Venturi effectFrom Wikipedia, the free encyclopedia

The pressure at "1" is higher than at "2" because the fluid speed  at "1" is lower than at "2".

A flow of air through a venturi meter, showing the columns connected in a U-shape (a manometer) and partially

filled with water. The meter is "read" as a differential pressure head in cm or inches of water.

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a

constricted section of pipe. The Venturi effect is named after Giovanni Battista

Venturi (1746–1822), an Italian physicist.

Contents

 [hide]

1     Background   

2     Experimental apparatus   

o 2.1      Venturi tubes   

o 2.2      Orifice plate   

3     Instrumentation and Measurement   

o 3.1      Flow rate   

o 3.2      Differential Pressure   

4     Examples   

5     See also   

6     External links   

7     References   

[edit]Background

The Venturi effect is a jet effect; as with an (air) funnel, or a thumb on a garden hose, the

velocity of the fluid increases as the cross sectional area decreases, with the static

pressure correspondingly decreasing. According to the laws governing fluid dynamics, a

fluid's velocity must increase as it passes through a constriction to satisfy the principle of

continuity, while its pressure must decrease to satisfy the principle of conservation of

mechanical energy. Thus any gain in kinetic energy a fluid may accrue due to its increased

velocity through a constriction is negated by a drop in pressure. An equation for the drop in

pressure due to the Venturi effect may be derived from a combination of Bernoulli's

principle and the continuity equation.

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. In choked flow the mass flow rate will

not increase with a further decrease in the downstream pressure environment.

However, mass flow rate for a compressible fluid can 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.

Referring to the diagram to the right, using Bernoulli's equation in the special case of

incompressible flows (such as the flow of water or other liquid, or low speed flow of gas), the

theoretical pressure drop at the constriction is given by:

where   is the density of the fluid, v1 is the (slower) fluid velocity where the pipe is

wider, v2 is the (faster) fluid velocity where the pipe is narrower (as seen in the figure).

This assumes the flowing fluid (or other substance) is not significantly compressible -

even though pressure varies, the density is assumed to remain approximately constant.

[edit]Experimental apparatus

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

[edit]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. To account for the assumption of an inviscid

fluid a coefficient of discharge is often introduced,.. which generally has a value of 0.98.

[edit]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.[1]

[edit]Instrumentation and Measurement

Venturis are used in industrial and in scientific laboratories for measuring the flow of

liquids.

[edit]Flow rate

A venturi can be used to measure the volumetric flow rate Q.

Since

then

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.

[edit]Differential Pressure

Main article: Pressure head

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.

[edit]Examples

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

Cargo Eductors on Oil, Product and Chemical ship tankers

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

Wine aerators , used to infuse air into wine as it is poured into a glass

The capillaries of the human circulatory system, where it indicates aortic

regurgitation

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.

Protein skimmers  (filtration devices for saltwater aquaria)

In automated pool cleaners that use pressure-side water flow to collect

sediment and debris

The barrel of the modern-day 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

Steam injectors  use the Venturi effect and the latent heat of evaporation

to deliver feed water to a steam locomotive boiler.

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

The diffuser on an automobile

Large cities where wind is forced between buildings

The leadpipe of a trombone, affecting the timbre

Foam proportioners used to induct Fire fighting foam foam concentrate

into fire protection systems

A simple way to demonstrate the Venturi effect is to squeeze and release a

flexible hose in which fluid is flowing: 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.[1]

[edit]See also

Venturi flume

Bernoulli's principle

De Laval nozzle

Bunsen burner

Choked flow

Orifice plate

Pitot tube

[edit]External links

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.

[edit]References

1. ^ a b "The Venturi effect". Wolfram Demonstrations Project. Retrieved 2009-

11-03.

Categories: Fluid dynamics

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Def

In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the

speed of the fluid occurs simultaneously with a decrease inpressure or a decrease in

the fluid's potential energy.[1][2] Bernoulli's principle is named after the Dutch-

Swiss mathematician Daniel Bernoulli who published his principle in his

book Hydrodynamica in 1738.[3]

Bernoulli's principle can be applied to various types of fluid flow, resulting in what is loosely

denoted as Bernoulli's equation. In fact, there are different forms of the Bernoulli equation

for different types of flow. The simple form of Bernoulli's principle is valid for incompressible

flows (e.g. most liquid flows) and also forcompressible flows (e.g. gases) moving at

low Mach numbers. More advanced forms may in some cases be applied to compressible

flows at higher Mach numbers (see the derivations of the Bernoulli equation).

