Faucet Physics Question

[TimM]

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Why do you have to have conservation of mass through any particular area of the stream? There's nothing about this system that requires this.

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Once you reach a steady laminar flow, the rate at which the water comes out of the faucet must match the rate at which it goes down the drain. You can say the same thing about the rate of water passing through any horizontal plane intercepting the stream.

[daryn]

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Why do you have to have conservation of mass through any particular area of the stream? There's nothing about this system that requires this.

[/ QUOTE ]

Once you reach a steady laminar flow, the rate at which the water comes out of the faucet must match the rate at which it goes down the drain. You can say the same thing about the rate of water passing through any horizontal plane intercepting the stream.

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in other words i am correct right? it IS conservation of mass.

[slickpoppa]

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Why do you have to have conservation of mass through any particular area of the stream? There's nothing about this system that requires this.

[/ QUOTE ]

Once you reach a steady laminar flow, the rate at which the water comes out of the faucet must match the rate at which it goes down the drain. You can say the same thing about the rate of water passing through any horizontal plane intercepting the stream.

[/ QUOTE ]

in other words i am correct right? it IS conservation of mass.

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I dont think it can be completely explained by conservation of mass. It must have something to do with the cohesive properties of water. Think about it, if it were just a conservation of mass thing, wouldn't sand have the same property of bunching together? I haven't done any lab experiments about sand coming out of a faucet, but I am pretty sure that a stream of sand does not get narrower.

Also think about it from a conservation of momentum standpoint. Picture a molecule of water that leaves the faucet from the edge of the faucet. If the stream gets narrower, that means that the molecule must have experienced an inward force. So at the very least the cohesive property of water must have something to do with it.

[Redeye]

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Well to be honest, I'm not sure if we're talking about it with it on or off any more now, but I believe the narrowing effect of the stream was essentially the same with or without the aerator. I don't think the effect we're looking for here has anything to do with it being laminar.

I think it's all in water's tendency to attract to itself. I don't have the fluid dynamics knowledge anymore to say anything real intelligent about it other than I think this is roughly where the answer is.

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It won't matter if its on or off, you'll observe the effect both ways. The fluid flow is really more laminar than turbulent with or without the aerator on. As Daryn said before, its all basically about the conservation of mass through a given cross-sectional area. As long as there is no significant turbulence, the amount of fluid that passes through a given cross-seciton of the stream remains constnat. As the velocity of the fluid flow increases, the amount of water entering and exiting a cross-section must be the same therefore the diameter of the cross-section must decrease. As the fluid flow reaches terminal velocity, the diameter will eventually stop decreasing.

[daryn]

well i'm sure it has to do with cohesion, clearly, but is what we're talking about in the first place, laminar flow. i just remember the prof. telling us basically what tim said. the same amount of water (mass) has to be going down the drain as there is coming out the faucet. he called it an application of conservation of mass.

so since it's going faster at the bottom, you can't have too much mass exiting, therefore the stream narrows.

[MelchyBeau]

only in OOTia can you go from one thread about a shitty ass poem to another thread about whiskey to another about fluid dynamics.

p.s. I'm right, it has to do with the attraction of water molecules to each other.

Melch

[stinkypete]

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in other words i am correct right? it IS conservation of mass.

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yeah, you're correct, and i'm surprised people on this forum aren't smart enough to understand the concept.

the amount of liquid/time falling through any part of the stream is constant. the water close to the floor/sink is falling faster than the water at the top of the tap. for the same amount of water to fall through some horizontal plane at the bottom, the cross section has to be smaller since the velocity is faster.

if the cross section stayed the same and the water still accelerated, you'd have water being created out of nowhere... so in that sense, referring to conservation of mass makes sense.

[TimM]

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in other words i am correct right? it IS conservation of mass.

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Seems pretty obvious. This plus the fact that gravity is accelerating the water, plus the cohesion, makes the stream narrow.

With sand, say going through a tall hourglass, you probably get some spreading of the stream due to collisions and air resistance. but you also get a less dense stream as you get further down, for the same conservation of mass reasons. Consider just two grains falling one just after the other. Over time the distance between them must increase, as the first grain has always been accelerating just a little longer than the second grain.

[slickpoppa]

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well i'm sure it has to do with cohesion, clearly, but is what we're talking about in the first place, laminar flow. i just remember the prof. telling us basically what tim said. the same amount of water (mass) has to be going down the drain as there is coming out the faucet. he called it an application of conservation of mass.

so since it's going faster at the bottom, you can't have too much mass exiting, therefore the stream narrows.

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Of course it must obey conservation of mass, but the most important part of the equation is the cohesion. Theoretically, if the water molecules were not attracted to each other at all, the stream would not narrow at all. And how much the stream narrows is a function of the viscosity of the liquid, which is a function of how much the molecules are attracted to each other. So to focus on the conservation of mass aspect of the equation is not particularly helpful.

[stinkypete]

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p.s. I'm right, it has to do with the attraction of water molecules to each other.

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sorry buddy, you're not. what he described will happen even if the water molecules are packed as closely as possible at the top.