanism by which the forces change magnitude and direction is not known. However, one can theorize that the stitches play a key role. The stitches will most likely cause the boundary layer to trip to a turbulent state. As we know, turbulent flow will stay attached longer than laminar flow. In fact, once the boundary layer becomes turbulent, a separated flow tends to reattach. This reattachment will dramatically alter the forces on the ball. Similarly, as the ball rotates, a region that was turbulent due to the position of the stitches, might now become laminar. The laminar flow will separate earlier than the turbulent flow. This altering of the state of the flow from laminar to turbulent, separated to attached, would cause the forces on the ball to fluctuate as shown by the experiments.

Furthermore, it is important to note that even if the pitcher throws the ball with no rotation, the flow asymmetry will cause the ball to rotate. The flow asymmetry is developed by the stitch pattern on a baseball.

The Spitball and the Vaseline ball

As previously mentioned, the knuckleball is very difficult for a pitcher to learn much less control. A simpler and more effective method is to use a lubricant such as saliva or vaseline. This causes the pitch to slide through the fingers and thus have little spin. Therefore, the ball moves like a knuckleball, but at the speeds of a fastball. This makes a spitball next to impossible to hit.

Other dirty tricks employed by pitchers involve scuffing the surface and/or polishing the surface. Just as a rough surface promotes turbulent and, therefore, attached flow, a polished surface will maintain laminar flow and hence separated flow. By roughening one side and polishing another, the effects of various pitches will be exaggerated. So, in reality, a pitcher is not just a ball player, but an amateur aerodynamicist as well.

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