What is a "venturi"?

The Venturi effect is a special case of Bernoulli's principle, in the case of fluid or air flow through a tube or pipe with a constriction in it. The fluid must speed up in the restriction, reducing its pressure and producing a partial vacuum via the Bernoulli effect. It is named after Giovanni Battista Venturi, (1746�1822), an Italian physicist.

A fluid passing through smoothly varying constrictions is subject to changes in velocity and pressure in order to satisfy the conservation of mass-flux (flow rate). The reduction in pressure in the constriction can be understood by conservation of energy: the fluid (or gas) gains kinetic energy as it enters the constriction, and that energy is supplied by a pressure gradient force from behind. The pressure gradient reduces the pressure in the constriction, in reaction to the acceleration. Likewise, as the fluid leaves the constriction, it is slowed by a pressure gradient force that raises the pressure back to the ambient level.

The limiting case of the Venturi effect is choked flow, in which a constriction in a pipe or channel limits the total flow rate through the channel, because the pressure cannot drop below zero in the constriction. Choked flow is used to control the delivery rate of water and other fluids through spigots and other valves.

Practical uses
The Venturi effect is visible in:

the capillaries of the human circulatory system, where it indicates aortic regurgitation
large cities where wind is forced between buildings.
gas jets that mix air and flammable gas in barbecues, gas stoves, and Bunsen burners
water aspirators that produce partial vacuum from a water spigot
atomizers that disperse perfume
foam firefighting nozzles and extinguishers
carburetors that use the effect to suck gasoline into an engine's intake air stream.
Buchner funnels that use the Venturi effect to provide the suction on force against the contents of the funnel
Protein skimmers, a filtration device for saltwater aquaria.
in automated swimming pool cleaning robots that use pressure-side water flow to collect sediment and debris.
the modern day barrel of the , which uses a reverse taper to speed the air down the tube, enabling better tone, response and intonation.
A simple way to demonstrate the Venturi effect is to squeeze and release a flexible hose that is carrying water. If the flow is strong enough, the constriction will remain even if the hose would normally spring back to its normal shape: the partial vacuum produced in the constriction is sufficient to keep the hose collapsed.