Friction factor and Turbulent flow

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Discussion Overview

The discussion revolves around the relationship between friction factor, head losses, and turbulent flow in fluid dynamics. Participants explore the implications of the Moody chart and the behavior of the friction factor in different flow regimes, including laminar and turbulent flow. The conversation includes theoretical aspects and mathematical relationships relevant to fluid mechanics.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question whether head losses in turbulent flow are always greater than in laminar flow, referencing the Moody chart.
  • One participant notes that the friction factor's decrease does not necessarily indicate less head loss, as it must be multiplied by other terms to calculate head loss.
  • Another participant expresses confusion about the relationship between increased friction losses and a decreasing friction factor, seeking clarification on its definition.
  • It is mentioned that the friction factor relates to pipe length, diameter, and dynamic pressure, with a reference to the squared velocity in the dynamic pressure equation.
  • Some participants highlight that the friction factor is a function of the Reynolds number and relative roughness of the pipe.
  • A participant points out that while the friction factor decreases with increasing turbulence, it eventually stabilizes at high Reynolds numbers, becoming primarily a function of relative roughness.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and agreement regarding the relationship between friction factor and head loss, with some confusion remaining about the implications of the Moody chart and the behavior of the friction factor in turbulent flow. Multiple competing views and interpretations exist, and the discussion remains unresolved.

Contextual Notes

Participants reference the Moody chart and its implications for friction factor behavior but do not fully resolve the mathematical relationships or assumptions involved in the definitions and calculations discussed.

GT1
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Do the head losses on turbulent flow are always greater then on a laminar flow? Why does it seem from the Moody chart that the friction factor becomes smaller when the flow becomes more turbulent?
 
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Look at the definition of the friction factor and where the velocity term is located in that equation.

It is a little bit confusing but the drop in friction factor does not need less of a head loss since you will have to multiply it by V^2/2 and L/d first in order to obtain the later.
 
I still don't understand why if there are more friction losses, the friction factor decreases. What is the reason to define the friction factor at this way?
 
The friction factor a I understand relates the length of a pipe, its diameter and the dynamic pressure with head loss. It is very typical to relate phenomena such as these in relation to the dynamic pressure (think of drag coefficient). Since the Pd = 1/2 V^2 rho,the velocity is squared in in it.
 
The friction factor is a function of Re number and the relative roughness of the pipe (e/D).
 
Ok your correct about the e/D part I just looked it up. Anyway, the friction factor is defined as follows

http://en.wikipedia.org/wiki/Friction_factor

Note that it is a function of velocity squared were as you plot it against Re, you plot it against a first order velocity dependency. Hence, there is a velocity term left which causes the friction factor to go down when velocity (turbulence) goes up. the fully turbulent part on a Moody diagram seems non linear and I am not sure what causes that do.
 
GT1 said:
Do the head losses on turbulent flow are always greater then on a laminar flow? Why does it seem from the Moody chart that the friction factor becomes smaller when the flow becomes more turbulent?

The friction factor gets smaller until the Reynolds number gets really large. Then it's just a function of the relative pipe roughness and the friction factor remains relatively constant. This is indicated by the almost horizontal line on the far right side of a Moody chart.

The Moody chart indicates this pretty clearly and all of the numerical calcs I've done agree with that (for Newtonian Fluids).

CS
 

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