How Does Siphon Work? Explanation & Principles

[davenn]

hi Andy

thanks for the video ... that lead me to a couple of hours of video watching from the Periodic Table of Videos
collection ... very informative !

Cheers
Dave

 

[sophiecentaur]

The 'waterfall' effect must be fascinating to watch. Is there a movie available to see?

 

[Jacobim]

The 'waterfall' effect must be fascinating to watch. Is there a movie available to see?

Yes there are at the bottom of the page: http://www.nature.com/srep/2014/140422/srep04741/full/srep04741.html#supplementary-information

It just appears as the force between a the water resisting a bubble being created in the tube and the force pulling the water down balance to allow a large bubble to form.

 

[rcgldr]

Ignoring compressiblity, pressure increases linearly with depth from the upper surface of a fluid. At any fixed height above the ground, the pressure at that height in the container with the higher upper surface of fluid is greater than the pressure in the container with the lower upper surface of fluid. This means that within the siphon, there's a decreasing pressure gradient component as fluid flows from the higher upper surface container to the lower upper surface container, providing the force that drives the fluid, even if the siphon rises above the fluid upper surfaces.

The height of the intake and outlet ports of the siphon doesn't matter, as long as both ports are submerged.

For the botany case, I would assume osmosis is the main factor in drawing water upwards (or outwards) through any plant, regardless of height.

 

[NihalRi]

Hello again, so I've conducted experiments measuring the rate of flow of a siphon changing various things like the ratio of the lengths of tubing and the level of water. I found that the only times I got exactly the same rate values was when the difference in height between the lever of water and the receptor end of the tube were equal. Therefore I deduced that this height must be important. Then I graphed the energy levels using this height. Because velocity is perportional to the root of the height. It yeilded a straight line graph which proves that this is true. Therefore the only thing that drives a siphon is it's potential energy. I realize this is not a complete explanation but I think it's close to the truth. An interesting thing is that the siphon still ran when the two lengths of tubing were the same or the shorter and longer sides were reversed as long as the water level was higher than the end of the recepter tube, so this led me to dismiss the gravity pulls on the longer arm more than the shorter arm theory. Any thoughts?

 

[rcgldr]

An interesting thing is that the siphon still ran when the two lengths of tubing were the same or the shorter and longer sides were reversed as long as the water level was higher than the end of the recepter tube, so this led me to dismiss the gravity pulls on the longer arm more than the shorter arm theory.

As long as both ends of the siphon are submerged, and the siphon is filled with fluid (no air or vacuum gaps), the fluid in both containers will flow until the height (above ground) of fluid in both containers is equalized (or until the surface of the higher fluid drops below the receptor port of the siphon). The common expression for this is "fluid (or water) seeks its own level" .

 

[CWatters]

Over the last few years there has been controversy over how siphons work3, 4, 5, 6, 7, 8, 9. Two competing models have emerged. In one model, water flowing out of a siphon generates a low-pressure region at the crown so that atmospheric pressure pushes water into the siphon. In another, the weight of water flowing out of a siphon pulls water into the siphon via liquid cohesion.

Why are they seen as "competing"? Isn't it the case that the maximum height of a syphon is the combination of the two?

 

[sophiecentaur]

so this led me to dismiss the gravity pulls on the longer arm more than the shorter arm theory.

The effect of gravity on each part of the fluid is the same, near the ground. It would be a good idea to start by reading the basics about hydrostatic pressure. The pressure in each column is due to its height and the fluid density. The factors governing the flow rate are not just the pressure difference but the fluid dynamics so you would not expect that the flow rate would only be a result of the two heights involved. Using different tube thicknesses could show that the height is not the only factor.

the difference in height between the lever of water and the receptor end of the tube were equal.

I don't understand why you got any flow at all when the two levels were equal. (?)

 

[NihalRi]

The effect of gravity on each part of the fluid is the same, near the ground. It would be a good idea to start by reading the basics about hydrostatic pressure. The pressure in each column is due to its height and the fluid density. The factors governing the flow rate are not just the pressure difference but the fluid dynamics so you would not expect that the flow rate would only be a result of the two heights involved. Using different tube thicknesses could show that the height is not the only factor.

I don't understand why you got any flow at all when the two levels were equal. (?)

