13 July 2001

Mystery Of The Shower Curtain Solved

by Kate Melville

There's no way to get rid of the daily annoyance of the shower curtain billowing in and sticking to an exposed body part, but there's now a way to explain the phenomenon, thanks to a researcher at the University of Massachusetts.

David Schmidt, assistant professor in the mechanical and industrial engineering department, decided to map the forces acting on a shower curtain. Schmidt's areas of expertise include computer modeling of sprays, and the shower curtain question is one he's run into several times during his career. "This is a popular question," Schmidt said. "It's nice to have the answer key."

It's not as simple as it first appears. Until now, the explanation for the shower curtain's movement has been theoretical. "It's been one person's opinion versus another's," Schmidt said. With software designed by Fluent Inc., a New Hampshire-based software company, and modified by Schmidt to include spray capabilities, he decided, "I can do this. I thought it would be fun to use these tools to say more definitively what the effect was."

Using the Fluent software and two weeks' of time on his home computer, Schmidt drafted a model of a typical shower, divided the shower area into 50,000 miniscule sections, and let the software run. The software applies a technology called computational fluid dynamics to solve conservation of momentum and conservation of mass for each of the 50,000 sections over 30 seconds of actual shower time.

"What makes the shower curtain suck in is that you have low pressure on the inside and high pressure on the outside," he said. Schmidt discovered that there are two forces creating the low pressure inside the shower - Bernoulli effect and driven cavity - and it's the combination of these forces that has never been put forward.

The Bernoulli effect is the principal behind flight and an airplane's wings producing lift. The Bernoulli effect is seen near the showerhead, as air moves faster on the shower side of the curtain and pressure drops to vacuum pressure.

Driven cavity involves the shower's spray. Though the drops are being accelerated by gravity, they're actually slowing down due to aerodynamic drag, Schmidt said. "And for every action there's an equal and opposite reaction, and the opposite reaction is the air has to start moving. That's what makes this whole flow go." The air begins moving in a stable circle, called a vortex, "just like a dust devil indoors. This one, unlike a dust devil, doesn't die out because it's continuously driven by the shower."

Anyone can try to test Schmidt's model. "The best way to see it is to turn on the shower - cold water will do fine. Use a light, thin shower curtain and a strong showerhead. Stand outside the shower, stick your head in, and blow in smoke."