Comparing pressure loss to Blasius

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Discussion Overview

The discussion revolves around comparing pressure loss in a turbulent pipe flow simulation to established equations, specifically the Blasius equation and the Darcy-Weisbach equation. Participants explore how to relate their CFD results to theoretical models in fluid dynamics.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents a CFD simulation of turbulent pipe flow and seeks guidance on comparing pressure loss to the Blasius and Darcy equations.
  • Another participant argues against using the Blasius equation, stating it is not representative for turbulent pipe flow.
  • A participant mentions the Blasius friction factor and questions how to calculate it from pressure drop for comparison.
  • There is a clarification regarding the reference to the Blasius boundary layer, indicating a potential misunderstanding.
  • One participant suggests using the Darcy-Weisbach equation and Blasius correlations to approximate theoretical pressure loss for comparison with simulation results.

Areas of Agreement / Disagreement

Participants express disagreement regarding the applicability of the Blasius equation to the turbulent pipe flow scenario. There is no consensus on the best approach to compare the pressure loss, with some advocating for the Darcy-Weisbach equation while others remain uncertain.

Contextual Notes

Participants have not fully resolved how to calculate the friction factor from the pressure drop, and there are assumptions regarding the applicability of the Blasius equation that remain unaddressed.

Isabella F
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Hi everyone. i have done a CFD smulation in fluent for a turbulent pipe flow and have plotted static pressure against distance x. from the graph i get a straight negtive gradient. i have been told that you should compare the pressure loss to Blasius equation and Darcy equation. How do i compare the pressure loss to this. thank you. the fluid is water, diameter is 50mm, inlet velocity is 1m/s and reynolds number 49760.
heres is the xy plot i get.

s63xox.jpg
 
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Well that seems awfully silly to compare to Blasius because (A) it is pipe flow, so Blasius is not representative, and (B) it is turbulent, so Blasius doesn't apply.

Comparing to Darcy-Weisbach should be straightforward though.
 
The Blasius friction factor f=0.079/(Re^0.25).
Is there a way to calculate friction factor from the presure drop? that is what i think i need to do and then compare to blasius friction.
 
Ah, I thought you were referring to the Blasius boundary layer. My mistake.
 
How do i do this then. I am a bit confused.
 
Just look up the Darcy-Weisbach equation. It is very straightforward. Use it and the Blasius correlations to approximate the theoretical pressure loss (head loss) and compare to your results.
 

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