speed of object through water 1

[imjasper]

How would I calculate the rate of ascent of an object traveling up through water using bouyancy as the propellant?

Example:
an open bottomed,8 cubic foot object at 197 ft deep with 1 cubic foot of air.

I know that the weight and shape of the object will affect it's rate of ascent. I also know that as the object rises, the air will decompress to approximately 7.8 times it's original volume (P1/P2=V2/V1). So I guess I need to find out how those variables play into the rate of ascent.
Does anyone know the formulas involved, or at least some books or web sites that will help me learn how to properly calculate???

 

[DeepBlues]

Wow - Pretty complicated. You'll also need to take into account the object's shape and the coefficient of drag. Also, what kind of currents are you likely to encounter as it rises? You'll need to determine the drag on the moving object, and the increasing velocity due to air expansion will definitely make that difficult. Don't think I can help much here. Sorry.

 

[avhell]

I think that I may be able to help Let me check to make sure that I have this right... Open bottomed volume (to allow excess air to escape through the bottom as it expands?) of some dimension, on its way back up, with no other forces on it other than the buoyancy (applied force)? If this is the case, the solution should not be toooo hard (this is a purely relative term ). The buoyant force acting on the body is equal to the displacement of the volume by air (or whatever gas it is that you are using). Now, herein lies the problem. The only other forces acting on the object of any significance are the drag forces generated on the body. This is extremely dependant on the geometry of the body, and the velocity of ascent! Fortunatlely, having an open bottom allows the expansion of air to just bleed out of the bottom (if i have that assumption correct). So there is no expanding volume of air to deal with to change the buoyant force (buoyant force can be considered constant throughout the ascent). your real problem is going to be deconstructing the drag components from the body geometry. (there are texts with drag coefficients for various shaped bodies listed. these may even be online, but i am not sure. these figures are usually given in terms of the drag coefficient to the reynolds number) If you have trouble finding the formulas, check back with me, i can look some of them up for you.

 

[imjasper]

This does help me. However, the problem I was having is that as I said above, the object holding the air is capable of holding 8 cubic feet of air but only 1 cubic foot is initially pumped in. This means that as the object ascends, the volume of air will increase thereby increasing the bouyancy.
You are helpful however in that I still need to figure out how to calculate speed given the geometry of the object so that I can figure the best design.
I think that I am over my head in this matter, but being that it is a very sensitive matter, I can't divulge too much about the aparatus to any outside source. This makes my task that much more difficult.
I will say that eng-tips forum is the best place that I've found on the net so far to get real answers to real questions. Most here are extremely helpful with a bare minimum of information.
So thank you very much for your reply, it is very appreciated.

 

[avhell]

I gotta start figuring out everything I mean to say before I hit that silly post button. More or less, you can assume a constant temperature during the process I would imagine. If this is the case, the ideal gas law can be used here to provide a solution to your problem.
(P1)(V1) = (P2)(V2)
or
V2 = ((P1)(V1))/(P2)
where V1 and P1 are the volume and pressure you are starting from, respectively, and P2 is the pressure at your point of consideration, which will yield your volume.
again, hope this helps
p

 

[imjasper]

David,
Your posts have been most helpful and are appreciated.

 

[fubar]

JASPER,
DON'T FORGET TO FACTOR IN THE WATER TEMP! TEMP=DENSITY=RESISTENCE=SPEED OF ACCENT. DEPENDING ON YOUR TIME/LENGTH OF ACCENT YOU MAY PASS THROUGH ANY NUMBER OF THERMO LAYERS PRIOR TO REACHING THE SURFACE. SORRY, DIDN'T MEAN TO BUST YOUR BUBBLE!

 

[imjasper]

Not busting it at all. Thanks for the info. I had considered the temp. factor but have been making my equations dependent on a constant temp. Your input made me start to think about some other factors that I really need to consider though. Thank you for your input, it did help.