John Welch

Cabrillo College

6500 Soquel Dr.

Aptos, CA 95003

831-479-6518

jowelch@cabrillo.edu

Pressure in Air Stream

Abstract (50-75 words)

A water manometer is used to show pressure differences that arise when a fast moving stream of air is directed over surfaces of different shapes. The commonly taught idea that “A fast moving stream of air has less pressure than the surrounding still air” is contradicted: over the flat surface, the pressure is the same as in still air. Pressure over the concave and convex surfaces is higher and lower, respectively, than the still air.

Construction of Apparatus:

An acrylic tube is used as the reservoir for the manometer. Flat pieces of acrylic are glued on to the ends of this tube to seal it off and also to act as feet for the apparatus. Three holes are drilled in a line along the tube, into which are glued 3 smaller tubes which will be the manometer arms. Acrylic solvent glue works well for all the gluing (available at plastic shops). O-rings are placed on each vertical tube to mark the ‘original’ water levels. Curved surfaces are made from sections of 3” plastic pipe (could be PVC or ABS), drilled to fit snugly on the vertical tubes. The flat and curved surfaces are best held on by friction rather than glued so they can be removed to change the O-rings or experiment with differently shaped surfaces. Water with food coloring is used as the manometer fluid. An air track blower works great for the air supply, but a hair dryer that can be set to no heat will also work.

Use of Apparatus:

The principal use for this apparatus is to test the hypothesis: “The pressure in a fast moving stream of air will be lower than that of the surrounding still air.” This is often taught as a direct statement of Bernoulli’s Principle. With this apparatus, students (and instructors) will be able to observe for themselves whether or not this statement is true, or under what conditions it is true. They will find that the hypothesis is false when the air is directed over the flat or concave surfaces. Students may experience the predicament of having to reject either their own observation or reject a ‘solid law of physics.’ Resolving this predicament should set up an excellent critical thinking activity. For example, we might ask students to list the assumptions made in the derivation of “Bernoulli’s Principle” and to try to identify any that are not met in this situation (e.g. the fluid is not incompressible and is viscous, the derivation compares 2 points on the same streamline, not the stream with the surrounding still air, etc.) Students can then be asked to think about a better hypothesis than the above in order to explain the observed result that the pressure drops over a convex surface such as an airlplane wing.

Using the apparatus is simple: put the O-rings at the level of the water to indicate the original height, then blow air horizontally over one of the tubes and see whether the water rises, falls, or neither.

This apparatus was inspired in part by a demo given by Evan Jones at the AAPT 2004 summer meeting.

Equipment and costs required to construct apparatus:

Item	Source	Part number	Cost
12” x 1.5” acrylic tube	Plastic shop		~$5
8” x 3/8” acrylic tube (3)	Plastic shop		~$5
O-rings, 3/8” (3)	Plumbing store		~$1
Hair dryer or blower	Drug store, or use air track blower		~$30
1.5 in length of 3” PVC pipe	Plumbing or irrigation store		~$1
Total Cost			~$42 including air source