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Let's say you have a garden hose connected to an ordinary water tap which is opened fully. If you pinch the end of the hose, water leaves the hose at a higher speed (and this can be useful while watering plants, to reach pots which are further away). However when a tap (with no hose connected) is opened only slightly, water flows out at a low speed, possibly even in drops.

The actions of pinching the end of a hose and of almost-closing an open tap seem similar, so why the difference in behaviour?

Prateek
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3 Answers3

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This diagram shows the difference between closing the tap and pinchng the end of the hose:

Hosepipe

In both cases you are reducing the area the water has to flow through, and this increases the water velocity in the constriction. The upper diagram shows what happens when you close the tap. Closing the tap increases the velocity of the water at the constriction, but as soon as the water is past the constriction is slows down again and it emerges from the end of the hosepipe with a relatively low velocity.

The lower diagram shows what happens when you pinch the end of the pipe. The constriction increases the velocity of the water but because the constriction is right at the end the water doesn't have a chance to slow down again so it leaves the end of the pipe with a relatively high velocity.

John Rennie
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    Good answer, but based on your drawing it appears that a tap will always give out the same amount of water regardless of the size of the constriction which is not true. What am I missing? – JeffDror Jan 10 '14 at 11:27
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    @JeffDror: the point of my diagram is that fluid always flows faster at a constriction. So if you reduce the area by 50% the flow velocity will be twice what it is in the rest of the hose. As you close the tap (or pinch the end) the overall flow rate will decrease but the ratio of the velocity at the constriction to the velocity in the rest of the pipe will remain the same. – John Rennie Jan 10 '14 at 11:32
  • This does not answer the question posed. It rather explains why pinching the hose in the middle does not increase the speed of the stream. link.

    The question explicitly says, slightly opened tap with no hose connected. The reason for the low speed of the water in this case is the viscosity and caused by it pressure drop.

    BTW, water out of the end of the hose also slows down if you pinch its end tight enough.

    I don't have a reputation to write a full-blown answer. :)

     – Alex Fainshtein Jan 19 '20 at 22:09
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enter image description here

As shown in the figure, the gap of the faucet is inside the faucet, and the direction of spraying water is shown by the red arrow. Although the speed of the water in the gap is relatively large, the arrows are opposite to each other, so the speeds cancel each other out, so you are at the exit of the faucet. No high speed can be seen. At the end of the hose, the speed of the water passing through the gap can be seen. Why is the gap velocity high? Because the gap reduces the flow rate upstream of the hose, the static pressure of the water inside the hose increases, so the water at the gap obtains a greater thrust. So the speed increases.

enbin
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    I think you are right. When I pinch the hose while watering the garden, I feel the water pressure in the hose building up. So the increase in velocity is not only due to the smaller area through which the water has to pass (as suggested in the previous answer). The previous answer is, for example, applicable to the long rows of slowly moving cars on a highway. When the road gets smaller the velocity of the cars increases, to slow down again if the road gets broader again. – Deschele Schilder Sep 29 '20 at 07:16
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    @DescheleSchilder If you block the end of the hose with your fingers, you will feel the pressure of the water in the hose increase a lot. The pressure difference is the cause of the increase in water velocity. – enbin Sep 29 '20 at 11:23
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When pinching the hose, the pressure slightly expands the diameter of the hose. When you release the pinch in the hose, the water has greater velocity for two reasons: 1) There is slightly greater volume in the hose because of the expanded volume of the hose 2) The elastic response of the stretched hose, pushing the water out.

user38449
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  • I was asking about the steady-state behaviour, not about the change caused by the pinching or releasing. Perhaps this was not clear from the question, sorry about that. – Prateek Jan 29 '14 at 06:48