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February 28, 2011

"An Experimental Study of Wiffle Ball Aerodynamics"


Published in the American Journal of Physics in December, 2007, the above-titled paper by Jenn Rossman and Andrew Rau will tell you everything you ever wished you had known about what makes a Wiffle ball dance.


Too bad it's decades too late for me to make use of it.

But not for you.


[Figures and tables from the paper appear above and below.]

From Dana Mackenizie's March 2011 Wired magazine story on the ballistics of the Wiffle ball: "The go-to researcher is Jenn Rossman, 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."


"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 40-60mph — 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."


"Rossman 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,' Rossman says."


The abstract of the 2007 paper follows.



An experimental study of Wiffle ball aerodynamics

We measure the aerodynamic forces on a Wiffle ball as a function of the Reynolds number and ball orientation. The effects of asymmetric flow outside the ball and flow within the ball are considered, and are both associated with the ball’s tendency to curve without pitcher-imparted spin. The problem of Wiffle ball aerodynamics is an accessible way to introduce topics such as boundary layer separation and transition to turbulence.

February 28, 2011 at 04:01 PM | Permalink


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Moving the ball in the air due to aerodynamics? Cricketers have been doing this for centuries, with the additional menace of using the seam of the ball to control the direction of the bounce.

Posted by: Fred | Feb 28, 2011 7:42:42 PM

Oh, Laws-a-day. Please don't let these be my tax dollars at work.

Posted by: Becs | Feb 28, 2011 7:30:30 PM

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