ABSTRACT
Pipelines, blood vessels, hallways, and ink-jet printers all contain examples of internal flows. A fluid’s “stiffness,” viscosity, has a significant effect on the flow of an incompressible fluid through a pipe or between parallel plates. Baseballs and other objects moving through fluids leave wakes behind them. These wakes can be either laminar or turbulent. Even in laminar flow, obstructions and protrusions such as wings and flaps on airplanes and rocks in streams can cause some rotational flow behind them. Zones of rotational flow are called vortices. The streamlines are crowded near the bottom of the ball, and the wake is deflected upward by the spin. This deflection is linked to a net downward force on the ball, which is why a pitch thrown in this way will drop or sink as it approaches the batter.
