Talk:Static pressure/Archive 1

Notion of static pressure

This article on static pressure is rather sad. It is in serious need of technical improvement. The claim that static pressure only refers to the pressure in a fluid at rest is incorrect. Even odder is the claim that the pressure adjacent to an aircraft is only static pressure if the aircraft is at rest! (An aircraft flies in the atmosphere which can be considered at rest so it is very odd to suggest that the aircraft must be at rest too.)

In Aerodynamics by L J Clancy, (1975) Pitman Publishing Limited, London, it states on page 21: "To distinguish it from the dynamic and total pressures, the actual pressure of the fluid, which is associated not with its motion but with its state, is often referred to as the static pressure, but where the term pressure alone is used it refers to this static pressure."

I am unaware of any source which supports the notion that static pressure only refers to the pressure in a fluid remote from the influence of a solid body. Conversely, it is extremely common to see the following equation:

$total\;pressure=static\;pressure+dynamic\;pressure$

This equation has universal applicability, not only at points in the fluid that are remote from a solidy body.

At present the article strongly suggests the following relationship:

$total\;pressure-dynamic\;pressure=pressure$
This is odd because the terms total pressure and dynamic pressure are sub-sets of the generic and over-arching term "pressure". To have the generic term equal to the difference between two of its sub-sets is unique, and therefore to be avoided.

I am willing to re-work the article Static pressure to make it compatible with other sources, including other articles on Wikipedia.

If anyone disagrees with my thoughts above, please respond and, most importantly, quote a reputable source or sources for your ideas.
Dolphin51 08:22, 1 December 2007 (UTC)

A few points regarding the question pressure vs. static pressure:

– First, meanings: Static basically means still. Static pressure therefore is, by its very definition, the pressure of a fluid at rest. See for example Merriem-Webster on static ("exerting force by reason of weight alone without motion") and Dictionary.com ("acting by mere weight without producing motion: static pressure").

– Second, misuse of static: take an aircraft wing, drill a hole at random on the surface and attach a manometer underneath it. What you will measure, at any given moment, is simply the pressure of the fluid at that particular point, i.e, by definition, the force that the fluid exerts normally onto the surface, divided by the area of the hole (dF/dA). If the aircraft is stationary, say it's sitting on the ground, that's when, on the manometer, you will read the static pressure (which is the atmospheric pressure, in this case). When the aircraft is in flight, the air flows, relatively to the hole; how can the pressure exerted there be the static pressure? It is not, and in fact you'll get a significantly different reading on the manometer. To call that pressure, in the in-flight case, static pressure is a misuse of language. That quantity should be referred to simply as 'the pressure', full stop. It's what Clancy calls, in your quotation, "the actual pressure of the fluid, [...] often referred to as the static pressure", although he then adds to the confusion by saying that "where the term pressure alone is used it refers to this static pressure". It should be the other way round! "where the term static pressure is (improperly) used, it refers to the 'actual' pressure".

– Third, Bernoulli: the statement that "total pressure and dynamic pressure are sub-sets of the generic and over-arching term pressure'" makes no sense, I'm afraid. This is what the terms in the Bernoulli equations are about: the term p is simply 'the pressure', the only term that expresses the force per unit area in the fluid at any time, regardless of the fluid being static or in motion. The term $1/2\rho v^{2}$ is dimensionally a pressure, but it has nothing to do with any force exerted on any surface; it's not "the pressure due to the velocity", as sometimes it's wrongly described; it's just the product of density and square of velocity, which, if added to p, gives a quantity that remains constant. In turn, the term p_stagnation is again dimensionally a pressure and it's defined simply as the sum of the previous two (which, incidentally, is constant and equal to 'the pressure' p at a stagnation point). There are no pressures being 'subsets' of other pressures.

– Fourth, sources: you are right when you say that it is extremely common to see the Bernoulli equation always written with P_static without specification, but that doesn't make it more correct, or consistent with the definition of static. In fact this confusion and misuse is so widespread that you'll find it even in NASA documents: check out this page on Bernoulli equation and follow the hyperlink on the 'static pressure' text. It links to, guess what, 'Gas pressure', which is just 'the pressure', dF/dA, or $\rho RT$. So they say static pressure but what they really mean is just pressure. There are other sources, on the other hand, that adopt the correct definition of static pressure.

