Flow Visualization Over a 74 MPH Curve Ball

A curve ball is not only one of the hardest pitches to throw, but it also creates some simulation challenges. Standing 60.5 feet from home plate, the pitcher simultaneously spins and hurls the baseball along a trajectory that depends critically on the release point as well as other factors. Simulation challenges for curve ball aerodynamics include careful geometric modeling of the baseball, the use of a rotating grid, and hybrid RANS/LES turbulence modeling.

A good curve ball must be thrown in such a way that the arc of the curve brings the ball over the strike zone by the time it reaches the plate. The release point (which determines the initial trajectory), the initial velocity, and the initial rotation rate are all critical parameters that determine the quality of the pitch. Once the ball leaves the pitcher's hand, aerodynamics and gravity take over for about the next half second.

The flow around a spinning baseball is complicated by a number of factors, not the least of which is the stitching that interrupts the smooth flow near the surface. But the main impact of the rotation is to alter the surface pressure distribution in such a way as to drive the ball down toward the ground faster than gravity alone. The Loci/CHEM solution for a spinning baseball uses overset unstructured grids and shows that the top to bottom unsteady pressure difference (red/green-high pressure and blue-low pressure) acts in the same direction as gravity to increase the downward force on the ball.

Note also that the particle traces behind the ball have a slightly upward trajectory, which means that the opposing momentum change acting on the ball due to the wake is also downward. This explains why one often sees pitches "thrown in the dirt" out in front of home plate: the pitcher did not correctly balance the required initial upward trajectory with the released ball velocity and rotation rate. It takes a lot of refined coordination and athleticism to throw a consistent breaking ball, which is one of the reasons why talented pitchers make a lot of money. Read more about the Physics of Baseball on Dr. Alan M. Nathan's page devoted to this topic.