How Can You Calculate a Parabolic Velocity Profile in 2D?

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Discussion Overview

The discussion focuses on calculating a parabolic velocity profile in two-dimensional laminar flow, particularly in the context of fluid mechanics and pipe flow. Participants explore the relationship between the shape of the velocity profile and the maximum velocity, as well as the mathematical derivation of this profile.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants propose that the velocity profile is parabolic with maximum velocity at the peak, seeking calculations to demonstrate this.
  • Others mention that the volumetric throughput rate is equal to the velocity integrated over the cross-sectional area of the pipe, and that average velocity can be derived from this integration.
  • One participant suggests that the derivation of the parabolic profile can be found in undergraduate fluid mechanics texts, specifically referencing the Navier-Stokes equations.
  • A later reply challenges an earlier claim about averaging velocities, stating that velocities must be weighted according to differential areas, and provides a formula for volumetric flow.
  • Another participant acknowledges a misunderstanding regarding the dimensionality of the profile being discussed.

Areas of Agreement / Disagreement

Participants express differing views on the correct approach to deriving the parabolic velocity profile, particularly regarding the treatment of velocities and areas in calculations. The discussion remains unresolved with multiple competing perspectives on the methodology.

Contextual Notes

There are limitations regarding assumptions made about the flow conditions, such as the applicability of the parabolic profile to laminar flow only, and the need for clarification on the integration process for calculating average velocity.

sifr
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sifr said:
I know it must relate to the velocity profile being a parabola shape and the max velocity being at the peak of the parabola -

I wanted to know whether there are actual calculations I can do to show this?

I only know the basics so as much details as possible would be great help

http://blogs.tlt.psu.edu/projects/accessibilitydemo/examples/VelocityProfileLaminar.png

The volumetric throughput rate is equal to the velocity integrated over the cross section area of the pipe. The average velocity is equal to the volumetric throughput rate divided by the total cross sectional area. See Bird, Steward, and Lightfoot, Transport Phenomena.

A parabolic profile only applies to laminar flow. In turbulent flow, the factor is much less than 2.
 


You are looking for the derivation of laminar flow through a pipe. Most undergraduate fluid mechanics books derive this velocity profile through the Navier-Stokes equations. My favorite is Granger's "Fluid Mechanics"; its a cheap dover book more detailed than many modern texts. Assuming its parabolic (that is to say, without justifying it) the sum of the velocities along the diameter of the pipe divide by the diameter will equal the average velocity. This is analgous to finding the vertex in high school precalx.
 


Aero51 said:
You are looking for the derivation of laminar flow through a pipe. Most undergraduate fluid mechanics books derive this velocity profile through the Navier-Stokes equations. My favorite is Granger's "Fluid Mechanics"; its a cheap dover book more detailed than many modern texts. Assuming its parabolic (that is to say, without justifying it) the sum of the velocities along the diameter of the pipe divide by the diameter will equal the average velocity. This is analgous to finding the vertex in high school precalx.

The line in bold above is not correct. You have to weight the velocities in terms of the differential areas.

dQ = 2∏r v dr

You then divide, not by the total radius or the diameter, but by the total cross sectional area ∏R2
 


Oh sorry i was thinking for a strictly 2d profile. I probably should have checked first.
 

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