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Spin around the thread ring

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

  • Make a hole in the side of the straw, pass the thread through, and then tie the end of the thread to form a loop. When you blow the straw, the thread circulates around.
  • There are science textbooks that say, "Air flows faster in the straw, the pressure drops according to Bernoulli's theorem, and the surrounding air is sucked in through the side hole. Together with this air, the thread is sucked in and the thread turns", but this is a mistake. If you place pieces of tissue paper near the hole, you'll see that air is blowing out of the hole, and you'll see that this explanation is incorrect.
  • The correct explanation is, "The thread in the straw experiences viscous friction caused by the flow of air and is dragged downstream and circulates." The fact that it is dragged by viscous friction even after it comes out of the straw also helps circulation.
  • Viscous friction is a type of friction that works with a fluid (gas or liquid), but in the case of this experiment, the flow drags the thread downstream and simultaneously acts as a brake to the flow with the inner surface of the straw, slowing the flow down. To proceed with this brake, the pressure must be higher toward the upstream side of the flow (pipe friction loss). At the outlet of the straw, the pressure is almost atmospheric pressure, and on the upstream side, the pressure is high, so air is blown out from the side hole.

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 not correct to compare the energy between the flow in the straw and the surrounding stationary air (i.e., not on the same streamline); furthermore, the inside of the straw is strongly affected by viscous friction, and the energy becomes smaller (resulting in lower pressure) as it moves downstream (pipe friction loss). At the outlet of the straw, the pressure is almost atmospheric pressure, so the pressure inside the straw on the upstream side is higher than the atmospheric pressure.

[Keywords] Viscous friction, tube friction loss
[Related items] Sprayer 2 (Often Misunderstood Principle), Short Hose and Long Hose, Is air sucked in or blown out?
[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