- Venturi effect
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 principleand the equation of continuity. The fluid velocitymust increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energyis 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 pressure cannot drop below zero in the constriction. Choked flow is used to control the delivery rate of waterand 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 .
The Venturi effect is named after
Giovanni Battista Venturi, (1746–1822), an Italian physicist.
*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. See
aspiratorand pressure headfor a discussion of this type of siphon.
*Orifice plate:Venturi tubes are more expensive to construct than a simple
orifice platewhich uses the same principle as a tubular scheme, but the orifice plate causes significantly more permanent energy loss. Aortic insufficiencyis a chronic heart conditionthat occurs when the aortic valve's initial large stroke volume is released and the Venturi effect draws the walls together, which obstructs bloodflow, which leads to a Pulsus Bisferiens.
The Venturi effect may be observed or used in the following:
capillariesof 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 grills,
gas stoves, Bunsen burners and airbrushes
Water aspiratorsthat produce a partial vacuum using the kinetic energy from the faucet water pressure
Steam siphons 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 gasolineinto an engine's intake air stream
Protein skimmers (filtration devices for saltwater aquaria)
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
Venturi scrubbers used to clean flue gasemissions
* Injectors (also called ejectors) used to add chlorine gas to water treatment chlorination systems
* Sand blasters used to draw fine sand in and mix it with air
bilgewater 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
scuba diving regulatorto assist the flow of air once it starts flowing
vaporizers to optimize efficiency
Venturi masks used in medical oxygen therapy
recoilless rifles to decrease the recoil of firing
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.
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