Shower curtain effect

Partly settled. Some of it is understood, some is still argued about.

The shower curtain mystery was discussed informally for decades, including in Scientific American, with the Bernoulli-principle explanation being the most commonly repeated but never rigorously confirmed. In 2001, David Schmidt, a mechanical engineer at UMass Amherst, built a computational fluid dynamics simulation of a running shower (done in his own time, unfunded) and found that spray droplets drag surrounding air into a horizontal, low-pressure-core vortex that pulls the curtain inward, work recognized with the 2001 Ig Nobel Prize in Physics -- whose own citation explicitly called it only a 'partial solution.' The vortex/drag picture is now the most commonly cited explanation, but because Schmidt's study was an informal simulation rather than a peer-reviewed experimental confirmation, and the Ig Nobel explicitly flagged it as partial, the everyday phenomenon is better described as partially, not fully, resolved.

An Ig Nobel Prize was given not for solving a mystery but for honestly admitting a shower curtain is only 'partially' understood.

Explanations that circulated

• The most popular explanation, Bernoulli's principle (fast-moving air inside the curtain has lower pressure, sucking it in), does not hold up: the effect persists with cold water and even without airflow from the showerhead driving a simple pressure drop, and simulations found the dominant driver is instead a recirculating vortex set up by droplet drag on the air.

References

• Shower-curtain effect, Wikipedia contributors, summarizing D. Schmidt's CFD simulation and 2001 Ig Nobel Prize citation, Wikipedia / Ig Nobel Prize in Physics, 2001