Velocity as a function of area in pipe flow

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Discussion Overview

The discussion revolves around the relationship between velocity and area in pipe flow, particularly how average velocity can be expressed as a function of area despite velocity being a function of radial position. Participants explore the implications of integrating velocity over area versus diameter and the effects on understanding flow characteristics.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how to conceptualize velocity as a function of area when it is already defined as a function of diameter, suggesting a potential confusion in the relationship.
  • Another participant clarifies that the average velocity across a cross-section averages out the radial dependence, indicating that it is distinct from the local velocity at a point.
  • A participant reiterates the initial question about the relationship between area and diameter, proposing that thinking of velocity as a function of area could be valid since area is dependent on diameter.
  • It is noted that integrating velocity over area loses information about the radial dependence, and while it is possible to express average velocity in terms of diameter, it may not capture the same nuances as using area.
  • Some participants highlight that using area in calculations stems from conservation of mass, which adds complexity to the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the implications of treating velocity as a function of area versus diameter. There is no consensus on whether one approach is superior or more intuitive than the other, indicating an unresolved discussion.

Contextual Notes

The discussion involves assumptions about the flow characteristics and the mathematical relationships between velocity, area, and diameter, which may not be fully articulated or agreed upon by all participants.

mech-eng
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There is an equation which perplexes me and it is about calculation of average velocity in a pipe but over area.


In the image, the velocity is already function of diameter, i.e, u=u(r) so how can we think velocity as a function of area?

Source: Fluid Mechanics, Fundamentals and Applications by Çengel/Cimbala

Thank you.
 

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You've sort of answered your own question. It's the difference between the velocity at a point, which is a function of ##r##, and the average velocity across the cross section, which averages that ##r##-dependence away and depends on ##A##.
 
mech-eng said:
There is an equation which perplexes me and it is about calculation of average velocity in a pipe but over area.In the image, the velocity is already function of diameter, i.e, u=u(r) so how can we think velocity as a function of area?

Source: Fluid Mechanics, Fundamentals and Applications by Çengel/Cimbala

Thank you.
u(r) is the local axial velocity at radial coordinate r. The axial velocity is maximum at the center of the tube u(0), and is zero at the wall, u(R) = 0. If you want to get the average axial velocity, you calculate the volumetric flow rate and divide by the total cross sectional area of the tube.
 
Can thinking of u(Area) instead of U(D) be the same? Because Area is function of diamter as well. This looks like a differential equation problem and while integrating over area it becomes integration over diameter because dA=2*pi*r*dr

Thank you.
 
In general, the velocity varies over the range of ##r##. Doing the integration so that it is ##u_{avg}(A)## loses all that information about the ##r## dependence. You could certainly cast it in terms of ##u_{avg}(D)## as long as you are just referring to ##D## as the inner diameter of the pipe and not some kind of surrogate for ##r##, but you couldn't get ##u(D)## just like you couldn't get ##u(A)##. It is less intuitive though, since using ##A## comes from conservation of mass.
 

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