– Fifth, WP articles: as you've seen, not all articles on WP are consistent about the usage of static pressure and pressure, but many are, like Bernoulli's principle and Stagnation point (before your edits).

– Conclusion: I strongly believe that we should work to make all articles consistent with the correct meaning and use of the term static. Your post made me realize that this article needs some work itself, to expand and clarify such crucial difference.

Giuliopp 00:37, 4 December 2007 (UTC)

Notes

Giuliopp, many thanks for your prompt and thoughtful response. I will digest it in the next few days and respond. In the interim, I can make a couple of comments for you to think about.
1. On 22 February you inserted the following statement "The term 'static pressure' ... refers strictly to the pressure in the fluid far upstream of any object immersed into it." This is inconsistent with the notion that static pressure is reserved for a fluid at rest. What if the fluid far upstream of an immersed object is not at rest? (Your insertion on 22 February did not give any reference to substantiate the statement.) My primary difficulty with the article Static pressure, as it presently stands, is the stark contradiction of the two statements - static pressure relates to a fluid at rest; and static pressure is strictly the pressure far upstream of any immersed object. What are readers supposed to make of this contradiction?
2. I wrote "total pressure and dynamic pressure are sub-sets of the generic and over-arching term pressure", and you wrote "it makes no sense". Let me explain. Pressure is simply force divided by area. It is a general concept with very wide application. In many different applications it is desirable to identify pressure but with a specialised meaning, so we have such pressures as:

• blood pressure
• hydraulic pressure
• osmotic pressure
• hydrostatic pressure
• stagnation pressure
• impact pressure
• dynamic pressure
• static pressure

They must all be compatible with the definition of pressure, but each one has a more precise meaning that distinguishes it from the others. By using an adjective to qualify the word "pressure", ambiguity is avoided. Dolphin51 04:30, 4 December 2007 (UTC)

I see what you mean in point 1 (and also 2). The thing is that the use of the expression static pressure in fluid dynamics is bound to create ambiguity, and probably should be avoided altogether. I agree that the paragraph you quoted isn't clear enough right now. I'll come back to it, possibly by the end of this week.

Giuliopp (talk) 23:52, 4 December 2007 (UTC)

Hi Giuliopp. I have examined your thoughts carefully. I think I now understand your point of view. I can make the following comments.

