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Why does the shower curtain billow in on me?


Shower to the People

Some might find it silly to study the shower curtain in such depth. Know this: The shower curtain effect is so pronounced a phenomenon that it did finally have to be put to scientific study due to controversial and contradictory theories.

Disclosure: All of that is an exaggeration. But the shower curtain effect did have some advocates with disparate theories, and one man finally had enough free time to tackle the subject with a computer model.

But before we get to the actual cause, let's talk about the two most popular hypotheses for the shower curtain effect.

  1. The Bernoulli principle says that as fluid speeds up, pressure drops. This creates an imbalance of air pressure: The air from the outside remains at the same pressure, pushing in on the lower-pressure air where you're showering, leading the shower curtain to have its way with you.
  2. The buoyancy theory: Hot water from the shower produces hot air. That hot air is less dense, so the colder, denser air from the other side heads for the lower-pressure area, causing the curtain to move inward.

Although they both would make great titles for the "Bourne Identity" franchise, they are both wrong. For one, the Bernoulli principle doesn't address the kind of droplets we'd find in a shower. The buoyancy theory sounds great -- until you realize that the curtain billows in on a cold shower, as well.

This is where our hero comes into the story. An assistant professor of mechanical and industrial engineering at the University of Massachusetts Amherst in 2001, David Schmidt stood up to the Bernoulli and buoyancy bullies. Schmidt created a model for the bathtub and ran a simulation for two weeks that recreated 30 seconds of shower time.

The second-most famous "eureka" moment to come out of a bathtub occurred when Schmidt saw that as the droplets decelerate, they transfer energy to the air around the bathtub, creating a swirling, twirling sideways vortex. The axis of the vortex is perpendicular to the curtain, and its center is a low-pressure region, so the curtain is pulled toward the eye of the hurricane, if you will [source: Schmidt].

Surely you're not done learning about showers and physics. To read more, explore the next page.


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