How Does Compressed Air Behave Underwater?

[Colby Lippincott]

Hello everyone,
I am designing a machine that will utilize jets of compressed air underwater. Does anyone have information about the physics of gas underwater? I would like to know how far the jet will travel before floating upwards, based on the pressure of the air and diameter of the nozzle. I also need to know how much pressure is needed to evacuate all the water from a container underwater.
Thanks!
Colby

 

[boneh3ad]

This is actually a very difficult problem that's still a very active area of research. This is kind of a hybrid problem combining multiphase flow (gas and liquid) and turbulent buoyant jets (air is certainly buoyant in water). Each of those problems individually are difficult and active research areas in their own right.

The short answer, though, is that the jet will float upwards right as it leaves the jet, so you will have the qualify that criteria with "before floating upward a specified distance." Once you decide on that, you are probably better off experimenting or trying to make some very basic estimates based on the various jet-related topics you can find in textbooks and papers.

Determining how much pressure is required to evacuate a container underwater is easier. Air will continue to flow out of the container as long as the pressure in the container is greater than the pressure in the water at the exit point. You will never fully evacuate the container based on air pressure alone since eventually its pressure will fall below the ambient water pressure and water will begin to come into it.

 

[Colby Lippincott]

Sorry, I meant evacuating the water out of a container using compressed air. The setup is a tank filled with water and immersed. There is an outlet hole where the water and air will flow out, and an inlet hole where the compressed air is hooked up. Assuming there is a continuous flow of air into the container, how much pressure would be needed to remove all the water from the container?

 

[boneh3ad]

Just more than whatever the ambient water pressure at the outlet is. That's all you need to maintain flow going outward.

The problem will be that unless the outlet is in a place such that all the water flows toward it naturally (e.g. if the tank was placed in air, all the water would evacuate itself), then eventually you will reach a point where there is still water in the tank and only air is coming out. Think of a standard compressed gas cylinder and its shape. If you just used that and placed it sideways, eventually you would lose enough water with your setup that the water level would fall below the level of the outlet and only air would escape. Just something to keep in mind when you design your system.

An alternative might be to essentially place a piston or some other movable barrier between your air and your water and let the air push the barrier, which pushes the water. That might get unnecessarily complicated depending on what you are trying to do, though.

 

[Baluncore]

A container being filled with compressed air at depth will become more buoyant and may begin to rise. If it rises, the air within will gradually expand and be progressively expelled as bubbles from the water drain hole.
If the tank begins to rise before all water is expelled, the remaining water will be expelled as it rises, which will progressively increase the buoyancy until all the remaining water has been expelled.
If the tank cannot vent air as it rises, the internal pressure will remain the same as the hydrostatic pressure at depth, which might result in bursting of the tank as the confining pressure is reduced.