Naked Science Forum
General Science => General Science => Topic started by: Atomic-S on 30/07/2009 00:25:07
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Consider a garden hose having a nozzle set for a direct stream, not a spray. If we wish to eject the water to the maximum possible height, what is the best position for the nozzle? At ground level? At hand level? At rooftop level?
What if instead we want to throw the water the maximum horizontal distance (over flat terrain)?
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I rather suspect that the water pressure will have some bearing on this.
Obviously, to some degree the level at which the nozzle is placed will effect the maximum height, but as you increase the height of the nozzle, so you decrease the pressure. As for maximum distance, again the higher your nozzle, the further the distance, to a point. But again the higher the nozzle, the lower the pressure, and you will eventually reach a point beyond which there would be no further gain. This point will vary according to pressure.
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If you want a maximum ejecting water, you can aim nozzle to down.
If you want maximum height, you can close a half of your nozzle with your thumb
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It depends entirely upon the pressure and the resistance of the supply pipe. It's a typical load matching problem and, somewhere in there, the 'Maximum Power Theorem' probably applies. I would not be surprised if the maximum power you get out (giving the maximum height with the maximum flow) would be when the resistance of the nozzle is the same as the resistance of the water supply. However, it depends on how much rate of flow you need, too. If the jet is too thin, then air resistance will limit the height you can reach.
There is no 'one word answer' to this one.
Better to use a proper nozzle than your finger - the flow would be better.
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This is another ballistics problem.
Domestic water supplies are nearly always gravity fed and in the UK at least, the water boards would not be very happy with you attaching a pump directly to the supply as this will not only affect other users but also risk over pressuring the supply pipes.
Thus, with a gravity fed supply, lifting the nozzle higher will have no effect regarding the height you can achieve for any particular angle of nozzle.
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You are right in that it's essentially a balistics problem, but the question isn't adequately specified. Is the velocity of the water leaving the nozzle significantly affected by the height of that nozzle above the ground?
Also as for "the water boards would not be very happy with you attaching a pump directly to the supply ".
They are perfectly used to it. The pumps are called jet washers.
A pump that was trying to get a bigger range on the jet would lower the pressure in the local mains rather than raise it.
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Also as for "the water boards would not be very happy with you attaching a pump directly to the supply ".
They are perfectly used to it. The pumps are called jet washers.
Not according to a friend of mine who designs fire suppression sprinkler systems. If they need to use a pump it can only be connected to a local reservoir/storage tank.
Jet washers do indeed use a pump, but to force a small amount of water at high speed through a small nozzle, not to draw more water from the supply - how are domestic jet washers normally connected to the water supply? Via a length of ordinary garden hose, which would collapse if the pump tried to draw too much water through it - they're designed and manufactured to withstand moderate internal pressure, not external pressure - try standing on one. The hose from the jet washer unit does have to be strengthened though, as once it's full of water it'll be under high pressure.
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Could you not just connect up a hose and vary the angle to see how far it goes?
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Could you not just connect up a hose and vary the angle to see how far it goes?
Heh [:D] this should have been the first response - lol.
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That would have involved us getting up off our arses, tho.
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The optimum angle would be around 45 degrees but the maximum distance would depend on the precise nozzle diameter etc. etc..
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Also as for "the water boards would not be very happy with you attaching a pump directly to the supply ".
They are perfectly used to it. The pumps are called jet washers.
Not according to a friend of mine who designs fire suppression sprinkler systems. If they need to use a pump it can only be connected to a local reservoir/storage tank.
Jet washers do indeed use a pump, but to force a small amount of water at high speed through a small nozzle, not to draw more water from the supply - how are domestic jet washers normally connected to the water supply? Via a length of ordinary garden hose, which would collapse if the pump tried to draw too much water through it - they're designed and manufactured to withstand moderate internal pressure, not external pressure - try standing on one. The hose from the jet washer unit does have to be strengthened though, as once it's full of water it'll be under high pressure.
So what?
The jet washer is still a pump, and it's connected to the mains and it doesn't upset the water board.
Also, the water leaving the nozzle of the jet washer is traveling faster than it would without the pump so, for a given angle of elevation, it could travel further.
