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Friction factor unsteady-state flow

Friction factor unsteady-state flow

Friction factor unsteady-state flow

(OP)
Hello everybody,

I trying to collect some informations for what is concering the friction factor behavior in unsteady-state flow of water in fittings, such as elbows, reducers and so on, because the friction factor is defined for steady-state flow and i'm developing a model for an hydraulic closed loop and i would like to study it also in unsteady-state . Is anyone of you so kind that could give me some information or at least tell me where a can find anything on this matter?
Thank you in advance i'd really appreciate

Luca  

RE: Friction factor unsteady-state flow

Friction factor is correlated to Reynolds number and hence velocity.  It doesn't matter if the velocity is changing, except when it is crossing over from laminar through the transition zone into turbulent flow.

"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************
http://virtualpipeline.spaces.live.com/

RE: Friction factor unsteady-state flow

The friction factor will just be one more variable you will have to include in your analysis. As the flow rate and pressure drop change, so will the instantaneous friction factor change. As BigInch has said, the friction factor is a function of the Reynolds number and in a hydraulic circuit the density, viscosity and pipe diameter will be steady so you only have to track the effect of the flowrate on the friction factor.

Have a look in the FAQ section of this forum for the article by quark which discusses the analytical expressions for friction factor.  This will make it easy to generate an algebraic expression to include in your analysis.

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RE: Friction factor unsteady-state flow

From Nonsteady Duct Flow-Wave Diagram Analysis by George Rudinger, Dover Publications.   Copyright 1969
pg 47
"From this brief review, it becomes apparent that friction effects in nonsteay flow depend not only on the instantaneous flow conditions but also on the history of the flow."

Regards

RE: Friction factor unsteady-state flow

Then the question becomes, when is that ever significant?

"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************
http://virtualpipeline.spaces.live.com/

RE: Friction factor unsteady-state flow

I have seen and correlated unsteady (oscillating flow) flow coefficients and seen them deviate significantly from the standard, steady-flow correlations, even in regions where the steady-flow correlations predict unchanging values w.r.t. Reynold's.  The amount of deviation (usually positive) was dependent on the %-age of the unsteady component relative to the steady component.  So, YMMV...
 

RE: Friction factor unsteady-state flow

No doubt there are specific situations that don't fit the typicals.  That's why I say hyd modeling within 10% total head is a good day.

"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************
http://virtualpipeline.spaces.live.com/

RE: Friction factor unsteady-state flow

"hyd modeling within 10% total head is a good day"

Yup! :)

RE: Friction factor unsteady-state flow

The original question does not relate to steady state conditions.

But:

In steady incompressible flow, the  local friction factor is constant when the velocity profile is fully developed (and not changing).
For a short pipe, say L/D< about 60, with turbulence, the velociy profile is not fully developed. Hence, use of a Moody chart will not give a reliable friction factor.

For very long pipes, the effect of the length to obtain fully developed flow is generally neglected.

Further, if the flow is compressible, the velocity profile is changing, along with the local friction factor.

Regards

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