UKC

Why don't the bubbles go straight up?

http://www.youtube.com/watch?v=RNUZm5dpxt8&list=HL1351850769&featur...

Andy

In reply to ablackett:
Is it an optical illusion: The glass is curved, bowed out toward the viewer making it seem like the bubbles are rising oddly?

In reply to ablackett:

It could be something you can't see, possibly an obstruction in the center of the tank with the same refractive index as the walls/water or some curvature in the tank walls. More likely I suspect it's to do with the currents set up in the water by the rising bubbles causing a higher pressure region in the center of the tank.

jk

In reply to Reach>Talent:

If it helps, the bubbles used to go straight up, then the spice lounge man increased the flow rate at the outer bubble jets which caused the curving.

In reply to ablackett:

Has anyone else come across the beer glasses with a milled section in the centre of the base of the glass? It looks like it's been done to provide a series of points to help with bubble formation.

I think it's for southerners who want bubbles in their flat beer.

ALC

In reply to ablackett:
The bubbles will drag the surrounding water upwards so there must be a downward flow of water somewhere. The bubble paths look consistent with a clockwise water flow in the left hand side and an anti-clockwise water flow in the right hand side - up at the sides, down in the middle, towards the middle at the top and away from the middle at the bottom.

In reply to a lakeland climber:

What about southerners who like non fizzy but lovely beer made in the south using hops and barley from the pleasant part of the country not that tar like crap they make oop norf. You do realise which county has Britain's oldest brewer ?

In reply to ablackett: Is it something as simple as the fact that the air jets are angled?

In reply to ablackett:

You and jkarran have between you answered the question.

The faster flowing bubbles at the outer edges are creating a pressure differential so the water is slightly drawn out left and right, taking the bubbles with it.
So why do they converge at the top? Possibly because the bubbles are all expanding and dispersing so the pressure differential decreases, decreasing the water current up there. But there will be loads of other complicated stuff going on with turbulence, but I reckon I have covered the basic concepts

In reply to elsewhere:

Ooh I had not read your reply when I was posting - must have overlooked it. OK so what is creating the clockwise and anticlockwise water flows? ablackett says they used to all go straight up. Are there some hidden paddle wheels creating "controlled turbulence"? Looks too neat for that.

We are all assuming that it is all water and there isn't some weird immiscible layering going on...

In reply to Blue Straggler:


Answering a couple of points.

The jets aren't angled.
It is water.
After looking up immiscible - I am assuming that there isn't any immiscible layering going on.

I was wondering how long it would be before someone mentioned Bernoulli's principle. It has been suggested to me twice as a reason for the path of the bubbles.

I don't like it as an explanation because it doesn't seem to explain why the bubbles come back intowards the centre a the top.
I'm not convinced bernoulli's principle would have such a strong effect with the turbulent flow in the tank.

Thoughts?

In reply to Blue Straggler:
No need for paddles or controlled turbulence, the bubbles set the liquid in motion and after that some kind of continuous cyclical motion like I described is the only way for the water to descend.

A bit like convection cells

https://www.google.co.uk/search?q=convection+cells&prmd=imvns&tbm=i...

In reply to ablackett: It's because they know they're going to die when they get to the top, so they try to wriggle and delay the inevitable.

In reply to ablackett:
Next time we're in, see if you can get them to turn it off for a while and then on again. Would be an interesting bit of extra information to know whether the pattern takes a while to develop

In reply to ablackett:

The rising bubbles don't move freely through the water, they're resisted by it and therefore also apply a force to it setting up a current. Different size bubbles rise at different rates and more bubbles will entrain more water. Either by creating bigger or more bubbles diverting more air to the outer jets is leading to a net up-flow of water at the edges of the tank and a net down-flow in the center setting up two weak counter rotating currents in the tank which are distorting the path of the bubbles.

jk

In reply to ablackett: Nothing to input into this thread except to say I hate you.

I've been thnking about bubbles for three days and have no real answers. It's bugging me so much that I am probably going to have to read about them.

Happy now?

In reply to Wonko The Sane:

Did you miss the fourth post in the thread?

"If it helps, the bubbles used to go straight up, then the spice lounge man increased the flow rate at the outer bubble jets which caused the curving."

In reply to deepsoup:

Aha!!
No, didn't read the thread, just the OP.
Ta for that!

Looks like its a similar type of thing to rip tides. As said above the bubbles are dragging water to the surface so they have to fall back down to the bottom somewhere. As there's less resistance to flow in the centre with less nozzles and less frictional resistance to flow from the sides then water takes this path. This adds pressure to the centre resulting in the circular flow path.

when the flow was lower, the water could return to the bottom between the nozzles as the upwards velocity (drag) only impacted adjacent to the bubbles, at the higher rate the impacted water covers the whole area between each nozzle so not enough calm water for the water the return. It was likely that at the lower flow rate the jet streams were actually slightly bent but not perceivable without measuring.