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In physics, the shower-curtain effect is the phenomenon in which a shower curtain gets blown inward with a running shower. The problem of the cause of this effect has been featured in Scientific American magazine, with several theories given to explain the phenomenon but no definite conclusion.

Contents 1 Theories 1.1 Rising hot air theory 1.2 Bernoulli effect theory 1.3 Horizontal vortex theory 1.4 The Coandă Effect 1.5 Condensation 2 Solutions 3 External links

One older theory is that the heat generated by the shower is transmitted into the air of the shower, causing it to rise. As hot air flows up and out of the shower, colder air is sucked in to replace it. The shower curtain blocks this inflow, and the resulting pressure differential pushes it inward.

This theory was called into question by some limited testing which did not show a difference between cold and hot showers.

Bernoulli's principle states that an increase in velocity results in a decrease in pressure. This theory presumes that the water flowing out of a shower head causes the air through which the water moves to start flowing in the same direction as the water. This movement would be parallel to the plane of the shower curtain. If air is moving across the inside surface of the shower curtain, Bernoulli's principle says the air pressure there will drop. This would result in a pressure differential between the inside and outside, causing the curtain to move inward. It would be strongest when the gap between the bather and the curtain is smallest - resulting in the curtain trying to wrap you when you get close to it.

One computer model of the typical bathroom found that the spray from the shower-head drives a horizontal vortex. This vortex has a low-pressure zone in the centre, which sucks the curtain.

David Schmidt of University of Massachusetts was awarded the 2001 Ig Nobel Prize in Physics for his partial solution to the question of why shower curtains billow inwards. He used a computational fluid dynamics code to achieve the results. Professor Schmidt, however, is adamant that this was done "for fun" in his own free time without the use of grant or other public monies.

The Coandă Effect, also known as "boundary layer attachment".

A hot shower will produce steam that condenses on the shower side of the curtain; lowering the pressure there. In a steady state the steam will be replaced by new steam delivered by the shower but in reality the water temperature will fluctuate and lead to times when the net steam production is negative.

An inventor solved the issue altogether with the ShowerBow™. The patented pivot-weight holds the curtain, then tucks away. Another solution, first proposed in a CSIRO publication, is to use a door, not a curtain. Also, a second (decorative?) curtain, that remains outside of the tub/rim during a shower, may be hung outside of the main curtain; and this blocks, or slows, inflowing air at the bottom. Another simple solution, if water damage can be avoided, is to not fully close the curtain, leaving a gap at one or both sides which allows for easy pressure equalization and decreases inflow at the bottom. Finally, in low risk settings, a space-heater outside of the shower can also counteract or minimize buoyancy effects. Any lowering of water pressure and/or temperature differential should help decrease billowing. Yet another possible solution is the curved shower curtain rod, commonly found in hotels. The rod itself has a bend in it to hold the shower curtain out and away from the person. They are sold in home improvement stores and related web sites.

• An inventor solved the issue altogether with the ShowerBow™. The patented pivot-weight holds the curtain, then tucks away.
• Scientific American: Why does the shower curtain move toward the water?
• Why does the shower curtain blow up and in instead of down and out?
• The Straight Dope: Why does the shower curtain blow in despite the water pushing it out (revisited)?
• 2001 Ig Nobel Prize Winners
• Fluent NEWS: Shower Curtain Grabs Scientist – But He Lives to Tell Why
• Arggh, Why Does the Shower Curtain Attack Me? by Joe Palca. All Things Considered, National Public Radio. November 4, 2006. (audio)