A practical application of Bernoulli’s theorem with which the pilot should be familiar is the Venturi tube, sometimes called a convergent / divergent duct The Venturi tube has an inlet which narrows to a throat, and an outlet section, relatively longer, which increases in diameter towards the rear.
For a flow of air to remain streamlined the volume passing a given point in unit time (the mass flow) must remain constant; if a Venturi tube is positioned in such an air stream then, for the air to remain streamlined, the mass flow through the Venturi must remain constant. Mass Flow is dependent on the Area x Density x Velocity and is a constant. This is known as the continuity equation.
To do this and still pass through the reduced cross-section of the throat the speed of flow through the throat must be increased. In accordance with Bernoulli’s theorem this brings about an accompanying pressure and temperature drop. The use of venturi tubes have many applications in aircraft systems. For example the pressure drop at the throat of the venturi forms the basic principle of operation of the carburetor.
Author – Mohamed Freihat