Bernoulli Effect Myths
A commonly taught idea is that "A fast moving stream of air will have a lower pressure than the surrounding still air."
This is often stated as "The Bernoulli Effect." The goal of this page is to show that:
(1) The statement is contradicted by observation, and
(2) It is not a predicted result of the Bernoulli Effect.
(1) In order to test the statement experimentally, we can use the following setup:
Blow a stream of air over the top of each tube (with a hair dryer or air track blower), and use the water level to measure changes in pressure.
The observation is that the pressure in a fast moving stream of air is the same as that of the surrounding air unless its direction is altered by some surface. The shape of the surface is an important variable in determining the pressure in the stream, though the effect will be larger as the speed increases.
Watch video of several experiments.
(2) Bernoulli's principle is statement of conservation of the energy of a mass of air at different points along a streamline. It compares pressure, speed, and height of a mass of air at points A and B, both of which are in the stream and along the same streamline. It makes no comparison at all between points in the stream and points outside the stream.
Construction details for manometer apparatus:
Curved surfaces are made of a section of 3 inch PVC pipe.
O-rings are used to mark initial water level.
Thanks to Evan Jones for the idea of the flat plate on top of a tube.