# Talk:Fluid dynamics

Active discussions

## Fluid mechanics vs. fluid dynamics vs. continuum mechanics

I don't like the definition: "Fluid mechanics is a branch of continuum mechanics." Continuum fluid mechanics is only a subset of the entire field of fluid mechanics. Statistical mechanics, in which a fluid is not assumed to be a continuum, is even mentioned later in the paragraph. A dictionary defines fluid mechanics as: "a branch of mechanics dealing with the properties of liquids and gases." I think that in an encyclopedia this should described in further detail, maybe: "a branch of physics, or mechanics, dealing with the properties of liquids and gases, specifically with stationary fluids, hydrostatics, and fluids in motion, fluid dynamics. The study of fluid mechanics is dependant on the principles of conservation of mass (see continuity equation) and conservation of momentum (see Navier-Stokes equations,) and often relies on the study of thermodynamics. The object of the study of fluid mechanics is most often to calculate the forces exerted by a fluid flow (aerodynamics), to calculate the mass flow rate at a certain region within a fluid flow (pipes and plumbing), to describe the motion of a solid within a fluid (such as boat or an airplane wing,) to describe the noise generated by or propagating through a fluid, or even to describe the properties of a fluid." I would change the article myself, but I feel such a major change should be discussed. --Sean

Better, Thanks. -Sean

To me, fluid mechanics (or dynamics if you prefer) is a subset of continuum mechanics (which also includes elasticity). So, there'll be an inconsistency at one level or another no matter what you do. I don't have a strong opinion, otherwise. -- hike395 21:37, 31 Dec 2003 (UTC)

I think the problem is in the way of defining the concept of "fluid". If you define "fluid" as "liquids and gases" you're in troubles. If you define "fluid" as a "continuum which is perfectly deformable..." Things are clear: liquids and gases are phases of matter and are studied by statistical mechanics; fluids are continuos media and are studied by fluid mechanics or dynamics (this alternative remains) --crodrigue1 29 Jun 2004

This is a GOOD idea, we could say something like "continuum which is perfectly deformable...", e.g. liquid and gases in usual conditions to be precise and clear to everybody. --achab 14:05, 15 November 2005 (UTC)
To be precise, the definition of a fluid is a substance which deforms continuously under the action of a shear force, however small. For clarity, both definitions could probably be given. -EndingPop 21:10, 28 February 2006 (UTC)
The term fluid dynamics is actually a subset of fluid mechanics. Whereas dynamics relates to motion, mechanics is a broader term that includes both dynamics and statics. So hydrostatics is not actually a subset of fluid dynamics, but it is a subset of fluid mechanics. Moreover, the course actually taught to undergraduate engineers is generally called fluid mechanics. Therefore, I'd like to change back the title. Unfortunately, Wikipedia is not letting me do so. Any suggestions, apart from involving an admin?COGDEN 02:46, Nov 5, 2004 (UTC)

Actually, I created a separate fluid mechanics page.COGDEN 20:52, Nov 13, 2004 (UTC)

I meant have Fluid Mechanics redirect to fluid mechanics instead of fluid dynamics - Reubot

Alright I've changed the redirect, also COGDEN, you might want to have a look at this Wikipedia:WikiProject Fluid dynamics/Top_draft - Reubot

## Fluidics

Michael Hardy suggested: The material at fluidics should be incorporated into this article, and then that page should be redirected to this one.

About incorporation of Fluidics into Fluid dynamics: it is hardly advisable IMO. Modern usage of the term fluidics indicates that it differs from Fluid dynamics like, say electronics from Electromagnetism: fluidics is about control devices based on fluid dynamics. Mikkalai 19:43, 7 Sep 2004 (UTC)

Agreed. A more appropriate solution is to expand the stub entry on fluidics into a complete standalone article. Kaszeta 20:36, 7 Sep 2004 (UTC)

## Continuity?

Any particular reason that the continuum assumption is being called "the continuity assumption"? I've always heard it referred to as a "continuum assumption" or "continuum approximation."

Continuity refers to the conservation of mass control volume equation, so it should not be used to mean a continuum specifically so we can avoid ambiguity. - EndingPop

## Compressible vs incompressible flow

It's much better now. Thanks. Roger Jeurissen.

I read "where D / Dt is the substantial derivative, which is the sum of local and convective derivatives" but Material derivative page states "substantial derivative" = "convective derivative." Personally, I like "total derivative" (as opposed to a partial derivative) but perhaps this is too mathematically oriented. Lots of legacy language to sort thru on first reading. IDave2 (talk) 05:30, 27 May 2009 (UTC)

I tried to sort out the iw links in fluid dynamics and fluid mechanics. It's very confusing because some wikipedias have only one article for both concepts, while others have separate articles. Someone could clear this up... -Easlak 08:11, 10 November 2005 (UTC)

## help

ok i need help which shape best travels through water?

a Streamlined shape.