1. Many people, including many authors, use the expression “static pressure” in relation to aircraft, fluid dynamics and in other contexts. It is the mission of Wikipedia to provide an explanation of what these people mean. I think your view of static pressure is entirely sound when considered in isolation, but it isn’t consistent with long-term useage of the expression. It isn’t the mission of Wikipedia to develop a new explanation of static pressure that is rigorously correct but inconsistent with the common useage.
2. One of the common applications of the expression “static pressure” is in aircraft design. Wikipedia contains information about the Pitot-static system that shows all aircraft have a static system for measuring static pressure in flight for the purpose of determining indicated airspeed and altitude. Your suggestion that the pressure measured on the surface of an aircraft is only static pressure if the aircraft is not moving is not consistent with the universal useage of “static pressure” to refer to the pressure in the static system of an aircraft.
3. I agree that, technically, there is no difference between pressure and static pressure. The expression “static pressure” is associated with the Bernoulli equation. When the gravitational potential term can be ignored, such as in gas dynamics, Bernoulli’s equation is frequently quoted as:
static pressure + dynamic pressure = total pressure
There are few word equations that can match the elegance of this one. The beauty of Bernoulli’s equation is that it is applicable everywhere in the field of flow, not just in the free-stream remote from obstacles, or where the fluid has come to rest. You can see the equation in exactly this form at Stagnation pressure. For many decades, authors have chosen this form of Bernoulli’s equation, rather than the following which you would probably prefer:
pressure + dynamic pressure = total pressure
4. Exactly why so many authors have historically used the expression “static pressure” (rather than “pressure”) in the Bernoulli equation is not known to me, but I do know that this is not the only situation where this occurs. A perfect example of a similar situation is “true airspeed”. True airspeed is identical to airspeed. The word “true” is redundant and could be omitted, but pilots rarely refer simply to “airspeed”. They almost always use “true airspeed” or tas. The reason is because pilots and others also refer to indicated airspeed, calibrated airspeed, equivalent airspeed, and other airspeeds. To refer only to “airspeed” is potentially ambiguous so it is rarely used in this context. It is the mission of Wikipedia to explain what people mean when they write “true airspeed”, not to reform the piloting fraternity so they stop using that expression and simply use “airspeed” instead.
5. Immediately above you have written that to use the expression “static pressure” in fluid dynamics is bound to create confusion. I see no evidence of confusion after a very long time. The oldest authoritative definition of “static pressure” I have been able to locate at short notice is from the British Standards Institution in 1950. BS 185: Part 1: 1950 Glossary of Aeronautical Terms includes the following:
4412 Static pressure. The pressure at a point on a body moving with the fluid.
4413 Total head. The sum of the dynamic and static pressures.
6. You referred me to a site within www.aerospaceweb.org, and you wrote that the site defines the static pressure as being the atmospheric pressure. That is not true. The sentence is “Aircraft also take another measurement of the atmospheric pressure that is called the static pressure.” That is not a definition of anything. Selecting "dynamic pressure" takes you to the following site: [1] At that site you will find the sentence: “In other words, no matter where you are in the flow, the static pressure plus the term ½(rho)V^2 is always the same!” There is no suggestion in this sentence that static pressure only applies in the remote free-stream. Nor is there any suggestion that static pressure is only applicable to an aircraft when the aircraft is not moving.
7. I have asked you for an authoritative reference to support the notion that the expression “static pressure” is reserved for a fluid at rest, or the pressure remote from an immersed object, but you haven’t yet provided one. Your only reference is to the word “static” in English language dictionaries. Dictionaries provide an explanation of the word “static” but they don’t provide any information about the expression “static pressure”. For an authoritative reference for the latter, a technical document of some sort is required.
8. If static pressure means the pressure remote from any immersed object, what purpose does it serve in fluid dynamics? Do you know of some principle, theorem or equation that has meaning only in the free-stream, remote from a solid object? I don't. In contrast to that, my view of static pressure is applicable in the Bernoulli equation because that equation has meaning everywhere in the field of flow (except boundary layer and shocks).

Conclusion. In the absence of authoritative information to the contrary, I am satisfied there is no reason for Wikipedia to publish information that “static pressure” is only meaningful at points in a fluid flow where the speed is zero; or at a location in the free-stream remote from any immersed object. Wikipedia should publish information that is consistent with other Wikipedia sites, and other literature on the subject. I think the article on “static pressure” should have one sub-section applicable to the useage of this expression in aircraft design, and another sub-section applicable to its useage in aerodynamics. I feel qualified to begin both.

Perhaps there should also be another sub-section dealing with useage of the expression in instrumentation technology and/or hydrodynamics. I will leave that to others much more knowledgeable in that field than me. Dolphin51 (talk) 04:57, 7 December 2007 (UTC)

I've largely re-written the article in a way that addresses many of the questions you raised. With regard to point 7, I think that the lexical definition of static cannot be dismissed as 'non technical therefore not authoritative'. Static means only one thing, and if static is applied to pressure, the resulting expression is equally unequivocal (and Dictionary.com even uses it as an example). If in aerospace engineering the term static pressure has been historically misused, that is a different matter. I think that WP should acknowledge the common usage of the expression (which I've tried to do in the new revision), but it doesn't necessarily have to endorse it, especially if it creates ambiguity and lacks logical rigor ("Static pressure: erhm... are we talking about the free-stream - atmospheric - pressure or rather the fluid pressure anywhere near a wing?"; ok, the meaning is usually clear from the context, but nevertheless the ambiguity does exist). I'll give you another example: in fluid dynamics, the expression total pressure is just as common as static pressure, but in WP the article about it is titled Stagnation pressure, less common but completely unambiguous.

As for point 2, I agree that the example wasn't properly qualified; I've re-written it, adding a footnote.

One final note, about the quotation in point 5 ("Static pressure. The pressure at a point on a body moving with the fluid"): I read it as consistent with the rigorous definition of static pressure. If a body moves with the fluid, to me it means that the body is not moving relatively to the fluid, i.e. to an observer sitting on the body, the fluid appears at rest and the pressure exerted on the body is the static pressure.

