Submersible buoy

[Torres88]

I need to determine the flowrate and line sizing for a submersible buoy operating using ballast tanks. The ballast tank initially contain trapped air and as the valves is open seawater rushes in to replace the air. This increases the density of the buoy and causes it to sink. How do I determine the depth to which it sinks to. Assuming the ballast tank is completely filled. For neutral buoyancy I would need the weight of the buoy to be equal to the weight of the displaced fluid. Basically, I need to know how to calculate how fast the buoy will sink and at what depth it will stabilize. It is required for the ballast tanks to be completely filled and at 50 meter depth but I do not know the necessary calculation to determine the flowrate and the line sizing. Please help!!!

 

[MortenA]

You could set up an excell sheet where you calculate the average density of the bouy over time and the use Navier Stokes to calculate "bubble velocity" and repeat the calcs in timesteps? You must of course also include a collumn of water density over dept - or else your bouy will continue to sink afetr it first acheives negative boyancy.

Best regards

Morten

 

[Torres88]

Can you explain in more detail how this should be done because I am kind of lost.

 

[MortenA]

If you are lost then maybe you should look for a consultant? How will you document anything if you expect to be able to do the job based on advise /even well meant) from the web?

Best regards

Morten

 

[BigInch]

Personally I wouldn't use NS. I'd simplify and estimate drag force vs velocity of the buoy moving in water and correlate that to the buoyant force on the mass of the buoy free body, calculate acceleration and resultant velocity to get distance traveled over time, iterating for solution to velocity.

I don't know if you're talking about a flowrate of air out, or water in, or some combination of both. Your water intake flowrate can be calculated by the flow possible due to inlet pressure of water at a given depth across some length of pipe or across an orifce opening, or whatever you have conducting the water to the air volume remaining inside, and the remaining pressure of that air volume at any given time. If its a hose for air, use water depth inlet and the outlet pressures (atmospheric?) with gas flow equations.

I wouldn't think stability will be easy to achieve. Look for lots of oscillations before that happens, especially if you do this fast. Its not very easy to do with something as small as a scuba diver, so with a buoy... take it very slow and easy to maximize control.


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[MortenA]

BigInch - that was what i was thinking about - maybe im a little rusty but isnt NS about drag force? Anyway isnt my point about that once it start to sink it will sink for quite a while correct?

Best regards

Morten

 

[Compositepro]

The bouy will never stabilize depth passively. It needs a way to measure depth and a controller to control bouyancy using either compressed air or a pump to pump water out of the ballast tank. Neutral bouyancy at any particular depth is an unstable equilibrium - like balancing one ball on top of another. The only passive solution would be to have a float at the surface attached to the bouy.

 

[BigInch]

Yes NS is about drag, but shape factor 0.6 * [ρ] * v^2 is easier.

An anchor on bottom with a line to a buoy at 50 m depth would be easier still, but then we wouldn't need all that air pressure measurement and calculating sink time and stuff.

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic
"Note: Make that 99.99% for pipeline companies" -

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[BigInch]

Sorry 0.8

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic
"Note: Make that 99.99% for pipeline companies" -

http://virtualpipeline.spaces.live.com/