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The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. The Venturi effect is named after its discoverer, the 18th century Italian
physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate caus ...
, Giovanni Battista Venturi.


Background

In
inviscid The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inter ...
fluid dynamics, an incompressible fluid's velocity must ''increase'' as it passes through a constriction in accord with the principle of mass continuity, while its
static pressure In fluid mechanics the term static pressure has several uses: * In the design and operation of aircraft, ''static pressure'' is the air pressure in the aircraft's static pressure system. * In fluid dynamics, many authors use the term ''static pres ...
must ''decrease'' in accord with the principle of
conservation of mechanical energy In physical sciences, mechanical energy is the sum of potential energy and kinetic energy. The principle of conservation of mechanical energy states that if an isolated system is subject only to conservative forces, then the mechanical energy is ...
(
Bernoulli's principle In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after the Swiss mathematic ...
). Thus, any gain in kinetic energy a fluid may attain by its increased velocity through a constriction is balanced by a drop in pressure. By measuring pressure, the flow rate can be determined, as in various flow measurement devices such as Venturi meters, Venturi nozzles and
orifice plate An orifice plate is a device used for measuring flow rate, for reducing pressure or for restricting flow (in the latter two cases it is often called a '). Description An orifice plate is a thin plate with a hole in it, which is usually placed in ...
s. Referring to the adjacent diagram, using Bernoulli's equation in the special case of steady, incompressible, inviscid flows (such as the flow of water or other liquid, or low speed flow of gas) along a streamline, the theoretical pressure drop at the constriction is given by: p_1 - p_2 = \frac\left(v_2^2 - v_1^2\right) where \scriptstyle \rho\, is the density of the fluid, \scriptstyle v_1 is the (slower) fluid velocity where the pipe is wider, \scriptstyle v_2 is the (faster) fluid velocity where the pipe is narrower (as seen in the figure).


Choked flow

The limiting case of the Venturi effect is when a fluid reaches the state of
choked flow Choked flow is a compressible flow effect. The parameter that becomes "choked" or "limited" is the fluid velocity. Choked flow is a fluid dynamic condition associated with the venturi effect. When a flowing fluid at a given pressure and temperat ...
, where the fluid velocity approaches the local speed of sound. When a fluid system is in a state of choked flow, a further decrease in the downstream pressure environment will not lead to an increase in velocity, unless the fluid is compressed. The mass flow rate for a compressible fluid will increase with increased upstream pressure, which will increase the density of the fluid through the constriction (though the velocity will remain constant). This is the principle of operation of a
de Laval nozzle A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube which is pinched in the middle, making a carefully balanced, asymmetric hourglass shape. It is used to accelerate a compressible fluid to supersonic speeds ...
. Increasing source temperature will also increase the local sonic velocity, thus allowing for increased mass flow rate but only if the nozzle area is also increased to compensate for the resulting decrease in density.


Expansion of the section

The Bernoulli equation is invertible, and pressure should rise when a fluid slows down. Nevertheless, if there is an expansion of the tube section, turbulence will appear and the theorem will not hold. In all experimental Venturi tubes, the pressure in the entrance is compared to the pressure in the middle section; the output section is never compared with them.


Experimental apparatus


Venturi tubes

The simplest apparatus is a tubular setup known as a Venturi tube or simply a Venturi (plural: "Venturis" or occasionally "Venturies"). Fluid flows through a length of pipe of varying diameter. To avoid undue
aerodynamic drag In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding flu ...
, a Venturi tube typically has an entry cone of 30 degrees and an exit cone of 5 degrees. Venturi tubes are often used in processes where permanent pressure loss is not tolerable and where maximum accuracy is needed in case of highly viscous liquids.


Orifice plate

Venturi tubes are more expensive to construct than simple orifice plates, and both function on the same basic principle. However, for any given differential pressure, orifice plates cause significantly more permanent energy loss.


Instrumentation and measurement

Both Venturi tubes and orifice plates are used in industrial applications and in scientific laboratories for measuring the flow rate of liquids.


Flow rate

A Venturi can be used to measure the volumetric flow rate, \scriptstyle Q, using
Bernoulli's principle In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after the Swiss mathematic ...
. Since \begin Q &= v_1 A_1 = v_2 A_2 \\ pt p_1 - p_2 &= \frac\left(v_2^2 - v_1^2\right) \end then Q = A_1 \sqrt = A_2 \sqrt A Venturi can also be used to mix a liquid with a gas. If a pump forces the liquid through a tube connected to a system consisting of a Venturi to increase the liquid 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), the gas will be sucked in through the small hole because of changes in pressure. At the end of the system, a mixture of liquid and gas will appear. See aspirator and pressure head for discussion of this type of siphon.


