Shower power. (Physical news).

When a flimsy shower curtain "attacked" David Schmidt one morning, he peeled the sticky plastic off his bare thigh and decided to solve the mystery of clingy wet curtains. Schmidt, a mechanical engineering professor at the University of Massachusetts in Amherst, may be the perfect sleuth to do it.

Until now, says Schmidt, scientists have offered two hypotheses, or educated guesses, to explain menacing shower curtains: "It always came down to one person's opinion versus another's," he says. Some scientists have claimed it's the Bernoulli effect: as water, air, and other fluids accelerate (speed up), surrounding air pressure drops, creating lift (upward force). In other words, the same forces that cause airplanes to soar also make shower curtains billow. Other scientists have said it's the chimney effect: as hot air in the shower rises, cool air outside the shower rushes in to fill the void, drawing the curtain with it.

To study the forces at work, Schmidt drafted a computer model of a typical shower and divided the area into 50,000 tiny tetrahedral cells, or pyramid-like sections. Then he let the software crunch numbers for 50 hours. To record what happens in each of the cells during 30 seconds of shower time, the program used computational fluid dynamics, complex math equations that explain how whirls of air and water react to pressure and motion. After 1.5 trillion calculations, the program spit out a colorful map of shower forces. To Schmidt's surprise, it looked like a tropical storm (see diagram, above).

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It turns out, a curtain billows and clings because a shower stall is what engineers call a "driven cavity." Imagine the tub bottom as a moving treadmill. The motion forces the air and spray in the shower to rotate. The result: a vortex (swirling cylinder of fluid) that spins like a sideways hurricane. The vortex in the shower has a low-pressure center--and that sucks in the shower curtain. The sucking force is weak, though. To thwart a shower curtain attack, use a heavy curtain or one weighted with magnets.

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Caption: (Top) 1. Spray causes air to flow down from the shower head. 2. Air bounces off hard shower walls; ceiling forces the air back toward shower head. 3. Air currents swirl into a vortex. Its low-pressure center sucks in nearby air (above)--and the shower curtain.