Man's physical universe

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AERODYNAMICS 369

The Venturi tube is an application of Bernoulli's principle, which

states that when the rate of flow of gases or liquids is increased the

pressure decreases. Venturi tubes

increase the rate of flow of a gas or

liquid under a given pressure in the

constricted portion of the tube.

The gauges shown in the Venturi

tube in Fig. 157 can be graduated

so as to show rates of flow of liquids

Fig. 157. Bernoulli's principle.

The pressure is reduced at the constricted

portion of the pipe.

or gases through the Venturi tube.

Meters can even show the amount of water used in any given time.

Such meters are called Venturi meters.

Figure 158 shows one type of Venturi "Pitot tube" used with the

air-speed indicator of an airplane. The two open ends of the Venturi

"Pitot tube" are mounted in the

Fig. 158.

Principle of the air-speed

indicator, an application of Bernoulli's

principle.

airplane so as to face the direction

to which the airplane is pointed.

The difference in pressure on the

two sides of the air-speed indicator

varies with the air speed ; the instrument

may be graduated to read in

terms of miles per hour.

Figure 159 shows how the streamlining

of the wing causes the air

above the wing to travel faster

than the air below the wing. The

increase in the velocity of the air above the wing thus decreases the

pressure above the wing, and the decrease in the velocity of the air

below the wing increases the pressure below the wing. This combination

of low and high pressures produces a pull and push on the wing

Fig. 159. The circulatory stream of air about

the wing of an airplane, which combined with the

sweep of the air from left to right yields a velocity of

air above the wing which is greater than that below.

which is called the lift.

low pressure above the airfoil,

From 65 to 100 per cent of the lift is due to the

depending upon the angle of attack.

Bernoulli's Principle Has Many Other Interesting and Practical Applications.

A "cut" tennis ball, a "sliced" golf ball, or a "curved" baseball

given a spin when pitched will follow a curved path because the rota-

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