pub question

[Guest kremlin]

can you guys help

 

Posted: Sun May 15, 2005 12:31 pm Post Subject: pub question

 

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a pint rests on the correct answer to this one

 

a 400 gallon water tank fitted at say 20 feet up ona a wall has two pipes coming from the bottom. one is 15mm the other is 40mm diameter

 

the pipes run verticaly down the wall to the floor, at the floor each has 2 90 degree bends (to make a u shape)

 

the tank then has a plug put in each pipe, plugs are chained together.

tank is the filled with water

 

when full, chain is pulled (so both plugs are removed at same time)

 

question.

 

will the water from one pipe (at the bottom) go higher up than the other, if so which and why.

 

 

cheers guys.. i look forward to answer.

[J.C.MacSwell]

can you guys help

 

Posted: Sun May 15' date=' 2005 12:31 pm Post Subject: pub question

 

--------------------------------------------------------------------------------

 

a pint rests on the correct answer to this one

 

a 400 gallon water tank fitted at say 20 feet up ona a wall has two pipes coming from the bottom. one is 15mm the other is 40mm diameter

 

the pipes run verticaly down the wall to the floor, at the floor each has 2 90 degree bends (to make a u shape)

 

the tank then has a plug put in each pipe, plugs are chained together.

tank is the filled with water

 

when full, chain is pulled (so both plugs are removed at same time)

 

question.

 

will the water from one pipe (at the bottom) go higher up than the other, if so which and why.

 

 

cheers guys.. i look forward to answer. [/quote']

 

I'll go with the larger due to less drag/volume of flow

[towjyt]

I say no. They both are under the same pressure pr. sq. inch or whatever.

[towjyt]

I'll go with the larger due to less drag/volume of flow

 

You might be right about that. There would be less friction pr.volumn in the larger pipe. I don't think it would make very much difference tho.

[[Tycho?]]

Hmm, I would have thought the smaller one would shoot higher at first, but now I dont think so. 1.5cm is pretty small, friction would play a signifigant role here. I'd say either the large one would go higher, or both the same. I dont know enough about fluids to calculate it though.

[Ophiolite]

All three answers are correct. (You may have to share the pint.)

Initially the higher flow rate possible in the larger diameter pipe will place it slightly higher

Once everything has reached equilibrium they must be at the same level.

Except when you take into account surface tension: the meniscus on the smaller pipe will be marginally higher..

[ski_power]

I suppose the one in the 15mm will go higher. Since it's diameter is lesser, the water will have greater velocity I guess...

[Callipygous]

I suppose the one in the 15mm will go higher. Since it's diameter is lesser, the water will have greater velocity I guess...

 

 

thats what i thought too, the thing thats throwing me off is i thought the velocity of water is supposed to be related to the area it has to pass through and the pressure involved. as a whole system the water in both pipes has the same area available to pass through. does the fact that it is the same system change the physics of this at all?

[uncool]

I'm taking this as what happens over time, and I'm assuming an ideal water. In those cases, the water will occupy the same level in the end. However, should the water be less than ideal (which we can assume it is), then, because of capillary action, it will rise more in the 15mm pipe than the 40mm pipe.

-Uncool-

[towjyt]

hi' date=' i originally posted on classical mechanics forum, but maybe this is a better arena for it

 

a pint rests on the correct answer to this one

 

a 400 gallon water tank fitted at say 20 feet up ona a wall has two pipes coming from the bottom. one is 15mm the other is 40mm diameter

 

the pipes run verticaly down the wall to the floor, at the floor each has 2 90 degree bends (to make a u shape)

 

the tank then has a plug put in each pipe, plugs are chained together.

tank is the filled with water

 

when full, chain is pulled (so both plugs are removed at same time)

 

question.

 

will the water from one pipe (at the bottom) go higher up than the other, if so which and why.

 

cheers guys [/quote']

 

Are you talking about water gushing up out of those 90 degree elbows? Or are you talking about screwing a pipe into those elbows and going back to the top of the tank?

[Guest kremlin]

hi ophiolite, i appreciate what youre saying, but if you could pad it out a bit with the principles you used to arrive your answer.

 

cheers

[Callipygous]

All three answers are correct. (You may have to share the pint.)

Initially the higher flow rate possible in the larger diameter pipe will place it slightly higher

 

that doesnt make sense.

 

the actually velocity of the water isnt higher, just the amount that passes a certain point in a given time. so the volume that can pass in a certain time should be higher, but i cant think of any reason the speed of the water should be higher, and therefore any reason it should reach a greater height. am i missing something?

[ski_power]

but some time i think that water will rise at the same level, as pressure at the bnase is same in both pipes, and initial and final pipe radius is also same

[Draco]

I guess the 40 mm pipe would produce more height as the volume is more combined with the liquid force = further distance travelled upward and say if both pipes ( bottoms ) were at 55 degrees the water in the 40mm pipe would achieve a greater height ? Yes..No ........Yes mmmm

[Callipygous]

why would volume contribute to height? greater volume does not mean greater velocity...

[gnpatterson]

by "how high the water goes up" I take it you are talking about the fountaining of the resulting jet of water.

 

the conditions you describe are the classical conditions to set up a quasi-steady flow. (you have a lot of water in a big tank)

 

the choice of water in and the diameters given indicate that the flow is a low renyolds number flow i.e. viscosity is not important (I could be wrong here as I havent done the calculation just made a guess.)

 

thus bernoulli is in effect

 

this tells us, in simple terms, that the velocity of the water coming out of the pipe will be given by the conversion of potential to kinetic energy.

 

Again bernoulli tells us how high the spouts will rise.

 

they should both rise to the height of top level of the tank again it is simple conservation of energy.

 

HOWEVER End effects do apply to the diameter of the pipe

 

I WILL neglect these in my argument.

 

to help you to "see" that the two spouts rise to the same hieght simply do a thought experiment, Imagine a thick pipe with water pouring out of it and mentally add a wall down the center of the of the pipe.

 

You can mentally divide the pipe and tank in half, as long as the tank remains "large" and the viscousity can be neglected then the flow of the thick pipe can be seen as the flows of a number of thin pipes running in parallel. (this is the arguement galilao used for dropping stones)

 

If you want to take viscousity and the surface tension into account it would require more than the value of a pint to motivate anyone.

[Karl1986]

My bad, I got it wrong :/