Talk:Darcy–Weisbach equation

Equivalence to Hazen-Williams equation

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 can`t get equivalence, the 2 values differ allot, 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?— Preceding unsigned comment added by 60.49.7.1 (talk • contribs) 08:15, May 24, 2006‎ (UTC)

It appears to me that the Hazen-Williams equation is not dimensionally correct, unlike the Darcy-Weisbach equation. This means two things:

1. the coefficient in the Hazen-Williams equation has units and cannot be directly compared to the dimensionless Darcy friction factor; and,
2. the Hazen-Williams equation is probably only useful for a particular range of Reynolds numbers.
   —Miguel (talk) 13:44, 15 March 2008 (UTC)

You may check the drop in pressure coefficient for an equation to find out if one is absolute pressure and others dynamic. I use the formula of Hencky to compare deformations of small volumes.

$\ln \left({\frac {p\inf {2}}{p\inf {1}}}\right)$ I suspect one pressure to lead to a out of the real space, infinity and second one to $\ln \left({\frac {p\inf {2}}{p\inf {1}}}\right)=\ln({\frac {\delta \cdot {p}}{1.013}})=\ln \delta \cdot {p}-\ln 1.013\cdot {f(dB)}$ , where p_atm is the pressure in air, 1.013atm , and f(dB) must be one for any EPS

188.25.48.61 (talk) 14:41, 1 August 2011 (UTC)

Quote for f

From the navier-stolkes equation there is the energz conservation:

${\frac {d{r}}{dt}}+U{\frac {du}{dt}}+V{\frac {dv}{dt}}={\frac {p}{\rho }}+f+4\mu \nabla {\frac {v^{2}}{2}}$ so f is force and none f-factor. I tagged f as unknown, hope to prove it as head looss in pressuse in the darcy eq. Since then please leave f to disambuation—188.25.48.61 (talk) 14:23, 1 August 2011 (UTC)

Anonymous butter-fingered typist, your <math> expressions are malformed gobbledygook, making it impossible to discern what point you were trying to make. — Quicksilver^{T @} 01:15, 27 May 2015 (UTC)

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?— Preceding unsigned comment added by 68.107.105.71 (talk • contribs) 17:58, 6 November 2006 (UTC)

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)— Preceding unsigned comment added by 59.92.247.111 (talk • contribs) 15:25, 6 April 2007 (UTC)

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)

As an answer to the first question, yes the Darcy–Weisbach equation is applicable to all flow types, turbulent, laminar, steady, and unsteady no matter the velocity or pressure distribution. As for the equation given for calculating the friction coefficient f for turbulent flow, it is completely incorrect or at least a very poor approximation and I don't know where it comes from. The factor f for turbulent flow is found using the implicitly defined Colebrooke–White transition formula or its close approximation the Swamee–Jain formula. An equally acceptable solution is to read the f value from the graphical solution to this formula the Moody diagram. —Preceding unsigned comment added by 68.238.133.228 (talk) 03:03, 5 January 2009 (UTC)

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)

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)

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)

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.

The Moody diagram is not only applicable to round conduits, it can be used for any arbitrarily shaped cross section. The difference in its use is that in the calculation of the Reynolds number and the relative roughness parameters one needs to use in place of diameter the quantity 4 times the hydraulic radius of the cross section where the hydraulic radius is defined as the cross sectional area divided by the wetted perimeter. —Preceding unsigned comment added by 64.126.190.120 (talk) 03:11, 5 January 2009 (UTC)

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)

Title

Should be Darcy–Weisbach (with en-dash), not Darcy-Weisbach (with hyphen). See WP:DASH. —DIV (128.250.80.15 (talk) 10:10, 20 August 2008 (UTC))

Phenomenological versus semi-empirical

I think that darcy-weisbach is better termed a "semi-empirical" equation than a phenomenological equation for two reasons. It contains a part that is solidly grounded in theory (the v^2/2g term, and to a lesser extent the L/D term) and the term "semi-empirical equation" is in more common use than "phenomenological equation"Mikejens (talk) 16:06, 12 November 2008 (UTC)

