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Sprayer 2 (Often Misunderstood Principle)

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What kind of experiment is this?

Experimental procedure and explanation:

  • There is a well-known, but incorrect, explanation for the sprayer principle. It goes like this: “When air is rapidly blown, the speed of the blown air is faster than that of the surrounding air and the pressure becomes lower than atmospheric pressure.” This is incorrect. Professor Isao Imai (one of the “Logergists”) and Professor Sadatoshi Taneda have been pointing this out for some time. This experiment validates those claims.
  • Just blowing with a straw will not make the pressure lower. In the first experiment (with a connected straw), blowing will not suck in water. This is because the flow that exits the straw remains at atmospheric pressure.
  • In the second experiment, when a vertical straw is inserted into the flow, the water is sucked in and a water spray is generated. This is because flow separation occurs and flow is detached from the object; this takes place at the head of the vertical straw, and the pressure becomes lower than atmospheric pressure.
  • Note: Many introductory science books explain that per Bernoulli’s theorem, pressure reduces to below atmospheric pressure when you blow air rapidly or when air or water spouts out from straws and hoses, because the flow is faster there. However, the pressure remains atmospheric when air or water is just blown, and the explanation that pressure reduces is incorrect. The first experiment verifies that.

    Bernoulli’s theorem describes energy conservation in a fluid at points upstream and downstream along one streamline. It is incorrect to compare blown air against surrounding, stationary air. Please look at the reference documents for details.
[Keywords] Separation, Bernoulli’s theorem
[Related items] Sprayer 1
[Reference] “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs pp. 186–191.
Logergist “Physics Promenade Continued” (1964), Iwanami-Shoten, pp. 153–157.
“Bernoulli’s Theorem that’s often Misunderstood,” Sadatoshi Taneda, Journal of Japan Physics Society, Vol. 50, No.12 (1995), pp. 972–973.
“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, pp. 206–209.


Last update: 1.21.2015