## Where are the governing equations?

Is there any page regarding governing equations of fluid dynamics (continuity, momentum, energy)? I would love to add some basic definitions if there is none. Cheers, MrMo 10:59, 30 December 2005 (UTC)

There's a whole section on the governing equations, which points to Navier-Stokes equations -- Kaszeta 14:01, 30 December 2005 (UTC)
I suspect the Navier-Stokes equations are what most people will be looking for, but they only provide the three momentum equations. There is a page for the continuity equation in fluid dynamics (conservation of mass) that should probably be linked to somewhere on this page as well. I have been unable to find a page for the energy equation for fluid dynamics, but it should probably be included in this article somehow. 134.117.72.229 (talk) 19:43, 12 August 2010 (UTC)

## Citation of "Pope, 2000"

Could someone who knows the reference matieral please properly cite this source. I can't find any materials published by (I'm assuming Alan) Pope in 2000. The closest I can find is "Low-speed wind tunnel testing" from 1999. --Ig0r 22:11, 21 August 2006 (UTC)

It seems most likely that "turbulent flows" by Stephen B. Pope, published in 2000 by cambridge university press, is the cited reference. I've not combed the book to search for this specific remark, but on page 357, in the chapter on DNS, I found the following sentence:"Computer times are typically on the order 200 h on a supercomputer, and then only flows with low or moderate Reynolds numbers can be simulated." This, together with the observation that most macroscopic aerodynamic or hydrodynamic flows are of high Reynolds number, supports the statement. I didn't write this part, so I cannot guarantee that this exact statement is not in the book. I've changed the citation, but not the quote. Roger Jeurissen.—Preceding unsigned comment added by 86.82.146.207 (talk) 16:38, 29 July 2007

## Other fundamental engineering topics

Why is there a separate section for Fluid dynamics#Other fundamental engineering topics? It would be better to make a template for those topics. -- Myth (Talk) 05:42, 9 March 2007 (UTC)

Pretty much everything in this section seems to be factually inaccurate, so I think it could do with a complete rewrite.

I'm not sure this is the place to discuss streamlines. Is there somewhere better to define them and also the related concepts of steaklines and pathlines? Also, it would probably be better to separate the issues of time-dependence and irrotationality too. The significance of irrotationality (i.e. following from irrotational initial conditions) should probably be explained with reference to Kelvin's circulation theorem.

-- Rjw62 21:28, 18 July 2007 (UTC)

I've rewritten it completely, but left the original text commented out. Streamlines, potential flow etc. are useful concepts that deserve mention, but they have no relation to steadiness of the flow in particular. They should be described elsewhere. This part still needs editing. Roger Jeurissen.

That's much better :-). Streamlines, streaklines and pathlines are already covered in their own article and linked from this one at the bottom. A new section for potential / irrotational flow would probably be worthwhile though (at the same level as the steady/unsteady section). -- Rjw62 18:59, 30 July 2007 (UTC)

## Repetition in Other approximations

The section on Stokes flow seems almost identical to part of "Viscous and Inviscid flow" further up the page but going into more depth. Is this intentional or would it be better to write these two sections together? -- Estantia

I agree that this section seems to be factually incorrect. Steady flow has little to do with streamlines or turbulence. The only important consideration is that the system does not change with time.

## Hydrodynamics merge proposal

Since hydrodynamics is only a list of people, references and links, without its own description of topics typical for hydrodynamics, I propose to redirect it to here (that is Fluid dynamics). I already included some of the references. Also the figure and the link to magnetohydrodynamics can be moved here. The part with the list of historic contributions can be added in this Talk page, for the case somebody is interested to start a History section. Kraaiennest (talk) 23:41, 17 February 2008 (UTC)

In my opinion it is better to keep a separte page for hydrodynamics. Although hydrodynamics is a subset of fluid dynamics, it is a major topic by itself. I agree with you that the present article mainly consists of list of people who contributed the developments of hydrodynamics. But it can be cleaned up to meet wikipedia standards. May be magnetohydrodynamics can merge with hydrodynamics.--Salih (talk) 12:57, 14 March 2008 (UTC)
Can you specify why hydrodynamics is a "major topic by itself". As far as I can see, it is mostly an old synonym for fluid dynamics, or a synonym for fluid dynamics applied to water motion. E.g. Lamb's Hydrodynamics (ISBN 0521458684), from 1892, does not only deal with water or liquids, but also viscous flows, air flows and sound waves (in air).
It has not been possible to convert the hydrodynamics article into a good article, since it's creation in 2001. Also no attempts have been made since the merge-tag was attached to it. Neither has there been, up to now, discussion on the proposed merger. To my opinion, this is because it is mainly a synonym for fluid dynamics (if you wish, applied to liquids or water).
Further, a merger of hydrodynamics into magnetohydrodynamics does not seem logical to me. Magnetohydrodynamics studies the interaction between fluid dynamics and electromagnetism, in salt water, plasma's and liquid metals. While hydrodynamics is a synonym of fluid dynamics (eventually: applied to water or other liquids). Searching for hydrodynamics, I would not expect to be redirected to magnetohydrodynamics.
Crowsnest (talk) 16:22, 14 March 2008 (UTC)
The ideas from hydrodynamics (study of ideal fluid flows) are still used in boundary layer theory (determination of pressure field outside the boundary layer) to match the inviscid solution and viscous solution at the edge of the boundary layer. However, since there are no objections from other editors apart from mine, you may proceed with the merger proposal. --Salih (talk) 13:15, 15 March 2008 (UTC)

