Talk:Darcy-Weisbach equation

From Wikipedia, the free encyclopedia

I am trying to compare the head lost due to friction value by using the Darcy-Weisbach Equation and the Hazen-Williams Equation,however, I cant get an equivalent value, the 2 value difference a lot, 1 is 1.2 bar (Darcy-Weisbach Equation), the other is 0.5 bar! How could this happened? Which will be more correct? Or are there any rules that stated which equation is suitable for which situation?

It appears to me that the Hazen-Williams equation is not dimensionally correct, unlike the Darcy-Weisbach equation. This means two things: one, the coefficient in the Hazen-Williams equation has units and cannot be directly compared to the dimensionless Darcy friction factor; two, the Hazen-Williams equation is probably only useful for a particular range of Reynolds numbers. Miguel (talk) 13:44, 15 March 2008 (UTC)

Contents

[edit] Range of use

Can this equation be used for both laminar and turbulant flow, or is there a particular range of renoylds number that this equations works for. for instance, what happens in low velocity flow with high pressure, can the head loss still be calculated with this equation, or is there another method? - 68.107.105.71 17:58, 6 November 2006 (was unsigned until April 2008)

solution:ya..this equation can be used for both turbulent and viscous(laminar)flow.the only change is that in the formula,value of co-eff of friction changes in each case.
for viscous flow, f=16/(reynolds no.)
for turbulent,f=.079/(reynolds no.)to the power of (1/4) - 59.92.247.111 15:25, 6 April 2007 (was unsigned until April 2008)

I don't know who wrote the above, but here's a comment:
For the Darcy-Weisbach friction factor for laminar flow is: f=64/Re, and for the Fanning friction factor, it is: f=16/Re.
I would consider "viscous" flow a poor choice of terminology for "laminar" because viscous refers to viscosity, not the velocity profile of the flow (e.g. smoke curls rising from a cigarette in still air are laminar at the bottom, and turbulent at the top--but the viscocity of the air has not changed).Mas-wiki 20:51, 30 July 2007 (UTC)

[edit] Merge Darcy friction factor

Seems like a reasonable merge to me. +mwtoews 22:42, 1 July 2007 (UTC)

Seems reasonable to me, too.Mas-wiki 20:52, 30 July 2007 (UTC)

Now merged. No discussion to merge. Miguel (talk) 12:19, 15 March 2008 (UTC)

[edit] Head loss form: American?

An anonymous user added "(American)" to the "head loss form" header. What does that mean? That engineers not trained in the US or influenced by US conventions don't use the head loss form? Miguel (talk) 19:52, 1 April 2008 (UTC)

Maybe this is related to the statement at the end of the next section: "The use of different symbols for the same numerical coefficient depending on whether head loss or pressure is considered is a historical accident due to different conventions being used by different communities of scientists and engineers."
I am not familiar with λ being used for f. Who are the "different communities of scientists and engineers" referred to in the statement quoted above? -Ac44ck (talk) 16:07, 7 April 2008 (UTC)
You can see λ used for the "D'Arcy-Weisbach friction coefficient" here.
Does the f stand for friction, or for Fanning?
I wrote that line about λ and f boiling down to historical accident after finding a large number of sources using either symbol. It is a common occurrence that engineers and physicists will use different symbols for the same equation and usage propagates by people using the symbols used by whoever taught them. In the case at hand, engineers are more likely to use the head loss form, and physicists the pressure form. The fact that someone felt compelled to call the head loss form with f an "American" form of the equation also supports the idea that there are issues of propagation of notation within subcultures. Miguel (talk) 16:05, 24 May 2008 (UTC)

[edit] Formula instead of slope

The previous Confusion with the Fanning friction factor section said: "... the slope of the linear relation between the friction factor and the inverse of the Reynolds number in the limit of small Reynolds numbers. If the slope is 16/Re, ..."

  • The first sentence seemed convoluted: "linear relation between the ... and the inverse". Saying "inversely proportional" would be more compact, but there is also:
  • The "slopes" are 16 or 64. But "16/Re" is the entire right-hand side of a formula.

-Ac44ck (talk) 20:12, 7 April 2008 (UTC)

[edit] removed Blasius etc.

Some good info in the last few edits. The placement for some of it might be better elsewhere.

The given Blasius formula is for the Fanning friction factor. The Darcy friction factor would be a better fit in this article. But friction factor formulae are compiled elsewhere. And the formula given seems to use slightly different values from the one here.

The section which focuses on distinguishing between the Darcy and Fanning friction factors assumes that the reader is familiar enough with at least one of them (Darcy or Fanning) to identify the laminar friction factor line in a Moody diagram.

Discussing the limiting Reynolds number for laminar flow, and the distinctions between various friction factors may be more fitting here.

The procedure in the "Confusion with ..." section works for any location on the laminar friction factor line where the Reynolds number is an integral power of ten. The line is usually plotted only in the laminar flow region, but it can be extrapolated to any convenient Reynolds number and the given procedure still works.

Moved the content of these sentences from the "Confusion with ..." a new section in the Fanning friction factor article: It should be noted that f=16/Re is the friction factor for flow in round tubes. For a square channel this becomes 14.227/Re for example.

A Moody diagram assumes (round) pipe flow. That the friction factor is defined for round conduits might be stressed in a section other than the one which is intended to heighten awareness about the distinction between the Darcy and Fanning friction factors.

The square-channel friction factor is interesting. It appears to be a Fanning friction factor. I moved it to a new section of the article on the Fanning friction factor.

Unhappily, a version of the shear-stress formula in the Fanning friction factor article is missing from the Darcy-Weisbach equation article. It could round out the Derivation section of this article. - Ac44ck (talk) 21:52, 9 April 2008 (UTC)