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Fountain with a dryer?

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What type of experiment is this?

Experimental procedure and explanation:

  • Attach the mouth of the plastic bottle to the dryer to speed up the flow. In addition, attach plastic strings and let the air flow with a dryer. The plastic strings flutter straight up and do not fall sideways.
  • When the strings attempt to bend sideways, the air flow also tries to bend along the strings. For example, if the strings bend to the right, as shown, the air flow also bends to the right. At this time, a force to the right of the strings acts on the flow, and as a reaction (i.e., recoil effect), a force to the left of the flow acts on the string. This will straighten the string.
  • The explanation that "the flow is fast where the dryer is blowing, the pressure is lower than the atmospheric pressure according to Bernoulli's theorem, and the string is sucked into the flow" is incorrect. Even if you just make a flow in an empty place, the flowing part will not become low pressure, and it will remain at almost atmospheric pressure. The force is generated by the relationship with the object (string in this case) in the flow.
  • The purpose of attaching the mouth of the plastic bottle to the dryer is to increase the flow velocity at the outlet, regulate the flow, and make it difficult to diffuse in the middle.
[Caution] 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.
Even if the flow is ejected into the air, the atmospheric pressure remains almost the same, and the low-pressure part cannot be created. If the explanation that "the place where the fluid is flowing attracts the surrounding air and things at a lower pressure than the atmospheric pressure" is correct, it will increasingly attract the surrounding air, and the speed will be higher downstream, which is a contradiction (Ishiwata, "Illustrated Trivia Fluid Dynamics", p. 206-209).
[Keywords] Momentum theory, the Coandă effect
[Related items] Blowing Air across a Paper, Down Force, Water hose

“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P218-219, P206-209.
“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P128-133.

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Last Update:4.13.2021