I merged hydrodynamics into this article, and changed it into a redirect. The old text of hydrodynamics, containing interesting information on its history, can be found here. Crowsnest (talk) 00:35, 24 March 2008 (UTC)

## What to do with Hydromechanics?

There also exists the article Hydromechanics, seemingly mainly a copy of the Encyclopædia Britannica (1911) article. In those days, fluid mechanics was called hydromechanics. It gives a nice background on the history of fluid dynamics until 1900. Any suggestions what to do with this? Leave as it is, or transform into an article on the History of fluid dynamics, merge into a History section over here, delete and change into a redirect? Crowsnest (talk) 23:22, 10 June 2008 (UTC)

• Hi Crowsnest. I am in favour of transforming it into a sub-article on the history of fluid dynamics, posting it into Fluid dynamics, and changing Hydromechanics into a redirect. Dolphin51 (talk) 02:52, 11 June 2008 (UTC)

## Anyone know about single-phase vs poly-phase flows?

Perhaps some chemical engineer should put that in. Commutator (talk) 06:53, 11 September 2008 (UTC)

I am in the middle of a gedanken experiment and need to find an article about fluid behaviour i came across earlier once.

But alas for the life of me, i can't remember the name of the principle involved. I thought this would be the perfect place to look but no luck.

It involved the behaviour of a small stream water, say coming from a restricted tap, following the contours of the back of a spoon when held in it's path. As you can see, i can't for the life of me work out how the heck to phrase that into a question the wiki search engine would recognise. I do remember though that the principle had a name associated with it's discoverer.

As i said before. Sorry to bother everyone, especially as this is a talk page, but i didn't know where else to go.

As a princess with Danish for hair once said " ...you're my only hope!"

Outofthewoods (talk) 21:35, 26 January 2009 (UTC)

I think you will find the information you want at Coanda effect. Dolphin51 (talk) 22:01, 26 January 2009 (UTC)

## Would it really do that?

Sorry if this is a bit lame, but this question has bothered me for a while, and I don't have the skill to answer it myself.

In the opening credits of Star Trek Voyager (I know, but try to keep reading anyway), there is a scene that shows the ship passing through, presumably, a nebula. YouTube has the opening credits if you want to look.

Anyway, the ship causes some nice vortexes, very similar to the Rayleigh–Taylor image in the article.

My question: would it really?

A nebula is, well, nebulous. Is it considered a fluid? I visualize it as sparse particles in a vacuum. At what point are there enough particle available that they interact and form those swirls. I think that you'd really see a perfect wake - a 'V' or cone spreading from the ship.

Where is the line between "dust particle scattered about in a near-vacuum" and "fluid"?

Any ideas? Thanks in advance. 208.38.59.162 (talk) 17:50, 13 January 2010 (UTC)

Questions of this kind are best posted at WP:Reference desk/Science.
I agree it is unlikely a nebula would behave like a fluid. Widely-distributed dust particles behave like individual bodies, not like a fluid continuum. I think it is more likely that, as a body (space vehicle) enters the nebula at very high relative speed, it would strike those particles in its direct path and propel them forward, a bit like a racing car leaving the track at high speed and entering a safety barrier made of dunces hats or used tyres. The particles that are not in the direct path of the body would be unaffected by the passing of the vehicle - they wouldn't move forwards, backwards or sidewards because there is nothing to transmit any force across the void between body and particle. Even if the body misses a particle by only one millimetre the particle will not move (unless it collides with another particle propelled forward by a collision with the body.) Dolphin51 (talk) 22:28, 13 January 2010 (UTC)

## Replace bong with more "academic" example

I undid someone's edit that deleted a section that is scientifically accurate, but contains an example (a bong) that is somewhat distracting from the overall mood of the page. In the edit summary for the undo, I ask for a different image that illustrates the same principle.

