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in a diagram, the pressure in "1" conditions is higher than "2", and the relationship between the fluid speed in "2" and "1" respectively, is the same as for pressure.

The Venturi effect is an example of Bernoulli's principle, in the case of incompressible flow through a tube or pipe with a constriction in it. 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 effect is named after Giovanni Battista Venturi, (1746–1822), an Italian physicist.

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.

Referring to the diagram to the right, using Bernoulli's equation in the special case of incompressible fluids (such as the approximation of a water jet), the theoretical pressure drop at the constriction would be given by .

This is a Venturi tube demonstration apparatus built out of PVC pipe and operated with a vacuum pump.

• Venturi Tubes

The simplest apparatus, as shown in the photograph and diagram, is a tubular setup known as a Venturi tube or simply a venturi. Fluid flows through a length of pipe of varying diameter. To avoid undue drag, a venturi tube typically has an entry cone of 30 degrees and an exit cone of 5 degrees.

A venturi can also be used to mix a fluid with air. If a pump forces the fluid through a tube connected to a system consisting of a venturi to increase the water speed (the diameter decreases), a short piece of tube with a small hole in it, and last a venturi that decreases speed (so the pipe gets wider again), air will be sucked in through the small hole because of changes in pressure. At the end of the system, a mixture of fluid and air will appear.

• Orifice plate

Venturi tubes are more expensive to construct than a simple orifice plate which uses the same principle as a tubular scheme, but the orifice plate causes significantly more permanent energy loss and is less accurate.

In Chronic Aortic Regurgitation, after the initial large stroke volume is released, the Venturi effect draws walls together, transiently obstructing flow causing a Pulsus Bisferiens.

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.
• inspirators that mix air and flammable gas in barbecues, gas stoves, Bunsen burners and Airbrushes.
• water aspirators that produce a partial vacuum using the kinetic energy from the faucet water pressure
• Steam siphon using the kinetic energy from the steam pressure to create a partial vacuum
• atomizers that disperse perfume or spray paint (i.e. from a spray gun)
• foam firefighting nozzles and extinguishers
• carburetors that use the effect to suck gasoline into an engine's intake air stream.
• Protein skimmers, a filtration device for saltwater aquaria.
• In automated pool cleaners that use pressure-side water flow to collect sediment and debris.
• The modern day barrel of the clarinet, which uses a reverse taper to speed the air down the tube, enabling better tone, response and intonation.
• Compressed air operated industrial vacuum cleaners
• Venturi scrubbers used to clean flue gas emissions
• Injectors (or sometimes called ejectors) used to add chlorine gas in water treatment chlorination systems
• Sand blasters use the effect to draw fine sand in and mix it with air
• Emptying bilge water from a moving boat through a small waste gate in the hull. The air pressure inside the moving boat is greater than the water sliding by beneath.
• A SCUBA diving regulator may use the effect to assist the flow of air once it starts flowing.
• modern vaporisers use the venturi effect to optomise efficiency
• the Venturi mask used in medical oxygen therapy

A simple way to demonstrate the Venturi effect is to squeeze and release a flexible hose that is normal shape: the partial vacuum produced in the constriction is sufficient to keep the hose collapsed.

Venturi tubes are also used to measure the speed of a fluid, by measuring pressure changes at different segments of the device. Placing a liquid in a U-shaped tube and connecting the ends of the tubes to both ends of a venturi is all that is needed. When the fluid flows though the venturi the pressure in the two ends of the tube will differ, forcing the liquid to the "low pressure" side. The amount of that move can be calibrated to the speed of the fluid flow.

• Bernoulli's principle
• Choked flow
• Venturi injector
• Orifice plate
• Bunsen burner

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