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Shower Curtains

January 14, 2003

A public radio commentary

Today I'm talking to you from a most unusual place. I'm about to hop into my shower and share how an engineer solved one of the great mysteries of our time.

As I shower, the curtain billows into the tub. After years of doubt about exactly why this happens, we now have an answer from one Mr. David Schmidt. Schmidt, an engineering professor in Massachusetts, is an expert in something called computational fluid dynamics.

The motion of many fluids, like water and air, is described by a single equation, called the Navier-Stokes equation. Discovered in the 19th century, it's so important to our world that there is a million dollar prize for anyone who can solve it completely.

Engineers, though, with a computer can approximate its solution. They use this equation to calculate the flow of air over a jet, or how blood flows through our bodies. Schmidt, himself, is an expert in using the computer to solve problems with spraying fluids, exactly the case in a shower.

There are two competing theories about why my shower curtain billows into the tub. The first is the Chimney effect, which says that the hot water heats up the air in the shower, causing the curtain to rise. The shower stall then sucks in cold air, pushing the shower curtain into the tub. The second theory is based on the Bernoulli effect: that air rushes past the curtain, lowering the pressure along the curtain, sucking it in. This is, by the way, why an airplane flies.

Now, Schmidt programmed his home computer to simulate the flow of water past the curtain. Using nearly $30,000 worth of software, he created a virtual shower that flowed for thirty seconds at about eight gallons a minute. It took almost two weeks of computing, and had to make over 1.5 trillion calculations. Schmidt's computer showed him that we create a hurricane in our showers every morning.

The shower pushes the air into a large swirling vortex with a low pressure center. The shower's water droplets decelerate because of aerodynamic drag, transferring their energy to the bathub's air, which then twists like a hurricane in the bottom of the tub, pulling in the curtain. You can even see this hurricane, Schmidt says, if you blow smoke into the tub.

Now, this might all seem kind of silly, but the study of how fluid drops move is of vital importance. For example, many researchers use Schmidt's computer programs to study the scattering of fluids in asthma inhalers. They want to find better ways to deliver drugs to the lungs. Now, that would be an important problem to solve, unlike the shower curtain. After all, if you don't want to be bothered by a curtain, just buy for your shower stall a door.

Reporting from my shower this morning, I'm Bill Hammack.

Copyright 2003 William S. Hammack Enterprises