Question about siphons.?

Question

I'm trying to learn more about siphons. We haven't learned much about them in school yet, however I've searched a bit on the web. So far, I know that the higher the level of the water, the more water pressure that will be exerted.

Here's the question: is it possible to rise the pressure WITHOUT changing the level of the water? So far, the only way that I have experimented with is changing the size of the tube on the receiving end of the water. This provided results, but I want to know if you can change the pressure by altering factors in the tank holding the water. (Again I want to keep the initial water level the same.)

Websites where I can find out more are helpful, but I prefer direct answers.

Sorry if this is hard to understand. I don't know much terminology on the subject.

Answer 1

See the first reference for some explanations, but beware, some of it is incorrect. A siphon will work just fine in a vacuum, even if the evaporation rate is higher than in an atmosphere.

Water molecules are attracted to each other due to hydrogen bonds (see second reference) between a hydrogen atom in one molecule and the oxygen atom in a nearby molecule. If you 'pull' on water, that's why the water tends to follow the pull. Sand doesn't work that way. These hydrogen bonds are the 'couplers' between the cars in the train analogy.

It is usually claimed that the height of the siphon above the intake surface can't exceed the height where water pressure equals atmospheric pressure. That's true for a static system. A siphon in operation, with water flowing, will continue flowing if that height is exceeded. That's why a siphon can continue to operate in a vacuum. But even for a static system, the adhesion of water due to the hydrogen bonds is enough to operate a siphon somewhat beyond what atmospheric pressure alone would permit.

Increasing the water pressure at the intake tank affects the balance of forces. You can do that by increasing the water level in that tank (which you disallowed), by increasing the 'atmospheric' pressure (with compressed air in an enclosed tank), or just by raising the water pressure (with a piston or a pump).

The behavior of an operating siphon, including the water pressure at various points along the siphon, is a fluid dynamics computation. If you suddenly block the outflow, you can cause a tremendous pulse of pressure. Imagine that train entering a tunnel that's only half-way dug through a mountain. This principle is used in the hydraulic ram pump (see third reference).

Answer 2

If you have an enclosed container for the water, you could add an air valve & pressurize the water tank to get the water too flow easier and faster.