Mechanical Enggineers explain the secrets of Wiffle Balls

Wiffle balls are very poorly behaved. They curve wildly, practically bouncing around in midair. No surprise, given the eight holes molded into one side. But to learn how the little plastic balls twist and spin through space, the go-to researcher is Jenn Rossmann, a mechanical engineer at Lafayette College in Easton, Pennsylvania, who specializes in how fluids, especially blood, circulate. In 2003, she started thinking that Wiffle balls might be more interesting for her students to study than the baseballs she’d been discussing in class. “It’s all about trying to trick them to do fluid mechanics, which I think is the most fascinating thing ever,” Rossmann says.
So over the past eight years, she and her students have built the world’s most advanced Wiffle-ology lab, using wind tunnels and computer models to measure aerodynamics. The key to the ball’s unpredictability? Air flowing over the perforated side of the ball is more turbulent, as you’d expect, which pulls it in the direction the holes are facing. But two vortices of air trapped inside push it in the opposite direction. Assuming the ball isn’t spinning, external airflow wins at low speeds; the internal vortices dominate at high speeds. At 40 to 60 mph—the speed at which most casual players throw—the two forces are about equal, making it hard to predict which way the ball will break.
Rossmann has figured out how to put all that science to use. Scuffing the ball, she says, disrupts the already turbulent external airflow. Result: The internal vortices become more predominant, which “can nudge it into that higher regime, where it’s more predictable,” Rossmann says. Competitive players—yes, there are those—often scuff, and some of them have sent doctored balls to Rossmann for analysis. But she doesn’t play the game herself. “I imagine I would enjoy it, but now I’m worried,” she says. “Everyone would assume that I’m really good.”