Water fountains and momentum cancellation

[Powers]

http://www.straightdope.com/columns/...merge-into-one

When the twin streams of the Double Bubbler meet, however, their upward momentums tend to cancel out no matter how high the pressure gets.

Come again? How can two momenta cancel each other out in the very vector that they share?


Powers &8^]

[Irishman]

It's very poorly worded, but what I think is meant is that since the mass flows are roughly equivalent, the low velocity stream will slow down the high velocity stream to somewhere near the average.

I'm less certain about the "less gas" part. The common technique for drinking from the fountain appears to be to put pursed lips against the stream and suck in, with some element of the stream continuing and falling to the basin. This water is collected in the mouth until a comfortable quantity is accummulated, and then that mouthful is swallowed. Extra air to be swallowed would seem to come more from air in the mouth than air in the stream of water. However, a larger water bubble does help ensure a good flow for sucking through the pursed lips, i.e. a "larger mouth feel".

[Vashbul]

Also, the Halsey Taylor folks don't mention in their pamphlet how the double stream accomplishes their founder's main goal, providing safe drinking water. Here's how it works: both streams may contain the typhoid bacillus (Salmonella enterica), but merging them causes the little buggers to bash their brains out against each other, instantly purifying the water.

[Joey P]

Originally Posted by Powers

Come again? How can two momenta cancel each other out in the very vector that they share?

But they don't share a vector. Their components are close enough that they don't completely cancel each other out, but far enough apart that they cancel each other out just enough to produce a nice gentle curve.

Think about an example on a grander scale. (Mind you, I haven't actually tried this) Take a garden hose with a sprayer or pressure nozzle on it). Set it so that it arcs a few feet into the air. Now take a second one, just behind it and a slightly flatter angle so they merge. You should get a new arc somewhere between the two independent arcs. If you turn the water up, the shallower stream should keep the other one in check. As the first one attempts to spray higher the back one, the one that's at a shallower angle will be pushing it down harder.

What the article is saying is that if one day the pressure is, say, 60 psi instead of 50 the second arc will keep it in check instead of it being 3 inches higher then normal.

Does that make sense?

[Joey P]

Originally Posted by Irishman

It's very poorly worded, but what I think is meant is that since the mass flows are roughly equivalent, the low velocity stream will slow down the high velocity stream to somewhere near the average.

I'm less certain about the "less gas" part. The common technique for drinking from the fountain appears to be to put pursed lips against the stream and suck in, with some element of the stream continuing and falling to the basin. This water is collected in the mouth until a comfortable quantity is accummulated, and then that mouthful is swallowed. Extra air to be swallowed would seem to come more from air in the mouth than air in the stream of water. However, a larger water bubble does help ensure a good flow for sucking through the pursed lips, i.e. a "larger mouth feel".

When I read that I pictured Andy from Parks and Rec. I had to read it twice to figure out what you meant.

[Irishman]

By the way, I ran into a water fountain from a different manufacturer that did not use two streams. It has a single port, single stream. It does have a nozzle cover that has slits to presumably help prevent spraying, but otherwise functions adequately.

The difference in drinking is the size of the water stream at the apex where one sips. The double stream is a bit larger, and does give a different "mouth feel" - more water.

I had to read it twice to figure out what you meant.

Well, how do you normally drink from a water fountain? Open your mouth and let the water flow in and fill your mouth like a cup?

The most common method is to lean over from the top, press mouth against the stream, and pull water in through the opening, i.e. suck.

[Powers]

Originally Posted by Joey P

What the article is saying is that if one day the pressure is, say, 60 psi instead of 50 the second arc will keep it in check instead of it being 3 inches higher then normal.

Does that make sense?

I understand the concept, but not the way the article tried to describe it. The article said the "upward momentums tend to cancel out". Given that both streams have a significant upward component to their momenta, I don't see how they can cancel each other out. They should reinforce, shouldn't they?

Of course the two momenta average out when they collide. But that average momentum still has an upward component.


Powers &8^]