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A device for measuring the flow rate of a fluid, consisting of a tube with a short, narrow center section and widened, flared ends. Fluid, when flowing through the center section, is compelled to move at a higher velocity than when moving through end sections. This creates a pressure differential that is a measure of the flow of the fluid.
The Venturi principle is used in automatic shut-off nozzles and other petroleum marketing applications. The Venturi effect is the fluid pressure that results when an incompressible fluid flows through a constricted section of pipe.
The Venturi effect may be derived from a combination of Bernoulli's principle and the equation of continuity. The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is supplied by a drop in pressure or a pressure gradient force.
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 local pressure in the constriction cannot drop below the vapor pressure in a liquid or exceed the speed of sound in a gas.
The Venturi effect is named after Giovanni Battista Venturi, (1746–1822), an Italian physicist.