Sorry I meant when the difference which was different from zero was the same in different experiments. By different experimemts I mean one where I just varied the level of water and another where I shifted the tube to change the ratio of heights on the two sides.

 

[NihalRi]

As long as both ends of the siphon are submerged,

I don't think that both sides need to be submerged just the one with the water you'd like to displace.
Can someone clarify what the meaning of negative pressure is? Is it less than atmosphereic pressure?

 

[sophiecentaur]

I don't think that both sides need to be submerged just the one with the water you'd like to displace.
Can someone clarify what the meaning of negative pressure is? Is it less than atmospheric pressure?

If the lower tube is not submerged, a bubble can form and let all the liquid out of the long arm of the siphon.
All pressures are greater than zero and a 'negative' value is relative to atmospheric pressure.

 

[jbriggs444]

If the lower tube is not submerged, a bubble can form and let all the liquid out of the long arm of the siphon.

This is correct, of course. But in practice, if flow rate is sufficiently high and the tube width (and, therefore the bubble size) is sufficiently low then the bubbles do not rise faster than the outflow and the siphon continues to work in spite of this possibility.

 

[rcgldr]

I don't think that both sides need to be submerged just the one with the water you'd like to displace.

If either end of the siphon is not submerged, then air could travel up the exposed opening and stop the siphon from working. This would be an issue of the siphon diameter and the rate of flow.

 

[NihalRi]

If the lower tube is not submerged, a bubble can form and let all the liquid out of the long arm of the siphon.
All pressures are greater than zero and a 'negative' value is relative to atmospheric pressure.

Thanks:D How would the air bubble form? If we are thinking of a practical siphon air can't push up the undubmerged receptor end because atmospheric pressure acts upon both ends so it is balanced. I'm thinking of an incompressible liquid here but even if the liquid was compressible I doubt it would mattter :/

 

[sophiecentaur]

How would the air bubble form?

If the tube diameter is big enough, a horizontal surface (water on top and air beneath) can go unstable and, whatever the flow rate, bubbles could form and bubbles could go up faster than the falling water could compensate. Once the total volume of bubbles is high enough, the pressure difference would go away and the upper leg would fall back into the top reservoir. It is easy to imagine it happening with a 1m wide tube and a 1m high siphon but not happening with a 1cm wide tube. But a lower reservoir would prevent this happening.

 

[NihalRi]

I see :) I guess I'd be going in too deep if I ask where the instability comes from but this explains why a siphon would fail if the tube diameter is too big:D I'm wondering if this is a fundamental siphon rule or something that arrises from the particular fluids properties.

 

[sophiecentaur]

Working or not depends on the total pressure difference between upper reservoir and lower reservoir. If the air were more dense, the siphon would fall faster, for a given height of water tubes. A more dense atmosphere would also mean that the maximum practical height of ∩ would be greater.
The operation of the siphon is governed both by atmospheric pressure differences AND hydrostatic pressure differences. I have a feeling that people want to choose between them.

 

[sophiecentaur]

if I ask where the instability comes from

There is no 'restoring force' to make the horizontal surface go flat, after some random disturbance. So, once a dent forms, the water at the top part of the dent will flow down to the bottom, making the dent deeper etc. etc.

 

[NihalRi]

I have a feeling that people want to choose between them.

Thats true I've been trying to choose for a while , but both play their roles I think it's just important to make a distinction where each is acting :)

 

[sophiecentaur]

Thats true I've been trying to choose for a while , but both play their roles I think it's just important to make a distinction where each is acting :)

Just imagine a water siphon that's been set up in a deep (say a few hundred metres deep) tank of light oil. It would be easier to accept that both fluids will contribute to the siphon effect. For a start, the limit to the possible height for the siphon to operate would be significantly greater because the ambient pressure could be several atmospheres.
Here's a thought. Imagine setting up a saltwater siphon in a fresh water tank. The water pressure differentials would be very small.

 

[rcgldr]

How would the air bubble form?

If the tube diameter is big enough ...

or the flow rate slow enough. What happens is water falls out of the tube and is replaced by air bubbles that travel upwards. Consider the case of a bottle of water with a 1 cm diameter opening: turn the bottle upside down and the water falls out, as air bubbles flow up through the water. Or consider the case of a garden hose with 1 cm diamter and zero or near zero flow rate. Take a section of the hose near the end and orient it downwards and you'll get the same results, water falls out, air bubbles flow up.