Atmospheric siphon effect

enzi

While I'm at it I might as well ask your help on another question that has been bugging me for a while.

Let's hypothesize a huge "straw" a hundred metres or more in diameter. One end of this straw is at sea level. The other end is at 10,000 metres in altitude. It's obvious that the air pressure inside the straw would equalize, but if (again hypothetically) it could be "started" somehow, would there be a siphon effect which would push the sea level atmosphere up the straw to the stratosphere and would this effect be self sustaining?

Thanks again in advance for any assistance.

Drakkith

No, that isn't how a siphon works. A siphon works because the fluid in the siphon is pulled down by gravity. This creates a suction effect that sucks more fluid into the siphon and the result is the draining of a reservoir that is higher in elevation than the other end of your siphon where the fluid is coming out. A siphon cannot work against gravity.

enzi

Ok, perhaps due to the fact that I slept through most of my high school science classes I'm not explaining myself properly. The "straw" is connecting two areas, one with about 100 kPa and the other one with almost zero. Would there be a way to get that atmosphere flowing from the high pressure end to the low pressure end (like a deflating balloon) and have it be self-sustaining?

A.T.

In zero-g it would flow from higher to lower pressure. But here the pressure difference is countered by the weight of the water. In fact, the pressure difference between sea-level and vacuum cannot push the water higher than 10m. But you can pull water higher than that, with negative pressure. Trees do this by evaporating a lot of water at their leafs.

http://en.wikipedia.org/wiki/Xylem

enzi

Would the lower density of the air work in a different way than the water you're discussing? The "straw" would only be connecting sea level air with stratospheric "lack of" air.

Drakkith

Gravity is still pulling all the air in the straw down. This is why you have a difference in pressure in the first place. A siphon works by using gravitational energy. It's like going up the side of a mountain and pushing a boulder off. You have to use some energy to get it going initially, but once it starts it rolls the rest of the way down. But you can't make it go up the mountain on its own.

enzi

Yes, definitely. So even given a hypothetical "starter" that would get the air flowing quickly from the bottom to the top, the Earth's gravity would still be pulling down on the air itself hard enough that it would not be self-sustaining, right?

A.T.

As I said: The pressure difference between sea-level and vacuum cannot push the water higher than 10m. But even if you make it less than 10m, the water will not pull itself up to a higher level, because the pressure difference is countered by the weight of the water.

The trees use Sun's energy to evaporate the water, which pulls more water. Without that energy input it wouldn't work continuously.

Drakkith

Kind of. It's not that gravity pulls down too hard to make this work, it's that you have the entire concept backwards. You are trying to work AGAINST gravity when siphoning works WITH gravity. A siphon is self sustaining BECAUSE of gravity. Does that make sense?

D H

No. There is a flaw in your original post, emphasis mine:

Equalize to what? You appear to be assuming that the pressure would be constant throughout the straw. That's not correct. The air in the straw would "equalize" (better: come to an equilibrium state) such that the at any point, the pressure balances the weight of all of the material above. Going up the straw starting from ground level, there is less and less overlaying material, and hence less and less weight that to be supported with increased elevation. Pressure decreases with height inside the straw.

enzi

OK, without addressing each reply individually let me be clear:

a) I am only talking about air not water
b) By equalization I mean to the air pressure at that point outside the straw
c) Is there any way to get this flow going in some way that I'm not imagining and most importantly be self-sustaining?

Thanks to all replies as I appreciate your time and effort!

Drakkith

Doesn't matter. Air is a fluid too. (but not a liquid) It obeys the same rules that water does in this case.

The air inside and outside the straw will be equal in pressure at all heights.

No, not at all. It would require external energy to be input into the straw, such as heating from the Sun. Self-Sustaining is not possible.

D H

What flow?

Let's make this a bit more concrete.

A pipe (a rather sturdy one at that) rather than a straw is needed. In fact, a concrete pipe won't do. Our pipe is going to need to be made of unobtanium to avoid the problem of progressive collapse.

Suppose we lay the pipe horizontally at sea level so that it fills with air, seal one end, and then hoist the pipe into a vertical position so that the sealed end is at the top. The open end at the bottom is exposed to the sea level atmosphere. Now simply break the seal at the top of the pipe and we should have a siphon, right? Wrong. Air will flow out of the bottom of the pipe as the pipe is being hoisted to the vertical. There will be no air at the top of the pipe to flow out of the top by the time the pipe is vertical and we break the seal.

Suppose instead we seal both ends and simultaneously break the seals on both ends once the pipe reaches a vertical orientation. This should work, right? Wrong again. Now air is going to flow out of both the bottom and the top of the pipe, with a whole lot more flowing out of the bottom.

You want air going into the pipe at the bottom and flowing out of the top, and that isn't going to happen.

enzi

Ok, fair enough! The answer is clear: No can do! Thanks to all for your valuable input!

Drakkith

But do you understand WHY?

enzi

Yup, no matter how much force you use to kickstart the flow gravity is going to overcome it and slow it down to a halt.

Do I get a passing grade? :)

Drakkith

I'll give you a B-.