Bernoulli's principle can be derived from the principle of conservation of energy. This states

that, in a steady flow, the sum of all forms of mechanical energy in a fluid along

a streamline is the same at all points on that streamline. This requires that the sum of kinetic

energy and potential energy remain constant. Thus an increase in the speed of the fluid

occurs proportionately with an increase in both its dynamic pressure and kinetic energy, and

a decrease in its static pressure and potential energy. If the fluid is flowing out of a reservoir

the sum of all forms of energy is the same on all streamlines because in a reservoir the

energy per unit mass (the sum of pressure and gravitational potential ρ g h) is the same

everywhere.[4]

Fluid particles are subject only to pressure and their own weight. If a fluid is flowing

horizontally and along a section of a streamline, where the speed increases it can only be

because the fluid on that section has moved from a region of higher pressure to a region of

lower pressure; and if its speed decreases, it can only be because it has moved from a

region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing

horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed

occurs where the pressure is highest.

A flow of air into a venturi meter. The kinetic energy increases at the expense of the fluid pressure, as shown by

the difference in height of the two columns of water.

Applications

Bernoulli's principle can be used to calculate the lift force on an airfoil if the behaviour of the fluid flow in the vicinity of the foil is known. For example, if the air flowing past the top surface of an aircraft wing is moving faster than the air flowing past the bottom surface, then Bernoulli's principle implies that thepressure on the surfaces of the wing will be lower above than below. This pressure difference results in an upwards lift force.[nb 1][20] Whenever the distribution of speed past the top and bottom surfaces of a wing is known, the lift forces can be calculated (to a good approximation) using Bernoulli's equations[21] – established by Bernoulli over a century before the first man-made wings were used for the purpose of flight. Bernoulli's principle does not explain why the air flows faster past the top of the wing and slower past the underside. To understand why, it is helpful to understand circulation, the Kutta condition, and the Kutta–Joukowski theorem.

The wings of an aeroplane are designed such that the upper surface has a greater curvature than its

lower surface. As the aeroplane moves, the air blows in the form of streamlines. The air travels a

longer distance over the upper surface when compared to distance to its lower surface, in a given

time. This creates a difference in the velocity of the air above and below, as shown. The pressure at

top of the wing is low and below the wing is high. This pressure difference gives it an additional thrust.

http://www.youtube.com/watch?v=bC8v6hlXnSk

The working of spray-gun is based on Bernoulli's theorem

When the rubber bulb is squeezed, air is blown into the tube A, due to which, low pressure and high

velocity is created. Since this pressure is less than the atmospheric pressure, the liquid is pushed up.

This rising liquid is sprayed out of the nozzle 'N', due to the blowing air.

Curve of a Baseball

A non-spinning baseball or a stationary baseball in an airstream exhibits symmetric flow. A baseball which is thrown with spin will curve because one side of the ball will experience a reduced pressure. This is commonly interpreted as an application of the Bernoulli principle and involves the viscosity of the air and the boundary layer of air at the surface of the ball.

The roughness of the ball's surface and the laces on the ball are important! With a perfectly smooth ball you would not get enough interaction with the air.

There are some difficulties with this picture of the curving baseball. The Bernoulli equation cannot really be used to predict the amount of curve of the ball; the flow of the air is compressible, and you can't track the density changes to quantify the change in effective pressure. The experimental work of Watts and Ferrer with baseballs in a wind tunnel suggests another model which gives prominent attention to the spinning boundary layer of air around the baseball. On the side of the ball where the boundary layer is moving in the same direction as the free stream air speed, the boundary layer carries further around the ball before it separates into turbulent flow. On the side where the boundary layer is opposed by the free stream flow, it tends to separate prematurely. This gives a net deflection of the airstream in one direction behind the ball, and therefore a Newton's 3rd lawreaction force on the ball in the opposite direction. This gives an effective force in the same direction indicated above.

Similar issues arise in the treatment of a spinning cylinder in an airstream, which has been shown to experience lift. This is the subject of the Kutta-Joukowski theorem. It is also invoked in the discussion of airfoil lift.

Airfoil

The air across the top of a conventional airfoil experiences constricted flow lines and increased air speed relative to the wing. This causes a decrease in pressure on the top according to the Bernoulli equation and provides a lift force. Aerodynamicists (see Eastlake) use the Bernoulli model to correlate with pressure measurements made in wind tunnels, and assert that when pressure measurements are made at multiple locations around the airfoil and summed, they do agree reasonably with the observed lift.

Illustration of lift forceand angle of attack

Bernoulli vs Newtonfor airfoil lift

Airfoil terminology

Others appeal to a model based on Newton's laws and assert that the main lift comes as a result of the angle of attack. Part of the Newton's law model of part of the lift force involves attachment of the boundary layer of air on the top of the wing with a resulting downwash of air behind the wing. If the wing gives the air a downward force, then by Newton's third law, the wing experiences a force in the opposite direction - a lift. While the "Bernoulli vs Newton" debate continues, Eastlake's position is that they are really equivalent, just different approaches to the same physical phenonenon. NASA has a nice aerodynamics site at which these issues are discussed.