Giuliopp (talk) 00:29, 8 December 2007 (UTC)

Giuliopp

We are in complete agreement on at least one thing: “it is very common to refer to the non-velocity-dependent term of the Bernoulli equation as static pressure”. There is a significant number of authors in the field of fluid dynamics who use the expression static pressure to refer to the pressure generically at every point in the flow field. (Most of these authors also use the expressions static temperature and static density). It is the mission of WP to provide an explanation of what these authors mean by static pressure.

The WP article on Static pressure is still contradictory in that it says static pressure is:
1. the pressure exerted by a fluid at rest, and
2. the pressure in the free-stream (remote from an object immersed in the fluid).
I believe both these sentiments are incorrect and incapable of being substantiated. I have posted “Citation needed” flags. In the absence of adequate substantiation these statements can be deleted. That will clear the way for the entire article to be progressively improved.

You have provided explanations of the word “static” from English language dictionaries to support the notion that static pressure is the pressure in a fluid at rest. (Many readers will be very surprised at this notion because the pressure exerted by a fluid at rest is called stagnation pressure.)

Apart from a website within www.aerospaceweb.org you have provided nothing to support your notion that the expression “static pressure” is reserved for the pressure in the free-stream remote from any object immersed in the flow. You clearly hold the notion that static pressure should be reserved for pressure in the free-stream. From your various postings on the subject I infer that this notion is nothing more than your personal preference. You have provided nothing to indicate that anyone else holds the same notion. WP is not intended as a platform for individuals to broadcast their personal preferences or their original research.

Your most recent edit of Static pressure has done nothing to resolve the contradiction. It has helped make the simple concept of static pressure into something ridiculously complex, in my view. For example, if the pressure in the free-stream far upstream of an immersed object is P1, far downstream is P2, and far abeam the object is P3, what is the static pressure? Is it all of P1, P2 and P3, or is it just one of them? If Bernoulli’s equation is equally applicable to each and every point on a streamline, why is there value in distinguishing between the pressure at points where the streamline passes through the free-stream, and points where the streamline passes through a region affected by an immersed object? Do you really think clarity would be enhanced by using static pressure for the free-stream, and thermodynamic pressure (or some other expression) for the region affected by an immersed body? (There are many more anomalies than just these three.)

There is an incredibly simple way to eliminate these problems and improve the quality of WP. Acknowledge that there is a large number of authors who use static pressure (and static temperature and static density) as having meaning at each and every point in the flow field. They do so with complete consistency and great effectiveness. There are no grounds for suggesting that all these authors are incorrect. Let’s provide WP with an explanation that is consistent with the way these authors use the expression. Let’s leave our novel ideas, original thinking and personal preferences for some other web site. Dolphin51 (talk) 03:04, 10 December 2007 (UTC)

No, sorry, this is neither my personal preference nor original research. I'm sticking to basic definitions and logics. I don't see how you can deny that static pressure first and foremost means the pressure of a fluid at rest, when:

• Two English dictionaries (besides common sense) give an unequivocal definition.
• The author I already quoted, which you seemed to forget, so defines it.
• Even this very WP article contained such definition from the start.

Instead, you come up with some astonishing statements like "the pressure exerted by a fluid at rest is called stagnation pressure". What?! To my knowledge this is unheard of. Stagnation pressure is the pressure in a point of a generic flow field where the fluid velocity is zero (that is a stagnation point), which is quite different from a fluid at rest (= where the velocity is zero everywhere). Now it's you who has to provide sources to back that up.

Until we agree at least on this fundamental definition of static pressure, there's little point in carrying on discussing the case of static pressure for fluids in motion.

I'll post again on this thread with more references (as if they were needed) when I manage to pop to my university's library, but that may not happen until after xmas. Best regards. Giuliopp out.

