Venturi effect

The Venturi effect is a case where fluid flows through a tube that is narrow. The velocity of the fluid increases as it flows through the narrower tube while the pressure decreases, due to conservation of energy. The effect is an example of Bernoulli's principle.

Venturi Effect Media

The upstream static pressure (1) is higher than in the constriction (2), and the fluid speed at "1" is lower than at "2", because the cross-sectional area at "1" is greater than at "2".
A flow of air through a Pitot tube Venturi meter, showing the columns connected in a manometer and partially filled with water. The meter is "read" as a differential pressure head in cm or inches of water.
Video of a Venturi meter used in a lab experiment
Idealized flow in a Venturi tube
Venturi tube demonstration apparatus built out of PVC pipe and operated with a vacuum pump

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With the B.E. a simple flow of either A. Fluids, or B. Air, will cause pressure. Within an amount of time, this will cause a decrease in pressure and have the potential of creating energy.