And, while I'm at it, how could any pump that was seeking to increase the range of the jet increase the pressure in the water mains (appart from water hammer if it wasn't set up right)?
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45 degrees is the answer unless the earth's curvature comes into play
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BC, I don't think the veto on pumping directly from the mains is because of excess pressure - it applies to high volume pumps which can reduce the mains pressure by nicking all the water. I think shower pumps and jetwashers have an input pressure cut out, in any case.
turnipsock
It's only 45 degrees if you ignore air resistance and you can't do that for a narrow stream of water because it is very likely break up into drops and spreads out, slowing down considerably. Surely you remember the infants' boys toilets and the record breaking heights for peeing when you were pre-pubic and high pressure.
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I think I have solved the first question. It is simply a matter of conservation of energy. A segment of water having mass m, exiting the nozzle, has the kinetic energy mg(htower - hnozzle) less frictional losses incurred in transit. It has no source of additional energy. This kinetic energy determines how high the water can rise once it exits the nozzle. Therefore, the position of the nozzle that will give the maximum height when projected vertically is simply the position which minimizes frictional losses. That will be the position of near-zero flow rate, which means that the tip of the nozzle must be located at an altitude just slightly less than that of the surface of water in the supply tower. As the nozzle is brought down lower, the flow rate increases, causing increasing frictional losses and lesser ultimate height. Note that the effect of air resistance will follow the same rule. So, fire departments take note: To raise the highest window that the water can reach, spray it from the highest available position.
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45 degrees is the wrong answer if you don't start at groung level so it's deffinitely wrong for two of the three cases asked about.
"BC, I don't think the veto on pumping directly from the mains is because of excess pressure "
I know that but Lee seems not to. He wrote "this will not only affect other users but also risk over pressuring the supply pipes."
I guess he doesn't know the difference between a supply pipe and a delivery pipe. Even if he did, the delivery pipes would be the consumer's problem so the water board wouldn't care.
I know that the water board would complain about too large a rate of consumption of water whether that's due to a pump or not.
That raised consumption would lead to a drop in the pressure in the local distribution system.
What I don't get is how LeeE thinks taking more water out of the supply pipes will raise the pressure in them.
AtomicS's analysis starts off soundly in that you can generally rely on the conservation of energy to give at least one boundary to the answer.
Unfortunately while energy is conserved in the long run, you can store it in the short term.
This toy
http://en.wikipedia.org/wiki/Hydraulic_ram
will bugger up the calculation.
More importantly he has forgotten that with a nozle at the same height as the water in the water tower, there is no available energy to raise the velocity of the water so it drips out of the nozzle, falls straight down and achieves zero horizontal range.
If you assume that the friction and air reesistance are small then the best range, starting from the ground is achieved with a 45 degree nozzle.
There really isn't enough information to answer the other parts of the question.
In particular, if the water is being supplied fron a tank in the roof then the water won't reach the rooftop and the range there will be zero. If it's fed at a higher pressure than that you get a different answer depepnding on the effective impedance of the supply system.
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This is atypical TNS discussion. Everyone seems to have a point but the question is so open that there is no definitive answer.
The max height attained will be when the open end of the tube is, indeed, within a gnat's crotchet of the level of the supply tank surface; the water will dribble out but reach the greatest height. A true but irrelevant fact.
The max range will be with nozzel as low as pssible UNLESS that involves having a long length of thin hose, in which case the optimum may be with less head.
But for some practicalities, 45 degrees gives best range.
There's little more to be said without specifying the conditions more tightly - then it gets rapidly harder!
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Oops - I should have said reduce the pressure in them, not increase the pressure. In any case, you still risk damaging the supply pipes and affecting the supply to other people.
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Oops - I should have said reduce the pressure in them, not increase the pressure. In any case, you still risk damaging the supply pipes and affecting the supply to other people.
I agree. You'll reduce the pressure to other users if you try to "suck" more out the pipe than it wants to supply. Much worse, if there are leaks in the water main, even small ones, contaminated ground water can be drawn into the water main. [:o]
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In the thing that i am setting it up,I am aiming a maximum height so, what is a best degree of its angel to achieve a good height or the maximum height?
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