The Darcy–Weisbach equation is exact for laminar flow and can be derived theoretically. The formula may be extended to turbulent flow by varying the friction factor. The Colebrook-White equation for the turbulent friction factor has bases in experiment. Reading the article on Phenomenology_(science) confused me a bit. At this point, I am not sure that it is proper to call the Darcy–Weisbach equation itself either phenomenological or semi-empirical. Its application may yield a well-founded exact value or a less-well-founded approximation, depending on the how the friction factor was determined. -Ac44ck (talk) 19:23, 12 November 2008 (UTC)

Confusion with the Fanning friction factor

"... the Darcy–Weisbach factor is more commonly used by civil engineers, and the Fanning factor by chemical and mechanical engineers..." From where Do you take that? I am a mechanical / aeronautical engineer and I have never used the fanning factor. Hence this statement should be completely deleted! Ok? —Preceding unsigned comment added by 195.126.105.15 (talk) 09:27, 27 November 2008 (UTC)

I prefer things that are close to correct be fixed rather than be deleted completely. Throwing things away and making someone else recreate similar content from scratch doesn't seem like a helpful way to "edit".

There is an interesting discussion here:

http://biz2.yellowdotworks.net/files/yellowadmin/AcrAE.pdf

It says that chemical engineers use the Fanning friction factor; mechanicals and civils use the Darcy friction factor. It also sheds light on the use of $\lambda$ as a symbol for friction factor. It is for a "European friction factor" which is half the Fanning friction factor, so it doesn't seem that this article should be using $\lambda$ in the section for the "Pressure loss form".

The author's background is here:

http://biz2.yellowdotworks.net/content/background

He is a pHD chemical engineer and he claims that they use Fanning friction factor. -Ac44ck (talk) 01:40, 28 November 2008 (UTC)

The Darcy-Weisbach equation can be used equivalently with either the fanning friction factor or the Darcy Weisbach friction factor, however if the fanning factor is used the diameter D in the equation must be replaced with the hydraulic radius.74.60.57.253 (talk) 04:16, 4 November 2009 (UTC)

Wetted area

My first question is, why is this a link to hydraulic diameter? Second, I have a comment about the term wetted area, it is incorrectly used in this article. The average velocity cannot in general be obtained by dividing the flow rate by the wetted area. Instead one must correctly determine the effective flow area. While the two areas are often equivalent, there are cases where a obstruction is immediately downstream of the cross section, causing a blockage or lack of motivation of fluid in that region. The effective area is the unrestricted flow area. 75.167.129.226 (talk) 05:46, 27 December 2011 (UTC)

The presence of an obstruction immediately downstream of the cross section is contrary to the implied assumption of "fully developed flow." See the first sentence here:

Reynolds_number#Reynolds_number_in_pipe_friction - Ac44ck (talk) 03:59, 28 December 2011 (UTC)

Practical applications

In this section (right at the end), an alternative expression for h_f based on volumetric flow rate rather than velocity is derived. If my algebra is correct, the friction factor corresponding to the other coefficients in the formula would be the D-W one, but the symbol used is f rather than f_D. I leave it to better experts to fix if it is wrong (given the dire warnings about confusing the two). — Preceding unsigned comment added by 83.67.40.215 (talk) 07:49, 29 May 2012 (UTC)

Where is Figure 4?

Figure 4 is mentioned twice in the text, but is not visible. — Preceding unsigned comment added by Aubrey Jaffer (talk • contribs) 21:15, 16 February 2017 (UTC)

The text should have referred to "Figure 3"; I corrected it. ArthurOgawa (talk) 08:27, 15 November 2017 (UTC)

Do you mean "Roughness Reynolds number"?

The title of Fig. 3 states that the "Roughness Function B" is being plotted against the "Friction Reynolds number R*". I believe this latter should read "Roughness Reynolds number R*". In the text, the "Friction Reynolds number" is identified as Re.sqrt(fD), whereas the quantity R* is identified as the "Roughness Reynolds number". Moreover, B is identified as a function of the "Roughness Reynolds number", not the "Friction Reynolds number".