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Water hose

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

  • Attach plastic strings to a blower to let the air flow. The plastic strings flutter straight without hanging down.
  • If the string bends down as shown, the air flow also bends down. At this time, a downward force acts from the string on the flow, and as a reaction (as a recoil force), an upward force acts on the string from the flow. This will straighten the string.

There is sometimes an explanation that "the flow is fast where the blower is blowing, the pressure becomes lower than the atmospheric pressure from Bernoulli's theorem, and the string is sucked into the flow", but this is incorrect. Many people misunderstand "Bernoulli’s theorem" as "where the flow is fast, the pressure is low". At this point (as of December 2016), it feels as though many introductory science books and online writing have mistakes regarding this aspect.

Even if the flow is ejected into the air without any obstacles, the atmospheric pressure remains almost the same, and the low-pressure part cannot be created. If the explanation that "The location where flow is occurring is lower than atmospheric pressure, and the surrounding air and objects are being sucked in (incorrect)" is correct, it will increasingly suck in the surrounding air and the speed will be higher downstream, which is a contradiction. (Ishiwata "Illustrated Trivia Fluid Dynamics", p206-209).
[Keywords] Momentum theory, Coandă effect
[Related items] Fountain with a dryer?, Bernoulli’s theorem
[Reference] “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