How Does Siphon Work? Explanation & Principles

[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.