I don't with to hassle with providing fair-use justification for a borrowed image, and have not yet had the chance to create a replacement of my own, but if someone knows of a free or GPL'd example of more or less the exact same thing (gas purification by bubbling through a liquid column) used in industry, the petroleum industry comes to mind, that would be ideal for the article, I think. Thanks! 70.162.59.107 (talk) 19:46, 27 February 2010 (UTC)

Yes the image of bong spoils the overall mood of the page. More than that, it is inappropriate to include an image, which illustrates only a multiphase flow situation, in a section on "Compressible vs incompressible flow". In fluid dynamics, compressible flow has only one meaning that is always defined in terms of Mach number of the flow. Considering these factors I feel it is better to remove the image of bong from the article. 06:16, 28 February 2010 (UTC)
Thats not exactly true overall, although I can see how you could make this point for specific applications that do not involve any liquid interface. Here is an example of what I am talking about.. An understanding of both the fluid dynamics of both compressible and incompressible flows is necessary to model the interaction between hydrogen gas under pressure being bubbled through a heated unsaturated hydrocarbon for the purposes of conversion of it to a saturated hydrocarbon. Fluid dynamics will play a direct role in modeling this because the surface area of hydrogen gas exposed to the fluid has a direct relationship with the reaction speed achieved in the reactor. (I presume surface area exposed also has an effect on water filtration as well.) What I would like to see replace the current image (a bong used to filter smoke) is a free image that illustrates this at least as graphically. I've been looking for one, but haven't found it yet. Zaphraud (talk) 01:44, 1 March 2010 (UTC)
I agree, but my point is that the image does not go well with the content of the section in which the image is displayed. 13:44, 2 March 2010 (UTC)

## Too technical too quick

There's a great graphic showing an aerodynamic shape and explaing how it works. However it doesn't say, simply, WHAT that shape is optimal. The article doesn't state simply and early why a pointy bit at the front isn't the best shape. Can anyone help explain this? LookingGlass (talk) 16:31, 9 November 2013 (UTC)

## Use of closed-volume integral signs for continuity equations

I find the use of   confusing because, being a volume integral, there's no higher dimensional volume which it could "wrap around" and enclose, unless time were considered that higher dimension (even then, this would not be a "closed volume", since there's no four-dimensional volume enclosed by that volume). The continuity equation article uses the normal ${\displaystyle \iiint }$  anyways, so I'd think it would be a good idea to use the loop-less sign here as well.--Jasper Deng (talk) 23:34, 16 November 2013 (UTC)

Agree that the loop symbol is unnecessary here. We can get rid of it and have only loop-less integral sign. 07:00, 17 November 2013 (UTC)
I boldly carried out the replacement because of the lack of objection.--Jasper Deng (talk) 10:51, 18 November 2013 (UTC)

## "Constant" viscosity for Newtonian fluids

In the section "Inviscid vs Newtonian and non-Newtonian fluids," a sentence reads "These Newtonian fluids are modelled by a constant viscosity, depending only on the specific fluid." Would it not be more accurate to say that the viscosity of a Newtonian fluid is merely independent of strain rate? Even for Newtonian fluids, viscosity is often a strong function of temperature for example. — Preceding unsigned comment added by 192.28.0.15 (talk) 23:07, 19 January 2015 (UTC)

You are correct. While it is common, in my experience, to model Newtonian fluids with a constant viscosity, this is always done under the assumption of small temperature changes. I suspect this is where the confusion comes from. I've gone ahead and made the change, although I tried to maintain the idea that the viscosity primarily depends on the fluid. 209.87.255.222 (talk) 19:07, 12 May 2015 (UTC)

## Assessment comment

The comment(s) below were originally left at Talk:Fluid dynamics/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

Last edited at 14:32, 2 December 2006 (UTC). Substituted at 15:17, 29 April 2016 (UTC)

## Terminology in Fluid Dynamics

I cleaned up the part of this section dealing with compressible fluids a bit, but it occurs to me that this whole section is focused on stagnation and static properties. I think that it would be appropriate to turn the whole section as it is into a single discussion on stagnation, dynamic, and static properties, rather than repeating essentially the same thing for incompressible and compressible flows. Additionally, more terminology could be added - several of the sections above would be more appropriate for a "Terminology" section than for an "Equations" section. In fact, the last six sections of the "Equations of Fluid Dynamics" don't have a single equation. — Preceding unsigned comment added by 209.87.255.222 (talk) 22:17, 17 November 2016 (UTC)

## Request for comment: Plasma (physics)

There is a request for comment on the lede of Rfc Plasma (physics) that might interest fluid dynamics editors. Attic Salt (talk) 14:25, 14 October 2017 (UTC)

The article is also being considered for demotion from "good article" status: [1]. Please consider weighing in. Attic Salt (talk) 14:50, 20 October 2017 (UTC)