Filling a pool with the hose end above or in the water

[michelcolman]

Don't reply too quickly, I've seen a lot of answers on this question already (differing ones of course, what would you expect), at first sight it would seem to be really simple but there are quite a few complicating factors.

So the question is basically: what's the quickest way to fill up a pool with a garden hose. Hose just above the water level, hose just below the water level, or hose well below the water level? Or does it make no difference? (Hose far above the water level will obviously be slower as the output pressure of the hose will decrease much more than the air pressure from altitude)

From a pure pressure point of view, submerging the hose should make no difference: as the end of the hose is placed deeper under water, the output pressure of the hose will become higher but the opposing pressure from the water in the pool will increase by the same amount.

However, intuitively it seems easier for water to push air out of the way rather than other water. The submerged hose also creates a vortex in the water, and that energy has to come from somewhere too.

Also, when water exits the hose vertically in the air, gravity on the open water stream will accelerate it further and perhaps create extra suction (although that effect is probably less than what you would get from a lower exit point).

Some sites actually say that the submerged hose will have a slightly higher rate because of some kind of siphon effect, but I don't really understand how that would work.

Does anyone have a reliable answer, or is this just one of those things where every expert has a theory but nobody has actually bothered to check it rigourously?

Thanks for any insights!
Michel

 

[Baluncore]

You are considering varying the hose outlet position, but surely it is more important to specify what regulates water flow into the hose. That will be what really determines the time to fill the pool.

 

[Dullard]

I'm not going to do the experiment. Assuming nothing exotic about the water supply, best flow will be accomplished with the hose just above/at present water level. As you've already noted, there is no static pressure advantage to submerging the hose and the exit back-pressure (dynamic) will increase in water (vs. air). How much it actually matters depends on a lot of unspecified parameters - typically, not much.

 

[michelcolman]

You are considering varying the hose outlet position, but surely it is more important to specify what regulates water flow into the hose. That will be what really determines the time to fill the pool.

It's from a standard water faucet, so I imagine the pressure at the faucet is determined either by the height of the water tower or the output of some huge pump supplying the neighbourhood. So I think we can assume the pressure to be constant at the faucet. Which means the pressure at the outlet will be higher if I hold the outlet lower. There will also be friction losses from the water flowing through the hose, but those will only lower the pressure if the flow is faster so they won't affect the answer to the question.

 

[michelcolman]

I'm not going to do the experiment. Assuming nothing exotic about the water supply, best flow will be accomplished with the hose just above/at present water level. As you've already noted, there is no static pressure advantage to submerging the hose and the exit back-pressure (dynamic) will increase in water (vs. air). How much it actually matters depends on a lot of unspecified parameters - typically, not much.

I just noticed something interesting: if I hold the hose immediately above the water and then slowly lower it into the water, it quite abruptly moves down into the water, appearing to be "sucked in". Of course no actual "sucking" is taking place, but this rather seems to indicate that the reaction force of the exiting water on the hose suddenly decreases (by a lot), which would mean the flow rate is much lower? Or is there some other explanation why the hose suddenly moves down into the water?

 

[michelcolman]

It just occurred to me that I could do a very simple experiment: timing the revolutions of the water meter. Turns out the flow rates are practically identical after all. I guess the hose being "sucked in" is due to the water flow dragging the surrounding water along with it, resulting in a flow pattern with a reduced pressure near the end of the hose.

 

[Baluncore]

So I think we can assume the pressure to be constant at the faucet.

If the delivery pressure is constant, and the faucet is a fixed restriction to flow, as is the hose, then the hose end should be kept just above the level of the water in the pool. That will maximise the pressure difference between the faucet and the hose end, that is needed to overcome the faucet and hose resistance.

 

[michelcolman]

If the delivery pressure is constant, and the faucet is a fixed restriction to flow, as is the hose, then the hose end should be kept just above the level of the water in the pool. That will maximise the pressure difference between the faucet and the hose end, that is needed to overcome the faucet and hose resistance.

But why would that be exactly? If you hold the end of the hose higher, the water will come out with less pressure (and at some height no water will come out anymore at all). If you lower the end, it will have more pressure. Under water, the pressure from the water in the pool will go up by the same amount ρgh as you go deeper. Those differences would appear to cancel each other out.

I have established by experiment that, if there is a difference at all, it has to be very small.

 

[Baluncore]

But why would that be exactly? If you hold the end of the hose higher, the water will come out with less pressure ...

The lower the end of the hose, the greater the hydrostatic pressure will be that helps overcome the flow resistance of the faucet and hose.

Cut the hose to the minimum length needed to reach the surface of the pool. That maximises the hydrostatic advantage, while minimising the hose resistance.

Use a thicker hose to reduce resistance to flow.

Heat the water to reduce the viscosity, and so increase the flow.

 

[AlexisBlackwell]

This is a really interesting question! At first glance it may seem that there is no difference, but in fact it all depends on several factors. A submerged hose can create additional pressure due to the depth of the submersion, but there are also other factors such as vortex movement of the water. The hose outlet also affects how water enters the pool - a vertical outlet can create additional acceleration due to gravity.

 

[jack action]

From a pure pressure point of view, submerging the hose should make no difference: as the end of the hose is placed deeper under water, the output pressure of the hose will become higher but the opposing pressure from the water in the pool will increase by the same amount.

If you put the end of the hose at the bottom of a pool, the level of the water in the pool will only be at the same height as in the tube when the pool is filled.