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B ERNOULLI ' S P RINCIPLE IN A CTION

As fluid moves from a wider pipe to a narrower one, the volume of the fluid that moves a

given distance in a given time period does not change. But since the width of the

narrower pipe is smaller, the fluid must move faster (that is, with greater dynamic

pressure) in order to move the same amount of fluid the same distance in the sameamount of time. Observe the behavior of a river: in a wide, unconstricted region, it flows

slowly, but if its flow is narrowed by canyon walls, it speeds up dramatically.

Bernoulli's principle ultimately became the basis for the airfoil, the design of an airplane'swing when seen from the end. An airfoil is shaped lie an asymmetrical teardrop laid on

its side, with the !fat! end toward the airflow. As air hits the front of the airfoil, the

airstream divides, part of it passing over the wing and part passing under. "he uppersurface of the airfoil is curved, however, whereas the lower surface is much straighter.

As a result, the air flowing over the top has a greater distance to cover than the air

flowing under the wing. #ince fluids have a tendency to compensate for all ob$ects withwhich they come into contact, the air at the top will flow faster to meet the other portionof the airstream, the air flowing past the bottom of the wing, when both reach the rear end

of the airfoil. %aster airflow, as demonstrated by Bernoulli, indicates lower pressure,

meaning that the pressure on the bottom of the wing eeps the airplane aloft.

&A"*+ A A%".

Among the most famous applications of Bernoulli's principle is its use in aerodynamics,and this is discussed in the conte-t of aerodynamics itself elsewhere in this boo.

iewise, a number of other applications of Bernoulli's principle are e-amined in an

essay devoted to that topic. Bernoulli's principle, for instance, e-plains why a showercurtain tends to billow inward when the water is turned on/ in addition, it shows why an

open window and door together create a draft.

#uppose one is in a hotel room where the heat is on too high, and there is no way to

ad$ust the thermostat. Outside, however, the air is cold, and thus, by opening a window,one can presumably cool down the room. But if one opens the window without opening

the front door of the room, there will be little temperature change. "he only way to cool

off will be by standing ne-t to the window: elsewhere in the room, the air will be everybit as stuffy as before. But if the door leading to the hotel hallway is opened, a nice cool

bree0e will blow through the room. 1hy2

1ith the door closed, the room constitutes an area of relatively high pressure compared

to the pressure of the air outside the window. Because air is a fluid, it will tend to flowinto the room, but once the pressure inside reaches a certain point, it will prevent

additional air from entering. "he tendency of fluids is to move from high3pressure to low3

pressure areas, not the other way around. As soon as the door is opened, the relativelyhigh3pressure air of the room flows into the relatively low3pressure area of the hallway.

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As a result, the air pressure in the room is reduced, and the air from outside can now

enter. #oon a wind will begin to blow through the room.

A WIND TUNNEL.

"he above scenario of wind flowing through a room describes a rudimentary wind tunnel.A wind tunnel is a chamber built for the purpose of e-amining the characteristics of

airflow in contact with solid ob$ects, such as aircraft and automobiles. "he wind tunnel

represents a safe and $udicious use of the properties of fluid mechanics. ts purpose is totest the interaction of airflow and solids in relative motion: in other words, either the

aircraft has to be moving against the airflow, as it does in flight, or the airflow can be

moving against a stationary aircraft. "he first of these choices, of course, poses a numberof dangers/ on the other hand, there is little danger in e-posing a stationary craft to winds

at speeds simulating that of the aircraft in flight.

"he first wind tunnel was built in ngland in 4564, and years later, aircraft pioneers

Orville (456434785) and 1ilbur (45963474) 1right used a wind tunnel to improve theirplanes. By the late 47;ual to ;

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output cylinders are reversed from those of a car $ac. %or the car $ac, the input cylinder

is long and narrow, while the output cylinder is wide and short. "his is because the

purpose of a car $ac is to raise a heavy ob$ect through a relatively short vertical range ofmovementC$ust high enough so that the mechanic can stand comfortably underneath the

car.

n the hydraulic ram, the input or master cylinder is short and s>uat, while the output or

slave cylinder is tall and narrow. "his is because the hydraulic ram, in contrast to the car$ac, carries a much lighter cargo (usually $ust one person) through a much greater

vertical rangeCfor instance, to the top of a tree or building.

PUMPS.

A pump is a device made for moving fluid, and it does so by utili0ing a pressuredifference, causing the fluid to move from an area of higher pressure to one of lower

pressure. ts operation is based on aspects both of @ascal's and Bernoulli's principlesC

though, of course, humans were using pumps thousands of years before either man wasborn.

A siphon hose used to draw gas from a car's fuel tan is a very simple pump. #ucing on

one end of the hose creates an area of low pressure compared to the relatively high3

pressure area of the gas tan. ventually, the gasoline will come out of the low3pressureend of the hose.

"he piston pump, slightly more comple-, consists of a vertical cylinder along which a

piston rises and falls. *ear the bottom of the cylinder are two valves, an inlet valve

through which fluid flows into the cylinder, and an outlet valve through which fluid flows

out. As the piston moves upward, the inlet valve opens and allows fluid to enter thecylinder. On the downstroe, the inlet valve closes while the outlet valve opens, and the

pressure provided by the piston forces the fluid through the outlet valve.

One of the most obvious applications of the piston pump is in the engine of anautomobile. n this case, of course, the fluid being pumped is gasoline, which pushes the

pistons up and down

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O#" @=?@# F*O1* . (Photograph byichard &ummins&orbis

.Reproduced by permission.)by providing a series of controlled e-plosions created by the spar plug's ignition of the

gas. n another variety of piston pumpCthe ind used to inflate a basetball or a bicycle

tireCair is the fluid being pumped. "hen there is a pump for water. @umps for drawing

usable water from the ground are undoubtedly the oldest nown variety, but there are alsopumps designed to remove water from areas where it is undesirable/ for e-ample, a bilge

pump, for removing water from a boat, or the sump pump used to pump flood water out

of a basement.

FLUID POWER.

%or several thousand years, humans have used fluidsCin particular waterCto power a

number of devices. One of the great engineering achievements of ancient times was the

development of the waterwheel, which included a series of bucets along the rim thatmade it possible to raise water from the river below and disperse it to other points. By

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