Differential pressure

As fluid flows through a Venturi, the expansion and compression of the fluids cause the pressure inside the Venturi to change. This principle can be used in metrology for gauges calibrated for differential pressures. This type of pressure measurement may be more convenient, for example, to measure fuel or combustion pressures in jet or rocket engines. The first large-scale Venturi meters to measure liquid flows were developed by
Clemens Herschel Clemens Herschel (March 23, 1842 – March 1, 1930) was an American hydraulic engineer. His career extended from about 1860 to 1930, and he is best known for inventing the Venturi meter, which was the first large-scale, accurate device for measur ...
who used them to measure small and large flows of water and wastewater beginning at the end of the 19th century. While working for the Holyoke Water Power Company, Herschel would develop the means for measuring these flows to determine the water power consumption of different mills on the
Holyoke Canal System The Holyoke Canal System is a system of power canals in Holyoke, Massachusetts. It is split into three canals based on elevation and distance from the inlet at the Holyoke Dam- the First Level Canal, Second Level Canal, and Third Level Canal. Con ...
, first beginning development of the device in 1886, two years later he would describe his invention of the Venturi meter to William Unwin in a letter dated June 5, 1888.


Compensation for temperature, pressure, and mass

Fundamentally, pressure-based meters measure kinetic energy density.
Bernoulli's equation In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after the Swiss mathema ...
(used above) relates this to mass density and volumetric flow, \Delta P = \frac \rho (v_2^2 - v_1^2) = \frac \rho \left(\left(\frac\right)^2-1\right) v_1^2 = \frac \rho \left(\frac-\frac\right) Q^2 = k\, \rho\, Q^2 where constant terms are absorbed into ''k''. Using the definitions of density (m=\rho V), molar concentration (n=C V), and
molar mass In chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance which is the number of moles in that sample, measured in moles. The molar mass is a bulk, not molecular, ...
(m=M n), one can also derive mass flow or molar flow (i.e. standard volume flow), \begin\Delta P &= k\, \rho\, Q^2 \\ &= k \frac\, \dot^2 \\ &= k \frac\, \dot^2 = k \frac\, \dot^2. \end However, measurements outside the design point must compensate for the effects of temperature, pressure, and molar mass on density and concentration. The ideal gas law is used to relate actual values to design values, C = \frac = \frac C^\ominus \rho = \frac = \frac \rho^\ominus. Substituting these two relations into the pressure-flow equations above yields the fully compensated flows, \begin\Delta P &= k \frac \rho^\ominus\, Q^2 &= \Delta P_ \frac \left(\frac Q\right)^2\\ &= k \frac \dot^2 &= \Delta P_ \frac \left(\frac\right)^2\\ &= k \frac \dot^2 &= \Delta P_ \frac \left(\frac\right)^2. \end ''Q'', ''m'', or ''n'' are easily isolated by dividing and taking the square root. Note that pressure-, temperature-, and mass-compensation is required for every flow, regardless of the end units or dimensions. Also we see the relations, \begin\frac &= \frac\\ &= \frac\\ &= \frac = \frac. \end


Examples

The Venturi effect may be observed or used in the following:


Machines

* During
Underway replenishment Replenishment at sea (RAS) (North Atlantic Treaty Organization/Commonwealth of Nations) or underway replenishment (UNREP) (U.S. Navy) is a method of transferring fuel, munitions, and stores from one ship to another while under way. First develop ...
the helmsman of each ship must constantly steer away from the other ship due to the Venturi effect, otherwise they will collide. * Cargo eductors on oil product and chemical ship tankers *
Inspirator An inspirator is a device, similar to a venturi tube and an orifice plate An orifice plate is a device used for measuring flow rate, for reducing pressure or for restricting flow (in the latter two cases it is often called a '). Description An o ...
s mix air and flammable gas in grills, gas stoves,
Bunsen burner A Bunsen burner, named after Robert Bunsen, is a kind of ambient air gas burner used as laboratory equipment; it produces a single open gas flame, and is used for heating, sterilization, and combustion. The gas can be natural gas (which is main ...
s and
airbrush An airbrush is a small, air-operated tool that atomizes and sprays various media, most often paint but also ink and dye, and foundation. Spray painting developed from the airbrush and is considered to employ a type of airbrush. History ...
es * Water aspirators produce a partial vacuum using the kinetic energy from the faucet water pressure * Steam siphons use the kinetic energy from the steam pressure to create a partial vacuum * Atomizers disperse perfume or spray paint (i.e. from a spray gun) * Carburetors use the effect to suck gasoline into an engine's intake air stream * Cylinder head in piston engine have multiple Venturi areas like the valve seat and the port entrance *
Wine aerator Wine accessories are things that may be used in the storage or serving of wine. Wine accessories include many items such as wine glasses, corkscrews, and wine racks. Glasses Wine glasses are a type of glass stemware that are used to drink and t ...
s infuse air into wine as it is poured into a glass *
Protein skimmer A protein skimmer or foam fractionator is a device used to remove organic compounds such as food and waste particles from water. It is most commonly used in commercial applications like municipal water treatment facilities and public aquariums. Sm ...
s filter saltwater aquaria *
Automated pool cleaner An automated pool cleaner is a vacuum cleaner that is designed to collect debris and sediment from swimming pools with minimal human intervention. History Evolution Swimming pool cleaners evolved from the water filter and early cister ...
s use pressure-side water flow to collect sediment and debris * Clarinets use a reverse taper to speed the air down the tube, enabling better tone, response and intonation * The leadpipe of a trombone, affecting the
timbre In music, timbre ( ), also known as tone color or tone quality (from psychoacoustics), is the perceived sound quality of a musical note, sound or tone. Timbre distinguishes different types of sound production, such as choir voices and musica ...
* Industrial
vacuum cleaner A vacuum cleaner, also known simply as a vacuum or a hoover, is a device that causes suction in order to remove dirt from floors, upholstery, draperies, and other surfaces. It is generally electrically driven. The dirt is collected by either a d ...
s use compressed air *
Venturi scrubber A venturi scrubber is designed to effectively use the energy from a high-velocity inlet gas stream to atomize the liquid being used to scrub the gas stream. This type of technology is a part of the group of air pollution controls collectively refe ...
s are used to clean flue gas emissions * Injectors (also called ejectors) are used to add chlorine gas to water treatment chlorination systems * Steam injectors use the Venturi effect and the latent heat of evaporation to deliver feed water to a steam locomotive
boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central h ...
. * Sandblasting nozzles accelerate and air and media mixture *
Bilge The bilge of a ship or boat is the part of the hull that would rest on the ground if the vessel were unsupported by water. The "turn of the bilge" is the transition from the bottom of a hull to the sides of a hull. Internally, the bilges (us ...
water can be emptied 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 uses the Venturi effect to assist maintaining the flow of gas once it starts flowing * In recoilless rifles to decrease the recoil of firing * The
diffuser Diffuser may refer to: Aerodynamics * Diffuser (automotive), a shaped section of a car's underbody which improves the car's aerodynamic properties * Part of a jet engine air intake, especially when operated at supersonic speeds * The channel betw ...
on an automobile * Race cars utilising ground effect to increase
downforce Downforce is a downwards lift force created by the aerodynamic features of a vehicle. If the vehicle is a car, the purpose of downforce is to allow the car to travel faster by increasing the vertical force on the tires, thus creating more gri ...
and thus become capable of higher cornering speeds * Foam proportioners used to induct fire fighting foam concentrate into fire protection systems *
Trompe A trompe is a water-powered air compressor, commonly used before the advent of the electric-powered compressor. A trompe is somewhat like an airlift pump working in reverse. Trompes were used to provide compressed air for bloomery furnaces ...
air compressors entrain air into a falling column of water * The bolts in some brands of paintball markers * Low-speed wind tunnels can be considered very large Venturi because they take advantage of the Venturi effect to increase velocity and decrease pressure to simulate expected flight conditions.


Architecture

* Hawa Mahal of Jaipur, also utilizes the Venturi effect, by allowing cool air to pass through, thus making the whole area more pleasant during the high temperatures in summer. * Large cities where wind is forced between buildings - the gap between the Twin Towers of the original World Trade Center was an extreme example of the phenomenon, which made the ground level plaza notoriously windswept. In fact, some gusts were so high that pedestrian travel had to be aided by ropes.


Nature

* In windy mountain passes, resulting in erroneous pressure altimeter readings *The Mistral wind in southern France increases in speed through the Rhone valley.


See also

*
Joule–Thomson effect In thermodynamics, the Joule–Thomson effect (also known as the Joule–Kelvin effect or Kelvin–Joule effect) describes the temperature change of a ''real'' gas or liquid (as differentiated from an ideal gas) when it is forced through a valve ...


References


External links


3D animation of the Differential Pressure Flow Measuring Principle (Venturi meter)
*{{cite web , url= http://www.ce.utexas.edu/prof/KINNAS/319LAB/Applets/Venturi/venturi.html, title= Venturi Tube Simulation, author= UT Austin , access-date=2009-11-03
Use of the Venturi effect for gas pumps to know when to turn off (video)
Fluid dynamics