Look at that, the stream of water is thick near the neck of the botle, then it gets thinner, then it bulges again, then gets thin,bulges again, gets thin, then bulges just before it reaches the water level in the glass.
Why does it do this? I would expect the width of the stream to be the same all the way down. But it isn’t.
Maybe the amount of liquid coming out of the spout varies from moment to moment. But the bulges seem to stay in the same place.
The stream of liquid is not circular in cross section. It’s thinner in one direction than the other, kind of like this: (). And as it pours, it twists. That’s what makes it appear as it does. Why it twists, on the other hand, I can’t say.
The stream gets narrower towards the bottom because it is accelerating due to gravity. The volumetric flow rate, V, at any point along the stream will remain constant but the velocity, v, of the water will increase as it falls. The cross-sectional area of the stream, A, will thus decrease to maintain constant volumetric flow rate, as given by V = A * v.
Surface tension pulls the water into a cylinder, drag pulls it into a ribbon, and the result is a twisted ribbon? There will probably a fluid mechanics expert along to explain shortly. Gotta love the Dope.
It’s not just that - it’s also that the rate of flow from the mouth of the bottle isn’t constant - if there’s any ‘glugging’ or even just gentle oscillation of the remaining contents in the bottle, then the stream will consist of more water one moment, less the next. The bulges in the water stream might just be from it flattening and twisting (in which case they often remain in place like a standing wave), but in anything poured by hand that way, it could also just be that the stream is essentially composed of a series of big blobs that are stretched out and partially overlapping (in which case the bulges move down the stream with the water).
ETA: It also might not only be the twisting ribbon effect - it could be a sort of springy oscillation (of the cross-sectional shape) similar to that seen in the water sphere experiments in this video:
(1) When you are pouring water you tilt the bottle until you see water coming out and then stop tilting the water, and perhaps even jerk the bottle up a little bit. The water in the bottle has some momentum and it continues in the same direction that it was going causing a surge towards the opening. That surge reflects off of the bottle wall near the opening and sets up an oscillating effect as the pour goes on.
(2) Air has to replace the volume of water that is leaving the bottle. This isn’t a constant replacement. Once a sufficient amount of water has left the bottle the resultant pressure difference allows a surge of air in, decreasing the flow rate of the bottle. Once the pressures are equalized, more water can flow out causing a surge. This can be most easily seen if you just turn a bottle upside down. It makes the “Ga-Glurg, Ga-Glurg” sound as periods of high liquid/low air flow, and low liquid/high air flow reverse.
(3) The water isn’t coming out of the entire bottle opening. This results in a flow with the dimensions of a cut off circle, like the letter “D”. However, surface tension in the water will pull it back into a circular shape. The area of the flow is the same, and a circle with the same area is narrower than the “D” shape.
(4) As water falls down it speeds up, but the volume flow is the same. Imagine two imaginary pieces of paper bisecting the flow with one paper a foot below the other. Say the velocity at the first piece of paper is X, and the second is (X+A) where A is simply the increase in speed of the water due to the acceleration from gravity. Volumetric flow rate is AreaVelocity, and is constant for the entire stream of water. Therefore, XArea[sub]1[/sub]=(X+A)*Area[sub]2[/sub]. Since (X+A)>X we know that Area[sub]1[/sub] has to be greater than Area[sub]2[/sub]. In other words, we know that the stream has narrowed between position 1 and position 2.
If you use your faucet you can eliminate 1-3 and will see that the flow is constant. You should also be able to see the effects of point 4 if you look carefully enough.
I don’t think it’s a case of the amount of water varying from moment to moment. If that happened, the thick and thin bits would constantly change, too quickly to see. But the thick and thin bits of the stream stay in the same place, though the actual water is constantly being replaced.
What’s causing the distortion of the stream is internal turbulance pushing outward at different points until surface tenstion bounces it back through the stream until it’s pushing out at another point.
Laminar water streams on the other hand have very little internal turbulance and have little or no noticable distortion.
Google laminar water for more information.
Sundance has one of the clearer pictures of the results. I wasn’t able to find anything really describing it in the short time I have to search right now.
When I visited a friend at MIT some 20 years ago, one of the buildings (physics or engineering, probably) had a demonstration of a stream of water, like a drinking fountain (with a single round hole, not two like a lot of them), and a strobe. In that case, the stream of water was actually a series of blobs. Adjusting the frequency of the strobe, they’d stay in place, or slowly move forward or backwards.
That’s Strobe Alley, up on the fourth floor of Building 10, outside “Doc” Edgerton’s old lab. It’s still there.
They used to have a demonstration of this at the Boston Museum of Science, but they lost it in the shuffle of exhibits a few years ago.
I’ve seen it in other museums – it’s a pretty easy demo if you’ve got a circulating system, some water with glowing stuff in it (like anifreeze) and an adjustable strobe (like one of the EG&G units manufactured by Doc’s company)
Here’s a webpage on it – “The Piddler”
