The Answer My Friend, Is Blowing In The Shower Curtain

Last Updated (Saturday, 24 July 2010 16:24) Written by Raymond Nakamura

 On vacation (wonders never cease, so science never rests), I stayed in a place with a shower that felt like I was being whacked with a hair brush, and not in a good way. The more interesting thing was how the shower curtain would mysteriously blow in. I'd noticed this in other showers before but didn't realize it was such a contested phenomenon until I looked into it further. Here are the top explanatory contenders I've found.

Buoyancy - Let Me Float This Past You

The hot shower water heats the air, which rises. Air rushes in at the bottom, pushing the curtain with it. 

Pros — Like old time fireplaces burning real wood.

Cons — The curtain effect still seems to occur with cold water. But it does seem stronger with hot water. So if this is at play, it does not explain everything.

Bernoulli Principle - Possibly Overblown

Shower makes air move faster resulting in lower pressure so the shower curtain gets pushed in by the higher pressure air on the outside.

Pros — The Bernoulli Principle tends to be invokes whenever weird air effects occur.

Cons — The Bernoulli Principle is supposed to apply to different parts of the same flow, not for comparing different regions of air (inside and outside the shower). If the angle of the spray is toward the curtain, then maybe the speed of the air will be changing along the length of it with changes in pressure resulting in curtain movement.

Coanda Effect - Go With the Flow

Moving fluids tend to be entrained to follow a curved surface. The spray changes the flow of the air and the curtain follows the flow.

Pros — Flying Circus of Physics guy Jearl Walker says it's so. Also, Peter Eastwell (on p. 6) played with a shower and curtain and found that the effect depends on the proximity of the shower spray to the curtain. He says many phenomena ascribed to Bernoulli are actually related to the Coanda effect.

Cons — I thought the direction of the spray would be toward the bottom of the curtain, so I still don't get how this works.

Horizontal Vortex - Give This A Spin

Shower spray moves air which bounces off the walls and forms a vortex of lower pressure.

Pros — Engineering professor Dr. David Schmidt won an Ignobel prize in 2001 ("Research that makes people laugh and then think") for a computer simulation using fancy software of the phenomenon. Curmudgeonly Know-It-All Cecil Adams backs him up.

Cons — Does not seem to account for a person standing in the shower, which I'd think would mess up a vortex.

So I'm not convinced the curtain can be lowered on this question yet, though maybe it can be pushed out. Some say curved shower rods solve the problem, but I'm not sure if this supports one theory over another. The problem seems to be a lack of experimental data. Computer simulations are helpful, but they depend on what information you put in. Maybe I'll apply to the National Science and Engineering Research Council for a grant to set up experiments that would exclude one explanation from another. Or maybe Science World's renovation will include some showers with curtains.