Boundary Layer:

The region near a surface with a flowing fluid where the fluid speed is reduced by the surface.  See our post on this.

Boundary Layer Transition:

When the flow near the surface of the ball stops being laminar (steady, smooth, small skin friction) to turbulent (chaotic, high skin friction).

Boundary Layer Separation:

When the boundary layer leaves the surface of the ball and forms a wake.

Discoball Changeup:

Stephan Strasburg’s changeup which spins on a nearly vertical axis (3:00 tilt). Term coined by Michael Augustine @AugustineMLB.


Surprising to most people, fluids are liquids AND gasses.  From wikipedia: In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases.

K-H Vortex:

A vortex generated by the Kelvin Helmholtz instability.  Common in laminar separated boundary layers and visible in the laminar case of the movie below.

Laminar Effect:

A hypothesized effect where the boundary layer transitions to turbulent on one side and remains laminar on the other resulting in an asymmetric wake and force on the ball.

Laminar Express:

A special 2-seam fastball that is thought to generate force in addition to the Magnus effect by generating an asymmetric wake. Term coined by Trevor Bauer. More generally, we call this effect a “seam shifted wake” since it happens on other pitches also, e.g. the “Discoball Changeup.”

Magnus Effect:

The force generated when a spinning object moves through a fluid (gas or liquid). The object can be a cylinder or a sphere. There are many online articles about this, including herehere, and one specific to baseball here.

Pressure Gradient:

The change in pressure as you move in some direction.  On a baseball, the pressure gradient negative (favorable) on the front of the ball and positive (unfavorable) on the back.  Unfavorable gradients amplify disturbances and normally lead to transition from laminar to turbulent and/or boundary layer separation.


The direction a pitch moves from the pitcher’s point of view as indicated on the hands of a clock.


The rate and direction that air is spinning.  See our post on this.


The region of air behind the ball that is being dragged along at nearly the ball’s speed. The wake has low pressure compared to the front of the ball, which causes drag.