Aerodynamic paradox

The aerodynamic paradox is a physical phenomenon.

To demonstrate it, one tries to blow a paper bag inserted into it out of a funnel . The bag is not blown out, but pressed against the walls of the funnel. This is due to the fact that the air between the bag and the funnel wall is partially entrained to the outside by the blown air flow; this creates a negative pressure between the bag and the funnel wall and the external air pressure drives the bag against the funnel wall .

This effect was made famous in 1826 by Charles Bernard Desormes and Nicolas Clément .