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
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 (P1 − P2) at the constriction
would be given by .
The Venturi effect is named after Giovanni Battista
Venturi, (1746–1822), an Italian physicist.
Experimental apparatus
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. See aspirator and pressure
head for a discussion of this type of siphon.
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.
Aortic insufficiency is a chronic heart condition that
occurs when the aortic valve's initial large stroke
volume is released and the Venturi effect draws the
walls together, which obstructs blood flow, which leads
to a Pulsus Bisferiens.
Practical uses
The Venturi effect may be observed or used in the
following:
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 grills,
gas stoves, Bunsen burners and airbrushes
Water aspirators that 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 gasoline into an
engine's intake air stream
Protein skimmers (filtration devices 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 (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
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 to assist the flow of air once
it starts flowing
Modern vaporizers to optimize efficiency
In Venturi masks used in medical oxygen therapy
In recoilless rifles to decrease the recoil of firing
Ventilators
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.