Giuliopp (talk) 23:54, 11 December 2007 (UTC)

Thanks Giuliopp. Perhaps things are becoming a little clearer. I am talking exclusively fluid dynamics and aviation. I think you are also talking fluid statics. I have a vision for Static pressure having a number of sub-headings:

• Static pressure in aviation
• static pressure in fluid dynamics
• static pressure in fluid statics

I claim no expertise in fluid statics. I would have thought the term 'pressure' would be entirely adequate in fluid statics, but if civil engineers and others sometimes use the term 'static pressure' in relation to fluids entirely at rest I have no objection to this being explained on WP. For example "In fluid statics, in a body of fluid at rest, the term 'static pressure' may be used as an alternative to 'pressure'." I do claim some expertise in aviation and fluid dynamics. In your most recent posting you referred to:

• two English dictionaries that give an unequivocal definition. I agree they give an unequivocal definition of 'static'. They don't mention 'static pressure'.
• The book 'Process Control Instrumentation Technology' by Curtis D. Johnson. This is a website devoted to process control instrumentation, not fluid dynamics. The only explanation of 'static pressure' I can find on the website says "The statements made in the previous paragraph are explicitly true for a fluid that is not moving in space, that is not being pumped through pipes or flowing through a channel. The pressure in cases where no motion is occurring is referred to as static pressure." This may be a satisfactory explanation for the purpose of fluid statics. (It includes "where no motion is occurring".) I don't accept this as a definition of anything, and it certainly isn't a definition that is binding on fluid dynamicists and authors in the field of aviation who are only interested in fluid where motion is occurring.
• WP may well have begun with an explanation of 'static pressure' that was appropriate for fluid statics, but WP is not infallible. That is why we devote our time to improving it.

I note that you will be able to find a suitable definition of 'static pressure' when you can get access to your university's library. There are numerous good references pertinent to fluid dynamics in WP, and in books on your bookshelf, but I infer from your comment that you might be able to find one or two that match your view of 'static pressure'. You will then quote them as supremely authoritative. I notice you regard authors on this subject in fluid dynamics as "incorrect" if they regard 'static pressure' as having meaning at every point in the flow field. You even quoted the respected NASA website [2] and suggested it represented "confusion and misuse". If a hundred respected authors in the field of fluid dynamics hold a consistent view about 'static pressure', why put WP in the position of claiming they are in error? Finally, in fluid dynamics, stagnation pressure is definitely the term used for the pressure in a fluid at the point where the velocity is zero, fluid is at rest etc. (In fluid statics, the fluid is at rest everywhere and the pressure is called pressure or, as I am discovering, static pressure.) Good luck at the university library, but there really isn't a need to look too hard. There are high quality definitions on your bookshelf, and numerous references to 'static pressure' on WP. Let's keep those WP references consistent with each other. Regards Dolphin51 (talk) 05:32, 12 December 2007 (UTC)

Giuliopp, I have summarised my thoughts on the subject and posted them in the article Fluid dynamics. See my new sub-heading Terminology in fluid dynamics. Have I found any common ground? Dolphin51 (talk) 12:51, 12 December 2007 (UTC)

Notion of static pressure - 2

Everyone: I was asked to weigh in on this topic. The discussion is rather lengthy and I apologize if I missed someone's points. The article seems way too confusing to me. If the confusion is a problem caused by common usage in different fields and countries then this will be difficult to resolve. I have done aeronautics for 26 years and there has never been any confusion in my mind what static pressure is. If you take a pitot tube, static pressure is what you measure with port holes normal to the flow (such that the ram air does not enter them). It is called static to indicate that the ram air component (dynamic pressure half rho vsquared) does not come to play. Thus, you have static pressure for moving flows and freestream static pressure refers to the pressure of the undistrubed flow field. Thus, the static pressure can vary at all points around an airplane. Thus, the static referers to the fact that the value does not include the ram air components. In my opinion, this is entirely conisistent. But as has been pointed out to me, what makes sense to a person after doing it for 25 years is not necessarily clear or consistent to outsiders.12:25, 9 January 2008 (UTC) —Preceding unsigned comment added by Mangogirl2 (talk • contribs)

Noted. I will add my comments at the weekend.Giuliopp (talk) 00:07, 11 January 2008 (UTC)

Hi Giuliopp. Welcome back, and best wishes for 2008. I note your intention to add comments over the weekend.

On 10 December I posted two "Citation needed" flags in Static pressure. I have notionally given other Wikis a month to support the article with some authoritative references. I have a substantially re-worked version of the article on my computer, complete with high quality references, ready to go. I'm sure you will approve of it if I get to post it.

