# Why does the shower curtain move toward the water?

David Schmidt, an assistant professor at the University of Massachusetts¿Amherst, provides the following explanation:

 Image: COURTESY OF DAVID SCHMIDTSHOWER SIMULATION shows how a vortex forms, creating a pressure drop and sucking the curtain toward the water.

Maybe it happened to you this morning: you entered the shower and the curtain moved in to engulf you. I have recently discovered a new explanation for this common phenomenon, thanks to modern fluid-simulation technology.

As an assistant professor in the mechanical and industrial engineering department, I research ways to accurately simulate sprays. Typically we use these spray simulations to help design better diesel and aircraft engines. The same analysis, however, is equally applicable to a bathroom shower. The shower is, after all, just a large spray.

Until now, explanations for the shower curtain's movements were theoretical. It was one person's opinion versus another's, with most ideas drawing on the Bernoulli effect or on so-called buoyancy effects. The Bernoulli effect is the principle that explains how an airplane's wings produce lift. It says that as a fluid accelerates, the pressure drops. But the Bernoulli effect is based on a balance between pressure forces and acceleration, and does not allow for the presence of droplets. Nor, according to my calculations, is it responsible for the curtain deflection.

The buoyancy theory supposes that the hot shower causes the temperature of the air in the shower to rise, reducing its density. In that case, the pressure on the shower side of the curtain will be lower than the pressure on the outside at the same height from the floor¿causing the curtain to move toward the lower pressure. The problem with this explanation is that the curtain will suck inward toward a cold shower, too.

A modern way to study fluid-flow problems is to use computers to solve the basic equations of fluid motion. These equations are based on conservation of mass and momentum. Because of the limitations of finite computer power and current mathematics, however, the solution process can be difficult and time-consuming. Also, spray simulations are a particularly difficult challenge because they involve two different phases of water: liquid and gas.

To attack the shower curtain problem, I used software designed by Fluent Inc., a New Hampshire-based software company that contracted my consulting firm, Convergent Thinking LLC, to add advanced spray models to their software. The simulation took advantage of the fruits of this project. I was able to include the effects of the drops breaking up. Even more important, the new spray models captured the distortion of the droplets, which tends to increase their aerodynamic drag. This drag is the force between the air and the water that imparts motion to the air and slows the droplets.

To do the calculation, I drafted a model of a typical shower and divided the shower area into 50,000 minuscule cells. The tub, the showerhead, the curtain rod and the room outside of the shower were all included. I ran the modified Fluent software for two weeks on my home computer in the evening and on weekends (when my wife wasn't using the computer). The simulation revealed 30 seconds of actual shower time.

When the simulation was complete, it showed that the spray drove a vortex. The center of this vortex¿much like the center of a cyclone¿is a low-pressure region. This low-pressure region is what pulls the shower curtain in. The vortex rotates around an axis that is perpendicular to the shower curtain. It is a bit like a sideways dust devil. But unlike a dust devil, this vortex doesn't die out because it is driven continuously by the shower.

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1. 1. Lightning Joe 09:24 PM 12/8/07

Mr. Schmidt, have you simulated the effect of having a human body standing in the shower while it's running? Seems as though it would break up your precious (two-week run of \$28,000 software package) vortex.

Seriously, best of luck with your endeavors, but remember that the premise makes the problem.

2. 2. desertman001 10:17 PM 9/22/08

The reason the curtain moves is simple. Put a wet hand between the curtain and liner and you will find a big updraft filling the space between the curtain and liner. the downward flow of water displaces air that is pushed to the bottom of the shower were it is directed up the side of the shower and under the curtain. If you put a yardsick below the curtain to block the air, then turn the shower on, the effect disappears

3. 3. Adam Laceky 12:40 PM 10/5/08

Utter hogwash. I tested this theory, and the only thing that makes any difference is the temperature of the water. Hot air rises, vents out the top of the shower stall and sucks cooler air in from the bottom.

4. 4. desertman001 11:01 PM 12/12/08

well yes hot air rises but this downward flow is much stronger. I placed a large board between the water flow and the liner(with a 3 inch gap at the bottom for air to flow to the curtian) and the effect still occurs!. that should have prevented the vortex from affecting the liner. anyone with a shower and curtain want to test this and weigh in?

5. 5. Franki 11:35 AM 11/13/09

Problem Solved. No, not the scientific question, but the technical solution to the real world problem. The annoying billowing-curtain effect is eliminated when a flap (about a foot deep) is sewn across the bottom of the curtain, and the curtain arranged so the side of the tub is between the flap and the curtain. I don't know which air-flow theory explains it, but it works. No more shower curtain sticking to your legs.
(I accidentally googled to this site while looking to replace such a shower curtain.)

6. 6. Annou in reply to desertman001 07:03 PM 12/29/09

Hello, desertman001!
I'm French and I'm carrying out a school project about "Why does the shower curtain move toward the water?" and I'll speak about the David Schmidt's vortex theory (which is very stricking then) so tried to highlight the lines of vortex by various experiences but in vain.
I read that you have tested an experience which worked but I didn't all anderstood...
Could you explain me, if it doesn't disturb you...
And just a little question: is the difference between a bathtub and a shower important for this experience because I only have got a shower at home!
Thank you...

7. 7. desertman001 10:41 PM 4/30/10

annou, sorry for the late response, just saw your post. for it to work there has to be overlap so the curtian hangs down below the side of the enclosure. You should also have a shower curtian liner and a shower curtian so the air has a place to gather in between the two. If the curtian covers the shower entry then the air will come up from the bottom of the shower and fill the curtian /liner area and push the liner outward. Good Luck

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