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How bad mornings go worse, demystified.

When you take your morning shower, you may or you may not have noticed how the shower curtain sometimes seems attracted to you. While you try to escape the first seconds of ice cold water spraying all over your sleepy cold body, you somehow get a feeling that the shower curtain is closing in on you. Maybe someone or something is outside the curtain, slowly wrapping it around you, as if to engulf and suffocate you. Sometimes the wet and cold curtain will cling to your body and your legs much like strange women do when you're not single. Images from Psycho flash before your eyes as you wrestle and hope that it was only a draft or something this time too...

This is perfectly normal, and nothing to be afraid of, it turns out. This phenomenon has been discussed by drunk physicists for decades, but it's always been a strictly theoretical discussion. Some blame the Bernoulli effect, which is used for describing why airplanes can fly. But that's not really similar is it? In the shower we have a stream of finite particles and a presence of air standing still. Others claim that it because of some kind of buoyancy effect, meaning that hot air will rise and produce a pressure difference that will account for the shower curtain's behavior. Well, obviously this is wrong, since the damn curtain attacks even before the water is hot! 

Well, it turns out that all the money spent on research isn't wasted. David Schmidt, an Assistant Professor at the University of Massachusetts, used a state-of-the-art fluid dynamics simulation program, in order to see what really happens in the shower. This program will numerically solve the basic equations of fluids, with a trade of in computer time against accuracy. The software can simulate spraying water, and by some sort of FEM-like division of the shower area, it was possible to render a reasonably accurate result, in a reasonably short time. These kind of differential equations tend to be hard to solve, even numerically, and usually require quite some time for even the simplest cases. 

What really happens in that shower is that the water stream creates a vortex of air. As you can see on the illustration below, the vortex will rotate horizontally. The eye of the vortex is at lower pressure than the surrounding room and the rest of the vortex. Compare with a cyclone. This low pressure virtually sucks the shower curtain right into the center of the vortex, which is also where you are likely to be standing. 

One way to avoid this is to shower under lower water flow, so that the vortex doesn't get strong enough. Another solution is to acquire an industrial strength shower curtain, not easily affected by tiny household showers.  

|                        _____    
|--< .                  /--<--\      
|     ..               /---<---\ 
|      ..             /  --<--  \
|       ...          / /  -<-  \ \
|        ....        \ | | o | | |    <--- Vortex
|         ....        \ \ ->- / /
|          ....        \ -->-- /  
|           .....       \-->--/ 
|            .....       \->-/ 
|             ......
|              ......
|               ......
|                .......     <--- Your shower 
|                 .......
|                  .......
Brought to you in the spirit of Physics FAQ
Source: Scientific American
