Talk:Fanning friction factor

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[edit] Placement of factor 2

http://en.wikipedia.org/w/index.php?title=Talk:Fanning_friction_factor&action=edit&section=1 I changed the equation to match the above source the two was on top when it would be on the bottom - 19:42, 3 April 2008 Orfintain

I reverted this change, since if I remember correctly, the formula with the Darcy (aka Moody) friction factor has the 2 on the bottom, while the formula with the Fanning friction factor has the 2 on the top. Bluap (talk) 19:58, 3 April 2008 (UTC)

The reversion agrees with a simplified version of the formula found here:

http://www.eng-tips.com/faqs.cfm?fid=1208

-Ac44ck (talk) 22:40, 3 April 2008 (UTC)

[edit] Factors 2 and g

g in the denominator must be replaced by g_c. g_c is for the conversion unit for various system.For SI system of units g_c is equal to 1.This details are as per the Mcabe and Smith's Unit Operation in Chemical Engineering.--Sakthi

Viscos friction head doesn't depend on acceleration due to gravity. g shall be removed from denominator and ρ (fluid density) placed in numerator.

Head does, indeed, depend upon the local acceleration of gravity. Refer to the simple version of the "manometer equation" in the example on page I-5 of this document:

http://www.cfdlab.ttu.edu/me3370/ChI.pdf

∆P = ρ g h

For a given pressure, the head will be higher on the moon (where there is less gravity) than the head on Earth.

I never liked the notion of g_c. If one pays attention to the units involved, it isn't needed. Realize that the lbf is defined using a mass unit of "slugs" and that 32.2 lbm = 1 slug, and all is well - no need for g_c, which is just a messy way of writing "1":

g_c = 32.2 ft lbm/(s^2 lbf) = 32.2 lbm/slug = 1

Perhaps g_c was an invention of pro-metric forces: dumb-down the users of English units (make them forget how the lbf is defined), introduce a complicating factor, and hope for the combination to exert pressure away from English units. -Ac44ck (talk) 03:43, 4 April 2008 (UTC)

The changes suggested above would convert the equation into an equation for pressure loss. For friction head, the "g" in the denominator is correct. I think that the "2" in the denominator is a mistake, however. Since the Fanning factor is 1/4 of the Darcy factor, the denominator in the friction head equation must be 1/4 of what it is in the Darcy equation. R is 1/4 of 2D. 2R is half of 2D.

The hydraulic radius of a pipe is 1/4 of its diameter. --Jagz 01:31, 15 March 2007 (UTC)

These are my suggestions: In order to have the friction head loss (hf) given with its correct dimension (metres, SI), the denominator would have to include the acceleration of gravity (g ~= 9,81 m / s^2 ). When using the hydraulic diameter approximation all one has to do is replace the D with 4Rh, keeping in mind that hydraulic diameter more often than not yields large variations in accuracy. JMJ 240707

[edit] g, not g_c

I changed the formula to use 'g' instead of 'g_c'.

The units for this factor must be those of acceleration (L/T^2) if the 'head' is to have units of length.

g_c in the English system is 32.2 ft lbm/(s^2 lbf).

But 1 lbf = 1 slug-ft/s^2

So g_c ultimately has units of lbm/slug – a ratio of masses, which does not have units of acceleration. -Ac44ck (talk) 22:49, 3 April 2008 (UTC)