Why Higher Friction Factor with Laminar Flow?

5/18/2022

Why do laminar flows typically have higher frictional factors (Moody Chart) than turbulent flows?

I understand that the head loss is ultimately bigger in turbulent flows but would it not be expected that turbulent flows would experience a larger loss due to friction (and hence, a larger frictional factor)?

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4 Comments

Jesse

5/18/2022 08:08:30 am

I read the question and was about to post an answer, but remembered a discussion in this forum at one time about it.

https://www.meyerfire.com/daily/why-laminar-flow-means-higher-friction-factor

Alex

5/18/2022 08:12:09 am

Hi,

Frictional factor does not directly relate to the friction loss of the pipe. For example:

For laminar flow, the primary friction loss is based on the viscosity of the fluid. Where, for turbulent flows, the primary friction loss is based on the friction between the fluid (our case water) and the pipe.

This is seen by reviewing the equations to calculate both. For laminar flow, you calculate the friction factor by dividing 64 by the Reynold’s number. When reviewing the units within calculating the Reynolds number, there is no reference to the pipe surface or properties of the pipe itself. Therefore, friction loss for laminar flow is independent of the pipe it is traveling through.

For turbulent flow, you use the Colebrook-white equation to calculate the friction factor and then the Darcy-Welsbach formula to calculate the fluid frictional loss in a pipe. As much as I loved Fluid Dynamics while in School, I never played with these formulas much. I would always jump over to the Moody Chart mentioned in the question to find the friction factor. In the end, you will see that for turbulent flow, it is now not only based on the Reynolds number (like laminar) but also the relative roughness of the pipe.

Now to answer your question, yes, turbulent flows experience a larger loss due to friction. BUT, for laminar flow where Re is less than 2,300, you will have a higher frictional factor.

I hope this helps in the slightest. Fun morning taking the old books off the shelf.

Thanks,
Alex

Steven H

5/18/2022 08:22:38 am

It's been a while since I've done Darcy-Weisbach calculations, so apologies if I say something wrong. Your expectation that turbulent flows would have higher friction loss compared to similar laminar flows is correct.

The frictional factor (Moody Chart), however, is just a multiplier in the equation. The Darcy-Weisbach equation says that the pressure loss due to friction is proportional to the *square* of the flow velocity. Because flow velocity is higher in turbulent flows than for laminar flows (with other factors held constant -- see Reynold's Number definition), the pressure loss due to friction is also much higher. The frictional factor being less for the turbulent flow does not have as significant impact on the resulting friction loss as the increased velocity does.

As an aside -- in the "transition region", where flow is somewhere between laminar and turbulent (Re = ±2000-3000), things are a bit stranger, and perhaps it is better to not worry about that for now.

Jack G

5/18/2022 01:14:51 pm

As a Chemical Engineer, 1969, Fp Contractor all my life, i believe
First: laminar flows are very slow flows. ( fluid mechanics)
Second: the fliws start out in layers, wuth layer closest to wall doesnt move.
Third: each layer after adheres to each other. Subsequent layers move faster, wirh center mivung fastest.
Fourth: As the center is doing most of the flow, hazen williams flow charts for hydraulic equations, formulas, charts are very misleading for laminer flows.
A 4 inch pipe, laminar flowing, may only produce the flow of a 2 inch pipe at turbulent flow.
Fluid dynamics courses are wonderful in helping you understand sprinkler system dynamics.
Also , something bob worthing taught me in 67 ( star sprinkler course ) is you can throw hazen williams hydraulics out the window , flowing water below 10 fps or above 35 fps.
In testing pumps over 53 years. I ve come to agree.
Calcs dont match flows over 35 fps, is my experiences, no matter what it says in nfpa.
Opinion.

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Why Higher Friction Factor with Laminar Flow?

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