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Is air sucked in or blown out?

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

  • Install a branch pipe in the middle of the hose to allow air to flow. From the movement of the tissue paper, you can see that air is blown out from the branch pipe. Anyone who has ever blown a recorder knows that air will blow out of the side hole, and this result is obvious.
  • However, some people misunderstand fluid mechanics and think that it will be sucked in. There is a science book that says, "Air flows faster in a pipe, lowers pressure according to Bernoulli's theorem, and draws in the surrounding air through a side hole," but this is incorrect.
  • Inside the hose, viscous friction acts between the inner surface of the hose and the air flow, acting as a brake to the flow. To flow against this, the pressure must be higher toward the upstream side and lower toward the downstream side. At the outlet of the hose, the pressure is almost atmospheric pressure, and in the middle of the hose, the pressure becomes higher than the atmospheric pressure, so air is blown out.
  • In a similar experiment, if the tube is partially narrowed near the branch (or side hole), the flow will be faster there, the pressure will be lower, and the surrounding air may be sucked in (“Spray 2 (easy-to-mistake principle)”). However, if the thickness of the pipe is constant, air will be blown out, as in this experiment.

Many people misunderstand "Bernoulli's theorem" as "the pressure decreases where the flow is fast". There are many science book writers and science teachers who misunderstand this. It feels as though many introductory science books have mistakes regarding this aspect.
"Bernoulli's theorem" is the energy conservation law of fluids (gas and liquid), and it states that when comparing the total energy of the upstream point (point A) and the downstream point (point B) along the flow, the total energy is the same if there is no energy loss or supply in the middle. What is important here is that (1) the two points to be compared (point A and point B) are upstream and downstream on the same streamline, and (2) there is no energy loss (e.g., loss from viscous friction) or supply (e.g., acceleration caused by a fan halfway through). If those conditions hold, the sum of the energies of the two points is equal. After satisfying the above two conditions, it is correct that "if the heights of the two points are equal (i.e., if the potential energy is equal), the pressure is low where the flow is fast".
In this experiment, it is incorrect to compare the energy between the flow in the hose and the surrounding stationary air (i.e., not on the same streamline); furthermore, the inside of the hose is strongly affected by viscous friction, and the energy becomes smaller (resulting in lower pressure) as it goes downstream (tube friction loss). At the outlet of the hose, the pressure it almost atmospheric pressure, so the pressure inside the straw on the upstream side is higher than the atmospheric pressure.

[Keywords] Tube friction loss
[Related items] Spin around the thread ring, Sprayer 2 (Often Misunderstood Principle)
[Reference] “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P206-209, P216-217.
“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P182-185

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Last Update:4.13.2021