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Someone explained the perfume bottle with the Bernoulli effect: squeezing the bulb over the fluid creates a low pressure area due to the higher speed of the air, which subsequently draws the fluid up. This is illustrated in the following figure.

someone explained

My question is: as shown in the figure, in this pipeline system, because the pipeline at the position of P2 is narrow, the fluid should accelerate, the pressure difference is the cause of acceleration, so the pressure of P1 must be greater than p2, and P2 must be greater than atmospheric pressure (p3), so the high speed can not make the pressure lower than atmospheric pressure, so the liquid level in the vertical pipeline can not be raised.

 can not be raised

My explanation is: The air is viscous, so when air flows horizontally, it will take away the air of the vertical pipe, so the pressure of the vertical pipe decreases, so the liquid level of the vertical pipe will rise. This is illustrated in the following figure.

My explanation

Is my explanation right, or is someone's explanation right?

Reference resources

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    $\begingroup$ Surface tension of the liquid probably account for the vast majority of force required to pull up the column of liquid. It is also the reason why the column of liquid stays in the tube and not empty into the container due to gravity. $\endgroup$
    – Melvin
    Commented Apr 15, 2019 at 10:45
  • $\begingroup$ @Melvin Can't the tension be guaranteed to rise so high? $\endgroup$
    – enbin
    Commented Apr 15, 2019 at 12:39
  • $\begingroup$ You mean is the surface tension force sufficient to pull the column to that height? It depends on the diameter of the tube and the inner surface of the tube, but in the case of perfume bottles it almost certainly can. $\endgroup$
    – Melvin
    Commented Apr 17, 2019 at 1:46
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    $\begingroup$ I think its the pressure difference created by fast moving air particles. And not surface tension. Because if it is surface tension then when we stop using this gadget the liquid level falls or else liquid leaks out. $\endgroup$ Commented Jul 3, 2019 at 7:57
  • $\begingroup$ @Melvin I mean, the viscosity of air causes the air to be taken away, so that the pressure is lowered and the liquid level rises. $\endgroup$
    – enbin
    Commented Jul 3, 2019 at 8:24

1 Answer 1

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It is not viscosity lowering the pressure in the center of that center section.

I know this is old, but a search turned this up. Cummon' people! This is the oldest misconception since Adam & Eve..

That Blue figure will NOT work.

Build it with a good tube TEE and you will see it blow bubbles in the reservoir. I've done it!

The horizontal tube starts at atmospheric pressure.

The higher pressure in the bulb pushes air down the tube and it MUST be HIGHER than atmospheric pressure to accelerate out the bulb into the tube and into the atmospheric pressure - which is lower than in the bulb and tube.

It is at the bulb-tube boundary where Bernoulli's Principle 'happens'. In the bulb you have zero speed and a higher pressure from squeezing it (above atmospheric pressure). Speed picks up because that bulb pressure pushes more than the outside atmospheric pressure, Accelerating air out of the bulb. The pressure in the horizontal tube is ABOVE atmospheric.

Bernoulli is about Acceleration, NOT speed. Fluid acceleration from one place on the flow to the next place, is accompanied by a decrease in pressure between those two points along the SAME FLOW! Between two DIFFERENT LOCATIONS along the fluid FLOW!

In the second figure, P1 must be above atmospheric. The pressure at P3 will be near P1 - as follows: P3 is slightly below P1 because it returns to the P1 pressure having the same diameter and speed (per Bernoulli), but with some Head-loss due to viscosity along the pipe wall.

Consider this: Make all the tube the large diameter. Start a flow and keep it CONSTANT. Now change ONLY the diameter in the center to a smaller diameter. KEEP EVERYTHING ELSE the same!

You will see the pressure at P1 INCREASE. This is caused by the sloping wall being a restriction. The kinetic energy of the flow "hitting" those sloping walls converts some to static (potential energy) Pressure. Think of it as partial stagnation pressure.

From P2 to P3, we have this faster fluid 'running into slower fluid, therefore pushing on it and increasing the pressure. From P3 needing to be atmospheric, working back wards you should see this is correct. This Bernoulli Principle misconception has gone on WAY TOO long!!

Try this: Understanding Bernoulli's Principle Correctly kyuoyckftflurrpq.quora.com/

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  • $\begingroup$ The pressure of P3 should be atmospheric pressure. $\endgroup$
    – enbin
    Commented May 5 at 12:37

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