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, Giovanni Battista Venturi.
The static pressure in the first measuring tube (1) is higher than at the second (2), and the fluidspeed 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.
By measuring pressure, the flow rate can be determined, as in various flow measurement devices such as Venturi meters, Venturi nozzles and orifice plates.
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:
where is the density of the fluid, is the (slower) fluid velocity where the pipe is wider, is the (faster) fluid velocity where the pipe is narrower (as seen in the figure).
The limiting case of the Venturi effect is when a fluid reaches the state of choked flow, 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. 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 sectionEdit
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.
Venturi tube demonstration apparatus built out of PVC pipe and operated with a vacuum pump
A pair of Venturi tubes on a light aircraft, used to provide airflow for air-driven gyroscopic instruments
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, 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.
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 measurementEdit
Both Venturi tubes and orifice plates are used in industrial applications and in scientific laboratories for measuring the flow rate of liquids.
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.
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 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, 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 massEdit
where constant terms are absorbed into k. Using the definitions of density (), molar concentration (), and molar mass (), one can also derive mass flow or molar flow (i.e. standard volume flow),
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,
Substituting these two relations into the pressure-flow equations above yields the fully compensated flows,
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,
The Venturi effect may be observed or used in the following:
During Underway replenishment 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
Foam proportioners used to induct fire fighting foam concentrate into fire protection systems
Trompe 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.
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.
^Nasr, G. G.; Connor, N. E. (2014). "5.3 Gas Flow Measurement". Natural Gas Engineering and Safety Challenges: Downstream Process, Analysis, Utilization and Safety. Springer. p. 183. ISBN 9783319089485.
^Herschel, Clemens. (1898). Measuring Water. Providence, RI:Builders Iron Foundry.
^"Invention of the Venturi Meter". Nature. 136 (3433): 254. August 17, 1935. Bibcode:1935Natur.136Q.254.. doi:10.1038/136254a0. [The article] reproduces a letter from Herschel to the late Dr. Unwin describing his invention of the Venturi Meter. The letter is dated June 5, 1888, and addressed from the hydraulic engineer's office of the Holyoke Water Power Co., Mass. In his letter, Herschel says he tested a one-inch Venturi Meter, under 210 ft. head: 'I am now satisfied that here is a new and pregnant principle to be applied to the art of gauging fluids, inclusive of fluids such as compressed air, illuminating or fuel gases, steam, etc. Further, that the shape of the meter should be trumpet-shaped in both directions; such a meter will measure volumes flowing in either direction, which in certain localities becomes a useful attribute...'
^Blasco, Daniel Cortés. "Venturi or air circulation?, that's the question". face2fire (in Spanish). Retrieved 2019-07-14.
^Anderson, John (2017). Fundamentals of Aerodynamics (6th ed.). New York, NY: McGraw-Hill Education. p. 218. ISBN 978-1-259-12991-9.
^Dunlap, David W (December 7, 2006). "At New Trade Center, Seeking Lively (but Secure) Streets". The New York Times.
^Dunlap, David W (March 25, 2004). "Girding Against Return of the Windy City in Manhattan". The New York Times.
^Dusk to Dawn (educational film). Federal Aviation Administration. 1971. 17 minutes in. AVA20333VNB1.
Wikimedia Commons has media related to Venturi effect.
3D animation of the Differential Pressure Flow Measuring Principle (Venturi meter)
UT Austin. "Venturi Tube Simulation". Retrieved 2009-11-03.