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Sprayer 1 (With Necked Midsection)

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

  • There are several different ways to make a sprayer. Here, we observe a method that uses a “choke.” A choke is a narrowing down of the midsection of a pipe.
  • First, we open a hole in the middle of the straw and blow into it. Very little water spray is generated. The air speed at the hole is about the same as that at the exit of the straw; therefore, the pressures at those two locations are almost the same. Viscous friction exists inside the straw and therefore the pressure is higher upstream (frictional losses in the pipe). Thus, the pressure at the hole is slightly higher than the pressure at the exit of the straw. (The pressure at the exit is atmospheric pressure.)
  • When the straw slightly upstream of the hole is slightly pinched with fingers, the speed of the flow in the pinched region increases relative to the speed at the exit of the straw. Therefore, by Bernoulli’s theorem, the pressure in the pinched region decreases (lower than atmospheric pressure), and surrounding water is sucked into the straw. This water, in turn, forms a spray.
  • The significant feature in this experiment is the choke in the straw.
[Keywords] Choke, Bernoulli’s theorem, Pipe friction loss
[Related items] Sprayer 2
[Reference] “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs pp. 186–191.
Logergist “Physics Promenade 2” (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.


Last update: 1.21.2015