The month is up and so far nothing has been posted in the way of citations or references. I will wait over the weekend to read your comments, but please be aware that in my view the time for philosophical discussions has passed. By now you and I know each other's viewpoint very well. Static pressure is a poor article technically and it needs either some very, very persuasive citations, or a complete re-work. Dolphin51 (talk) 01:21, 11 January 2008 (UTC)

Dolphin51, you are entitled to think that this article is technically poor. The fact is that it is as rigorous as it can get, although at first it may come across as confusing. It's not 'philosophical'; it is an attempt to lay down an encyclopedic definition of static pressure, consistent throughout the whole of Fluid Mechanics. As Mangogirl2 says, it is a problem of common usage in different fields and the article points out the only way to use that expression without incurring in any ambiguity (by the way, you gave a brilliant example of correct usage here, while discussing the Kutta condition and repeatedly using fluid pressure instead of static pressure; so maybe there is common ground?). As of me, I'll contribute to WP when I find the right time, so you can dispense with waiting times and add your edits right away.

Mangogirl2, your interpretation of what static pressure refers to sounds a bit simplistic to me, in that it contains a subtle fallacy, which I explain here.

Giuliopp (talk) 16:25, 13 January 2008 (UTC)

Comprehensive re-write January 2008

When I first visited “Static pressure” I was struck by the extent to which its contents were starkly inconsistent with widespread use of the term "static pressure" by several generations of authors in the fields of aviation and fluid dynamics. Its contents were also inconsistent with use of the term “static pressure” in other related Wikipedia articles such as here and here. The article was not supported by any in-text citations or references other than an explanation of the word “static” from a dictionary, and a reference to the Greek root for the word “static”. On 1 December 2007 I initiated a debate on “Talk:Static pressure” about the sad state of the article.

On 10 December 2007 I posted “Citation needed” flags on the two key claims that were at the heart of the inconsistencies. A month later no attempt had been made to post citations or references in the article to support its claims.

Today I have inserted a comprehensive re-write of the article. I have attempted to produce an article which explains the various meanings of “static pressure” intended by the many authors who use the term. I have included numerous citations and references to support my re-write. If anyone visiting the article disagrees with anything they read, or wants to see a citation to support one of the statements, please leave a comment on this Talk page, post a “Citation needed” flag, or leave a message on my User page. If anyone knows of another application of the term “static pressure” please incorporate an explanation of that application into the article and support it with an appropriate citation. Dolphin51 (talk) 11:39, 15 January 2008 (UTC)

I did some research and I have to say that I could not find any authors adopting the rigorous definition of static pressure I was supporting. That definition – which I didn't make up – had been adopted by my professor of Fluid Dynamics, back at uni, and, I remember, that was the first time I (and my course mates) had ever heard such a coherent, unambiguous definition of what static pressure means; a definition that makes it impossible to get confused and misapply the Bernoulli equation, for example by mixing up free-stream velocity and local pressure, while writing one side of the equation.

Clearly (and unexpectedly to me) that professor of mine is an isolated voice and he also hasn't written any mainstream textbook of Aerodynamics, therefore I won't insist anymore with my point – at least until some author decides to put a bit more rigor in formal definitions.

Giuliopp (talk) 00:09, 2 April 2008 (UTC)

Hi Guiliopp. Thanks for providing closure on this one. I also went looking for a source that might match what you were saying. The only one I found was Bill Gracey's book Measurement of Aircraft Speed and Altitude. I added it as one of the references, and inserted a pertinent quote - currently Note No. 5.

I found that the majority of authors from the 1950s and later use the expression static pressure but very few of them provide an unambiguous definition of what they mean. Lawrie Clancy in Aerodynamics and Bill Gracey are two who tackled the task with any seriousness. The majority seem to work on the principle that there is no risk of ambiguity. That is a pity. Best regards. Dolphin51 (talk) 12:08, 4 April 2008 (UTC)

• All the above topics and discussion are now of historical value only. It relates to a version of the article Static pressure that has been replaced. I have asked Crowsnest to move all the above discussion into an archive. Is there any disagreement about that? Dolphin51 (talk) 12:08, 20 May 2008 (UTC)