# Shower-curtain effect

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.

## Theories

### Buoyancy theory

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.[1]

### Bernoulli effect theory

The most popular explanation given for the shower-curtain effect is the Bernoulli's principle.[1] 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.[citation needed]

### 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.[2][1]

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.[3]

### The Coandă Effect

The Coandă Effect, also known as "boundary layer attachment", is the tendency of a moving fluid to adhere to an adjacent wall.[4]

### Condensation

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.[citation needed]

## Solutions

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, however these often simply are used to add some weight to the bottom of the shower curtain, as most showers are made out of non-magnetic materials.[citation needed]