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-   -   confused with Darcy and Fanning (http://www.cfd-online.com/Forums/main/8548-confused-darcy-fanning.html)

 queram January 14, 2005 11:53

confused with Darcy and Fanning

hello guys, I am doing some research and have realized there`s nice turmoil in the definiton of pipe wall friction factor. One defines it as f=8*tau/rho/u^2, another one as f=2*tau/rho/u^2 (due to Fanning`s use of hydraulic radius in the equations of head loss instead of hydraulic diameter). That means, for gas in pipe, hydraulic radius is only half of geometrical radius, while, for the same gas in the same pipe, hydraulic diameter is the same as geometrical diameter. For me, this means Fanning`s statement is against common sense and there`s no reason to follow him. But, many follow him. Whehther f for laminar flow 16/Re or 64/Re is only the minor problem. Thousands of another equations for f that can be found in two versions with different coeffs....... a bit greater problem. But what really irritates me, is that two men are using the momentum equation of 1D unsteady compressible gas flow in the same form, with friction member defined in the same way (G=2/D*u*abs(u)*f) and one of them takes f according Fanning and the second one according to Darcy. Who the hell is right??

 Angen January 14, 2005 12:20

Re: confused with Darcy and Fanning

We have no choice but live with that. There is an article by Churchil in Chem.Eng.Prog.(~1960 or so) that makes some order with all these definitions.

Angen

 Rami January 17, 2005 04:04

Re: confused with Darcy and Fanning

queram,

I think it is merely a matter of nondimensionalization. The dimensional momentum equation is <html>

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If you consistently non-dimensionalize it, you should get the appropriate nondimensional stress terms and then compare them to those resulting from the wall friction factors (which themselves are merely non-dimensional stress factors).

I hope this will clarify your doubts,

Rami

 queram January 17, 2005 11:34

Re: confused with Darcy and Fanning

well guys... as I`m looking at it again and again... where do you think the problem itself originates: 1. in Fanning`s use of hydraulic radius as characteristic dimension or, 2. confusion in the use of terms "friction factor" (related to pressure drop, also called head loss coeff.) and "friction coeff" (related surface drag / surface roughness)

 queram January 28, 2005 04:11

Re: confused with Darcy and Fanning

well, I`ll put the self-answer: the main problem lies in the misuse of terms "friction factor" and "friction coefficient"; both are usually denoted as f, Cf or lambda, one is related to aerodynamic drag of the wall surface roughness, while the another one is related to pressure loss due to friction. Therefore, I suggest to use the clear and unambigious term when talking about friction: you mean friction? talk/write about friction coefficient, sucinitel trenia. you mean pressure loss? talk/write about friction loss coeff, head loss coeff or sucinitel tlakovej straty

 mike February 5, 2005 16:26

Re: confused with Darcy and Fanning