Water Velocity Question: Impact of Pipe Length on Flow Rate

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Discussion Overview

The discussion centers on the impact of pipe length on the velocity of water flowing through a vertical pipe. Participants explore the relationship between flow rate, pressure, and the effects of gravity and friction in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that if the pump produces a constant flow rate, the flow rate out of the top will equal the flow rate into the bottom, regardless of pipe length.
  • Others argue that if the pump produces a constant pressure, the flow rate out of the top may diminish with increased pipe height, potentially stopping if the height exceeds certain limits.
  • It is proposed that water, being incompressible, maintains a constant flow rate in the tube, but the velocity at the exit may vary based on other factors.
  • Some participants mention that the pressure difference between the top and bottom of a water parcel is the driving force for upward movement, while questioning the role of air pressure in this process.
  • Concerns are raised about whether the velocity decreases due to gravity and friction, with some asserting that this would violate conservation of mass if the liquid is incompressible and the tube's cross-sectional area is constant.
  • A participant describes an observable phenomenon with a faucet, noting that the velocity must remain constant in a uniform cross-section to maintain consistent flux.

Areas of Agreement / Disagreement

Participants express differing views on how pipe length affects water velocity, with some asserting that velocity remains constant under certain conditions, while others highlight the influence of pressure and gravity. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants reference various assumptions, such as the incompressibility of water, the constancy of cross-sectional area, and the nature of the pump's operation, which may affect the conclusions drawn.

  • #31
jbriggs444 said:
The question that I'd heard and answered was whether the length of the tube affected the difference between inlet velocity and outlet velocity. It does not. That's not the same as asking whether it affects velocity.
I do not understand that. So velocity will change with length then?
(sorry if I didn't make sense earlier)
 
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  • #32
russ_watters said:
In general longer pipes will have more friction loss, but which pipe are you referring to? For example, if you lower tank C, the flow rate will increase.
Lowering tank C will affect the water heights - that is why flow rate will increase.But will increasing the tube from B to A or C to A affect the velocity of the water at the top of the tube from B to A where the water comes out.
 
  • #33
Physicist1011 said:
Lowering tank C will affect the water heights - that is why flow rate will increase.But will increasing the tube from B to A or C to A affect the velocity of the water at the top of the tube from B to A where the water comes out.
Increasing tube lengths without increasing the height? That will reduce flow rates due to added friction loss
 
  • #34
Ok, so this will happen to tubes B to A and C to A but not from B to C right? (why)
 
  • #35
Physicist1011 said:
Ok, so this will happen to tubes B to A and C to A but not from B to C right? (why)
The air tubes are generally considered unrestricted here so their length doesn't introduce added loss.
 
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  • #36
Have the answers put your mind at ease, that this scenario as flawed, and won't sustain the desired action?
 
  • #37
Physicist1011 said:
Lowering tank C will affect the water heights - that is why flow rate will increase.But will increasing the tube from B to A or C to A affect the velocity of the water at the top of the tube from B to A where the water comes out.
Initially assume the system is in equilibrium. So nothing's happening. Now you want to experiment by changing the length of the tubes. So you have telescopic water carrying tubes (I don't know whether such a thing is available). I don't see any way you can control the length of the tubes independently. Assume you found some way to do that too.
When you increase the length from A to C, the length of the water column would increase. This would pump out water as in the diagram. When you decrease this length, it will not only cause the spurting to slow and reverse direction, the spurting will occur underwater from the other pipe in tank A.
When you control only the length from A to B, increasing the height would decrease the velocity and vice versa.
If both the lengths are changed simultaneously, it isn't an easy answer.
 
  • #38
russ_watters said:
The air tubes are generally considered unrestricted here so their length doesn't introduce added loss.
How does decreasing the height between A and B increase the speed of the water coming out of the fountain in A.
 
  • #39
Physicist1011 said:
How does decreasing the height between A and B increase the speed of the water coming out of the fountain in A.
If you raise B without touching A, you increase the static head between B & C; and that's the driving force for the flow.
 
  • #40
The length of the pipe will of course effect the velocity of water at the bottom. let the point from where water is coming be a and from where it is going out be b. then for streamline flow of water apply bernoulli's theorem at a and b. That is Pressure + 0.5 (rho)( v^2) + h (rho) g = constant
here rho= density
v=velocity
h=height (length in your case)
After applying bernoulli's theorem you will find that velocity of waste at bottom is dependent on length
 

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