# Bernoulli’s Principle and the Venturi

## Bernoulli’s Principle

Bernoulli’s principle describes the relationship between speed, pressure, and temperature of a fluid (in our case, the fluid is air).

Imagine a tube with air flowing through it. As long as the flow of air is constant, the static and dynamic pressure through the tube will be constant.

However if the size of the tube is reduced in the middle, some interesting things happen. Since the airflow into the tube is constant, the air must speed up through the smaller section in order for the same amount of air to come out the other end of the tube. So through the constricted centre section the velocity increases.

In the last lesson we learnt that faster air has a higher dynamic pressure, so through the smaller section of the tube the dynamic pressure will increase. Since the airflow into and out of the tube is constant, the total pressure must also be constant. As dynamic pressure has increased, the static pressure must reduce in order for the total pressure to remain constant.

A tube shaped like this is known as a Venturi. Air flowing through the constricted section of the Venturi has increased dynamic pressure and reduced static pressure.

## Bernoulli and the Wing

We can see how this resembles the shape of an aircraft wing. The airflow over the top and bottom of the wing is forced to increase in speed as it passes, leading to a decrease in static pressure.

The wing is shaped such that the air over the top of the wing is forced to accelerate more than the air flowing below the wing. Since the air below the wing now has a lower velocity, it must also have lower dynamic pressure and higher static pressure when compared to the air flowing above the wing. This difference in static pressure is one of the reasons the wing creates lift.

Note: the air under the wing has a higher static pressure compared to the airflow over the wing – but both have a lower static pressure than the surrounding air since both have been forced to accelerate.

We will talk more about how lift is created in the next chapter. The important thing to remember here is the pressure differential above and below the wing, with higher static pressure below the wing compared to above the wing.
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