Talk:Hagen–Poiseuille equation

"Before we move further, we need to simplify this ugly equation. First, to get everything happening at the same point, we need to do a Taylor series expansion of the velocity gradient, keeping only the linear and quadratic terms (a standard mathematical trick)." This simplification is misleading and shouldn't be used. The way it's written makes it seems like there is some approximation going on, while in fact viscosity is a linear function of r, which mean all other higher order terms in the Taylor series are zero. However, the linear relationship isn't obvious without knowing the final result. Therefore, Taylor series expansion shouldn't be used at all. —Preceding unsigned comment added by 24.2.124.121 (talk) 22:19, 1 October 2009 (UTC)Reply

The compressible flow equation is only valid for the isothermal case and with the assumption that density differences between the ends of the pipe are not too large (a density difference of a factor of four gives a ~4% error in volume flow rate). I also beleive that it is only valid for an ideal gas. I've edited the section to indicate the assumptions made, but have not had time to provide full details. If I have time I will but if anyone else wishes to do so a full explanation is given at http://www.cheresources.com/compressible_flow.shtml (but note an error of a factor of 4 in equation 7).

192.171.146.5 (talk) 08:37, 7 August 2008 (UTC)Phil RosenbergReply

I think these articles should be merged. They are about the same thing.

Also, I think it should be "Hagen-Poiseuille flow" (as opposed to "Poiseuille's law")-- I think that is the better name.

1. Hagen derived it independently.
2. It is an equation and rarely is valid (read: useful) in real applications; so, not a "law" in my opinion-- c.f. laws of thermodynamics, law of gravity.

Nephron  T|C 11:44, 10 April 2008 (UTC)Reply

1. Merge Poiseuille's law into Hagen-Poiseuille flow for reasons stated above. Nephron  T|C 11:44, 10 April 2008 (UTC)Reply

Since there was no voiced opinion... I decided to merge, move and re-work.

What was originally here is now in Hagen-Poiseuille flow from the Navier-Stokes equations‎. Going forward it might make sense to split-out the derivation into a separate article and merge what is in Hagen-Poiseuille flow from the Navier-Stokes equations. Nephron  T|C 04:21, 20 April 2008 (UTC)Reply

The Poiseuille's law discussion page prior to the merging suggested some improvement and simplification in the notation and derivation http://en.wikipedia.org/wiki/Talk:Poiseuille%27s_law. It might be worth implementing this. —Preceding unsigned comment added by 216.164.50.50 (talk) 19:23, 26 July 2008 (UTC)Reply

This article needs a good edit to remove all the instances of "Let's" and other "how-to" or pedagogical language. It might also be improved by mentioning which practical methods of flow measurement might be used to determine flow (thermistors, etc.). Robert K S (talk) 18:33, 4 December 2008 (UTC)Reply

Pressure should have the symbol p (not P), as per IUPAC (see the entry for pressure in the Gold Book) and other standards. —DIV (138.194.12.32 (talk) 04:44, 9 April 2010 (UTC))Reply

The electrical circuits analogy section gives the idea that the resistence of wire is inversely proportional to the fourth power of the radius. However, this is not true. According to the Ohm's law, the electrical resistance of a uniform conductor is inversely proportional to the square of the radius. 187.42.126.59 (talk) 22:15, 17 December 2009 (UTC)Reply

Yes and I have a further question. I use as a rule of thumb that an extensive variable divided by an extensive variable yields an intensive variable, while the product of two extensive variable is extensive, the product of two intensive variables is intensive, and the product of an intensive variable and an extensive variable is extensive. In the Poiseuille equation:

${\displaystyle \Delta p={\frac {8\pi \mu LQ}{A^{2}}}}$ 

Δp, L, Q, and A are all extensive variables, while μ is intensive. The left hand side Δp is extensive, but the right hand side, being the product of the ratios Q/A and L/A times μ is intensive. Extensive=Intensive is wrong. Ohm's law does not have this problem. PAR (talk) 05:19, 16 July 2022 (UTC)Reply

Hi, someone recently removed Q from the equation for resistance. I had previously assumed that "resistance" is a fluid dynamics term for pressure loss, in which case Q should still be there. However, if "resistance" is defined by analogy with electrical resistance it should be pressure loss per flow rate, in which case the removal of Q was correct.

In any case, I think it is confusing to use resistance - it isn't nearly as widely used and understood as in electronics. Better to be explicit in this case I think. I think my edit removed the section about tapered pipes - feel free to add it back! 87.194.20.81 (talk) 13:13, 22 June 2013 (UTC)Reply

I propose that Hagen–Poiseuille flow from the Navier–Stokes equations be merged into Hagen–Poiseuille equation. I think the current derivation in Hagen–Poiseuille equation is basically re-deriving Navier-Stokes for this particular problem from the first principle, which is unnecessary. This derivation can be removed and a derivation from the Navier-Stokes can be considered, which is shorter and less complicated and more relevant. — Preceding unsigned comment added by Prajaman (talk • contribs) 21:56, 2 April 2017 (UTC)Reply

For a gas line with pressure squared drop, is there a page describing the governing equations?--86.12.160.194 (talk) 10:16, 18 October 2018 (UTC)Reply

When the Reynolds number is introduced at the beginning of the article the symbol d is not defined: is it the diameter of the pipe? Reynolds number is extremely useful but there are several definitions where the typical length varies such as the hydraulic radius, the hydraulic diameter, the pipe diameter, not to mention the definitions for sedimentology and grain entrainment. Personally, I do appreciate the hidden paragraph about te derivation of the Poiseille law. — Preceding unsigned comment added by Dinobito (talk • contribs) 13:59, 11 July 2020 (UTC)Reply

The article says currently:

[...]the flow is bounded by Bernoulli's principle, under less restrictive conditions, by
${\displaystyle \Delta p={\frac {1}{2}}\rho v_{\text{max}}^{2}={\frac {1}{2}}\rho \left({\frac {Q_{\max }{}}{\pi R^{2}}}\right)^{2}\,\,\,\rightarrow \,\,\,Q_{\max }{}=\pi R^{2}{\sqrt {\frac {2\Delta p}{\rho }}}}$ 
[...]

Admittedly ${\displaystyle v_{\text{max}}}$  is not defined explicitly, but the implication of the notation is that it should be a local maximum. However, I suspect that what is actually intended is that it is the area-average maximum. That conclusion follows by simplifying the given equation to find
${\displaystyle v_{\text{max}}=\left({\frac {Q_{\max }{}}{\pi R^{2}}}\right)}$ .

Therefore it would be better to use a notation like ether ${\displaystyle {\overline {v}}_{\text{max}}}$  or ${\displaystyle \left(v_{\text{area-ave.}}\right)_{\text{max}}}$ . And either way the notation should ideally be explicitly defined.

—DIV (137.111.13.4 (talk) 08:01, 26 May 2021 (UTC))Reply

At the first Equation the article says:

μ is the dynamic viscosity,

Clicking this "dynamic viscosity" brings me to the Kinematic viscosity section just below the actual Dynamic viscosity section. I am unable to locate or identify where this misdirection is happening. 212.233.46.140 (talk) 09:37, 4 August 2023 (UTC)Reply