Demos: 2C-03 Venturi Tubes

A Venturi tube consists of two sections of a glass tube with a larger diameter connected by a section having a smaller diameter. The smaller and larger sections are connected by a tube filled with a liquid, serving as a manometer. Air flows through the tube and at steady state the manometer shows that the pressure in the larger cross-section has a higher pressure than the smaller tube, in accordance with Bernoulli’s Principle. (Bernoulli’s equation assumes incompressible flow and although air is compressible, at steady state it behaves like an incompressible fluid.)

Directions: The rubber tubing is connected to the lecture desk air supply. Start the air flow SLOWLY so that you do not drive the manometer liquid into the main tube. Increase the flow until there is a visible difference in the heights of liquid in the manometer tube.

Suggestions for Presentation: Remind the students of the continuity equation which shows that the fluid moves faster in pipes of smaller cross-section. What does Bernoulli’s equation tell us then about the pressure in each cross-section?

Applications: Venturi tubes are used often in boats and airplanes as water-and air-speed indicators.

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