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Puppy-Zwolle t1_irpci2q wrote

Bubbles are ellipsoidal in shape, motion is irregular, and velocity is independent of bubble diameter (U is approx. 28 - 30 cm/sec) for bubbles having radii up to 0.75 cm. For larger bubbles their velocity tends to increase to 35 - 40 cm/sec, but they are not stable and tend to subdivide into smaller bubbles. › stenstro › Bubble


mfairview t1_irpddt7 wrote

are air bubbles the most efficient mechanism for unaided traveling to the surface through water or is their some other material or form that can do so faster (eg and I'm making this up as an example: styrofoam ball, helium bubble, etc.)?


crazunggoy47 OP t1_irpfue9 wrote

It seems like an evacuated container with high volume and low mass could feel greater buoyant force than air. Especially because an air bubble should reduce in volume due to the surrounding pressure, and therefore reduce its buoyancy (I think).

I’m still looking to better understand how ambient liquid pressure affects bubble velocity. It feels like on the one hand, higher pressure should impact greater force to the bubble. But on the other, higher pressure would contract the bubble and reduce its volume and buoyancy. Does that mean there is a particular optimal water depth that causes the greater bubble velocity?


JimmyDean82 t1_irqt6a9 wrote

In a rigid container the air pressure and thus density would not change nearly as much as an air bubble at depth. So even a rigid container filled with air should work just as well (or close enough). Something like a balloon though would shrink at lower depths and thus rise slower at the start with increasing velocity as it rises and expands.


HighRelevancy t1_irqvhyy wrote

> Especially because an air bubble should reduce in volume due to the surrounding pressure, and therefore reduce its buoyancy (I think).

Yup. In fact, given that a lot of materials are more compressible than water, most things become less bouyant the deeper you go exactly because the water pressure compresses them. Objects that float on top of water can, at a sufficient depth, be compressed to the density of the water around them and begin to sink instead of floating!


TexasPop t1_irr2olk wrote

If you release the bubbles in the deepest parts of ocean, below 8000 m, the bubbles would have so high density that they will sink instead of rise.

This because the air compressed to more than the pressure at that depth will have a density higher than water.

But probably will the bubbles dissolve rather quick, but a baloon will definitely sink. You could fill the Marianer trench with air! Maybe.


crazunggoy47 OP t1_irrkn04 wrote

Wow, I plugged in a pressure of 16000 PSI into this calculator and you're right! It exceeds the density of water. That's weird.


oodelay t1_irran5a wrote

So is there a depth line where bubbles would become buoyant?


crazunggoy47 OP t1_irrkvo7 wrote

It looks like it. See my comment to TexasPop. Air matches the density of water at a water depth of around 5.3 miles.


Tex-Rob t1_irqdwk7 wrote

Now you have me wondering how high water would need to be for the water layer to meet the vapor layer where the air is too thin?


[deleted] t1_irqfo69 wrote



ltblue15 t1_irqnc7s wrote

High pressure alone at room temperature will only get you to supercritical fluid, which has continuous density changes with temperature and pressure. If you want liquid (which can phase change, aka boil), you need to drop the temperature below the critical temperature as well. Now, phase diagrams really only apply to pure gases because each element acts differently, and air is a mixture of elements. But, it’s mostly nitrogen and oxygen, and they behave relatively similarly, so we can sort of think about a phase diagram for it:

Anything below and to the right of the line is a gas. Anything to the left of the line is a liquid. Anything above the critical point is a supercritical fluid, which will totally fill its container like a gas and can no longer boil.


BigPickleKAM t1_irrccvq wrote

Or how hot a given water body needs to be to sublimate to vapor directly. At sea level the answer is 100 degrees celsius.

But at La Rinconada elevation 5,100 meters water boils at 82.5 degrees Celsius or so.


UEMcGill t1_irscflh wrote

>I’m still looking to better understand how ambient liquid pressure affects bubble velocity.

Stokes law

In a stable volume the pressure wouldn't affect the rate at which it 'fell' in this case falling is negative. But in the case of a bubble, it's pressure is equal to the liquid at the level it is in. But as the bubble ascends through the liquid column, the bubble diameter will increase to maintain pressure.

Now if we take Stokes law and the fact that pressure will vary linearly through the column, but velocity will vary by the square, and terminal velocity will vary by the cube, Id venture to say that the optimal water depth is farthest away from the point where the bubble was created, as R would be the greatest (until structure is no longer supported).


TheKingManBear t1_irrafrd wrote

The weight of the air doesn't really matter, as it's much much smaller than weight of the water it displaces. The main factor is the shape of the bubble, which is the same at different depths. So the bubbles should rise essentially at the same rate regardless of pressure.


frix86 t1_irpuf6g wrote

An educated guess would be an object with the lowest density that could hold a hydrodynamic shape and withstand the changing pressures.

Something like a torpedo, but very light for its size.


ShelfordPrefect t1_irreqr9 wrote

The upwards force from buoyancy is proportional to volume and the difference in density, drag is complicated but largely proportional to cross sectional area and shape. The best thing would be a light, long and narrow, streamlined torpedo shape.


Cephandrius17 t1_irt8lbe wrote

Larger size correlates with more speed since the resistance is probably related to radius squared, but volume is related to radius cubed. However, since large air bubbles are unstable, something like a submarine should be able to rise faster since it is larger.


cuicocha t1_irx7tky wrote

Correct link:

The source says that this only applies to bubbles >0.1 cm radius.

The source appears to be class notes, and does not cite sources itself. The idea that ascent rate is independent of bubble radius is counterintuitive to me given how Rayleigh drag works (normally appropriate at high Reynolds number). A quick search I did failed to find anything to confirm this. So, I'm leery of accepting this claim uncritically, and I'd like to see a better source here and explanation of the physics.


seicar t1_irqpbl3 wrote

wonderful. Do you know some specifics? Like Temp and Salinity? Would some of the averages (spd. size) be higher / lower in the Dead Sea vs Lake Huron? Would a thermocline layer make a "bump"?