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\title{How to Avoid Being Attacked in the Shower}
\author{Anthony Ramirez\thanks{IgNobel Laureate, 2001}}
\maketitle
\begin{abstract}
Like millions, perhaps even billions, of people, David Schmidt of
Amherst, Mass., takes a shower every morning. For the last few years,
between first spritz and final drain, a question has vexed him: why
does the shower curtain suck in? 
\end{abstract}
Today, thanks to \$28,000 worth of high-powered computer software, a
Ph.D. in engineering and too much free time, Dr.~Schmidt, 31, a
mechanical engineering professor at the University of Massachusetts,
believes he has an answer. It has to do with why airplanes fly,
hurricanes twist and apples fall.

True, the mystery of the sucking shower curtain does not rank high on
the ladder of mankind's challenges, and he himself never encountered a
sucking curtain until he showered one day at his mother-in-law's
house. There, with the showerhead hissing away, the corner of her
gossamer-thin shower curtain billowed in and clung clammily to his
leg. `It sucked beautifully,' he recalled.

For years, apparently, engineering cognoscenti and amateur scientists
have wrestled with clinging shower curtains. In 1994, Mr.~Schmidt
encountered the seemingly humdrum problem on his doctoral examinations
at the University of Wisconsin. One camp favors something called the
Bernoulli principle, which holds that as water, air and other fluids
accelerate, their pressure drops, leading to lift. Like an airplane
wing, the shower curtain moves, the Bernoulli backers say, because
water from the showerhead accelerates air on one side, letting air
rush in and move the curtain.

Another camp favors the buoyancy theory. Hot air on the shower side
rises, leading to cooler air pushing the curtain in. But no single
theory clinched the question. Curtains, for example, still billow in
cold showers.

Enter Professor Schmidt, with an industrial-strength computer program
he has helped develop. He is an expert in the computer modeling, or
imaging, of sprays, which is useful in diesel engines and the
like. First, he drafted a computer image of his mother-in-law's
bathtub, with its typically curved shapes, then filled it with 50,000
tetrahedral cells, or pyramid-like structures, that sense velocity and
pressure in three dimensions. Finally, the `shower' flowed for 30
seconds at about eight gallons per minute, a bracing blast.

In the end, Dr.~Schmidt's home computer crunched numbers for the
better part of two weeks, or 1.5 trillion calculations, leading to
some surprising answers --- surprising, at least, for engineers.

The shower's water droplets decelerate under the influence of
aerodynamic drag, transferring energy to the bathtub's air, which
begins to twist like a miniature hurricane turned on its side. As in
the eye of a hurricane, the pressure in the center of this disturbance
is low, pulling on the shower curtain. Curtain rods keep the top of
the curtain in place, but below the showerhead the bottom of the
curtain\dots sucks in.

A shower door, heavy shower curtain, or no curtain at all, solves the
problem, Professor Schmidt acknowledges. But you knew that already.

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