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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. As a second meaning, the fact that nearby phase front distortions of an optical wave are more severe than remote distortions of the same amplitude is called shower-curtain effect.
Also called Chimney effect or Stack effect, observes that warm air (from the hot shower) rises out over the shower curtain as cooler air (near the floor) pushes in under the curtain to replace the rising air. By pushing the curtain in towards the shower, the (short range) vortex and Coandă effects become more significant. However, the shower-curtain effect persists when cold water is used, implying that this cannot be the only mechanism at work.
See also Cooling tower.
Bernoulli effect theory
The most popular explanation given for the shower-curtain effect is the Bernoulli's principle. 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 attaching to the bather.
Horizontal vortex theory
A computer simulation of a typical bathroom found that none of the above theories pan out in their analysis, but instead 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 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
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.
Colder dense air outside and hot less dense air inside causes higher air pressure on the outside to force the shower curtain inwards to equalise the air pressure, this can be observed simply when the bathroom door is open allowing cold air into the bathroom.
Many shower curtains come with subtle features to prevent the shower-curtain effect. Most have adhesive suction cups which are applied to the edges of the curtain, and then pushed onto the sides of the shower when in use. Others may also have magnets at the bottom.
- "What Is The Shower Curtain Effect?". 2010-11-17. Retrieved 2011-06-29.
- 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)
- Experimental Investigation of the Influence of the Relative Position of the Scattering Layer on Image Quality: the Shower Curtain Effect
- The shower curtain effect; ESA