- #1
threy
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Fluid flows faster in a narrow tube which results in low pressure and high pressure in a large tube?
Why does fluid flow faster in a narrow tube?
- Context: High School
- Thread starter threy
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Discussion Overview
The discussion centers on the behavior of fluid flow in tubes of varying diameters, specifically why fluid flows faster in narrower tubes compared to wider ones. It explores concepts related to flow rate, pressure changes, and the principles governing fluid dynamics, including Bernoulli's equation. The scope includes theoretical explanations and conceptual clarifications.
Discussion Character
- Technical explanation
- Conceptual clarification
- Debate/contested
Main Points Raised
- Some participants note that fluid must flow quicker in a narrow pipe to maintain the same flow rate when transitioning from a larger pipe.
- Others emphasize the importance of mass flow rate and the continuity equation, stating that volumetric flow rate applies primarily to incompressible fluids.
- One participant introduces Bernoulli's equation to explain the relationship between kinetic energy and pressure, suggesting that an increase in speed results in a decrease in pressure.
- Another participant discusses the net forces acting on the fluid, indicating that higher pressure in the larger tube drives the fluid into the narrower section.
- Some participants highlight the distinction between a single tube with varying widths and multiple separate tubes, noting that flow rates may differ in the latter case.
- There are repeated assertions about the necessity for flow rate consistency, with some participants reiterating that what goes in must come out, leading to changes in velocity with changes in cross-sectional area.
Areas of Agreement / Disagreement
Participants generally agree on the principle that fluid must flow faster in a narrower section to maintain flow rate, but there is disagreement regarding the implications of this in different scenarios, such as single versus multiple tubes. The discussion remains unresolved on some aspects, particularly regarding the conditions under which these principles apply.
Contextual Notes
Some limitations include the assumption of incompressibility for fluids and the dependence on specific conditions related to the flow setup. The discussion does not resolve the complexities involved in different tube configurations.
- #2
mfb
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Some context would be helpful.
If you have a fluid going from a large pipe to a narrow pipe (or vice versa), the fluid has to flow quicker in the narrow pipe to get the same flow rate ([strike]volume[/strike] mass per time).
If you have a fluid going from a large pipe to a narrow pipe (or vice versa), the fluid has to flow quicker in the narrow pipe to get the same flow rate ([strike]volume[/strike] mass per time).
Last edited:
- #3
gauss44
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mfb said:
This is correct. And I'll add that there is a distinction to be made between this (a single tube with variations in width), and multiple (different) tubes.
In the later scenario: If you have 2 separate tubes which are not connected, there is no guarantee that water will flow faster through a narrower tube.
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mfb said:
More correctly, the mass flow rate must be maintained (continuity). Volumetric flow rate only works here for an incompressible fluid.
- #5
tiny-tim
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hi threy! 
pressure is not only force per area, it is also energy per volume
(1 Pa = 1 J/m3)
therefore conservation of energy requires that if the kinetic energy increases (ie if the speed increases), then the pressure must decrease
(mathematically, this is Bernoulli's equation … P + 1/2ρv2 + ρgh = constant along any streamline)
threy said:
pressure is not only force per area, it is also energy per volume
(1 Pa = 1 J/m3)
therefore conservation of energy requires that if the kinetic energy increases (ie if the speed increases), then the pressure must decrease
(mathematically, this is Bernoulli's equation … P + 1/2ρv2 + ρgh = constant along any streamline)
- #6
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Or put another way: the fluid speeds up when it enters a narrower portion of the tube. Since it speeds up, it has an acceleration, therefore a net force, in the direction it is moving. This net force must result from a higher pressure behind the fluid (in the larger tube portion) and a smaller pressure ahead of the fluid (in the smaller portion).
- #7
john bastian
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but why the fluid go faster in narrow tube than a thicker tube?gauss44 said:
- #8
CWatters
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Perhaps it's this simple...
If the fluid is incompressible the flow rate (in cubic meters per second) must be the same at all points along the pipe. What goes in must come out.
If the cross sectional area (in square meters) changes the velocity (in meters per second) must change to maintain the same flow rate.
If the fluid is incompressible the flow rate (in cubic meters per second) must be the same at all points along the pipe. What goes in must come out.
If the cross sectional area (in square meters) changes the velocity (in meters per second) must change to maintain the same flow rate.
- #9
enorbet
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There are some really great graphical and mathematical descriptions at http://en.wikipedia.org/wiki/Bernoulli's_principle
- #10
mfb
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Right.CWatters said:
This thread is from 2013. If there is a new question, please open a new thread.
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