Increasing the angle of attack gives a larger lift from the upward component of pressure on the bottom of the wing. The lift force can be considered to be a Newton's 3rd law reaction force to the force exerted downward on the air by the wing.

At too high an angle of attack, turbulent flow increases the drag dramatically and will stall the aircraft.

A vapor trail over the wing helps visualize the air flow.

Bernoulli's Equation tells us how much the pressure within a moving fluid increases or decreases as the speed of the fluid changes. Here is Bernoulli's equation:

where    is the first point along the pipe    is the second point along the pipe    is static pressure in newtons per meter squared

    is density in kilograms per meter cubed    is velocity in meters per second    is gravitational acceleration in meters per second squared    is height in meters

Bernoulli Equation

The Bernoulli Equation can be considered to be a statement of the conservation of energy principle appropriate for flowing fluids. The qualitative behavior that is usually labeled with the term "Bernoulli effect" is the lowering of fluid pressure in regions where the flow velocity is increased. This lowering of pressure in a constriction of a flow path may seem counterintuitive, but seems less so when you consider pressure to be energy density. In the high velocity flow through the constriction, kinetic energy must increase at the expense of pressure energy.

Bernoulli calculation

There are several other interesting illustrations—sometimes fun

and in one case potentially tragic—of Bernoulli's principle. For

instance, there is the reason why a shower curtain billows

inward once the shower is turned on. It would seem logical at

first that the pressure created by the water would push the

curtain outward, securing it to the side of the bathtub.

Instead, of course, the fast-moving air generated by the flow of

water from the shower creates a center of lower pressure, and

this causes the curtain to move away from the slower-moving air

outside. This is just one example of the ways in which Bernoulli's

principle creates results that, on first glance at least, seem

counterintuitive—that is, the opposite of what common sense

would dictate.

Another fascinating illustration involves placing two empty soft

drink cans parallel to one another on a table, with a couple of

inches or a few centimeters between them. At that point, the air

on all sides has the same slow speed. If you were to blow directly

between the cans, however, this would create an area of low

pressure between them. As a result, the cans push together. For

ships in a harbor, this can be a frightening prospect: hence, if

two crafts are parallel to one another and a strong wind blows

between them, there is a possibility that they may behave like

the cans.

Then there is one of the most illusory uses of Bernoulli's

principle, that infamous baseball pitcher's trick called the curve

ball. As the ball moves through the air toward the plate, its

velocity creates an air stream moving against the trajectory of

the ball itself. Imagine it as two lines, one curving over the ball

and one curving under, as the ball moves in the opposite

direction.

In an ordinary throw, the effects of the airflow would not be

particularly intriguing, but in this case, the pitcher has

deliberately placed a "spin" on the ball by the manner in which

he has thrown it. How pitchers actually produce spin is a

complex subject unto itself, involving grip, wrist movement, and

other factors, and in any case, the fact of the spin is more

important than the way in which it was achieved.

If the direction of airflow is from right to left, the ball, as it

moves into the airflow, is spinning clockwise. This means that the

air flowing over the ball is moving in a direction opposite to the

spin, whereas that flowing under it is moving in the same

direction. The opposite forces produce a drag on the top of the

ball, and this cuts down on the velocity at the top compared to

that at the bottom of the ball, where spin and airflow are moving

in the same direction.

Thus the air pressure is higher at the top of the ball, and as per

Bernoulli's principle, this tends to pull the ball downward. The

curve ball—of which there are numerous variations, such as the

fade and the slider—creates an unpredictable situation for the

batter, who sees the ball leave the pitcher's hand at one altitude,

but finds to his dismay that it has dropped dramatically by the

time it crosses the plate.

A final illustration of Bernoulli's often counterintuitive principle

neatly sums up its effects on the behavior of objects. To perform

the experiment, you need only an index card and a flat surface.

The index card should be folded at the ends so that when the

card is parallel to the surface, the ends are perpendicular to it.

These folds should be placed about half an inch (about one

centimeter) from the ends.

At this point, it would be handy to have an unsuspecting person—

someone who has not studied Bernoulli's principle—on the scene,

and challenge him or her to raise the card by blowing under it.

Nothing could seem easier, of course: by blowing under the card,

any person would naturally assume, the air will lift it. But of

course this is completely wrong according to Bernoulli's

principle. Blowing under the card, as illustrated, will create an

area of high velocity and low pressure. This will do nothing to lift

the card: in fact, it only pushes the card more firmly down on the

table.