Talk:Bernoulli's principle

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Help with this template Comments: edit – history – watch – refresh version reviewed This is a very good article. The intro mentions Euler might have got there first but I have read the contrary. More historical perspective in the introduction. The lift section has more space rubbishing an incorrect theory. Has the basic theory could do with some more explanation in the maths. Rex the first talk | contribs

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/Archive 1 – March 30, 2008

Contents 1 Talk Page Archive 2 Real World Application 3 Intended replacement "A common misconception about wings" 4 Move 5 A common misconception about wings 5.1 Stick and Rudder 5.2 Understanding Flight 5.3 Equal transit-time fallacy 5.4 Notes 6 Misunderstandings about the generation of lift 7 Proposal regarding the article structure 8 Request for help at Siphon

[edit] Talk Page Archive

Archive 1 has been created with a link at above right. It is an exact copy of the talk page as it was before this edit. Archive 2, when needed in the future, should be a new subpage (same as creating an article) titled "Talk:Bernoulli's principle/Archive 2" and the link added to the template on this page's code. For further information on archiving see Wikipedia:How to archive a talk page. See also User:5Q5 for the used archiving procedure. Thank you. Crowsnest (talk) 19:46, 30 March 2008 (UTC)

[edit] Real World Application

On 25 March the use of wing lift to illustrate Bernoulli's principle was deleted from Real World Application by an anonymous editor. (This is not the first time it has been deleted.) I reinstated the deletion, but it was deleted again, with the suggestion it was a dubious example (although no explanation of why it might be dubious.) The deleted text contained the following reference:
When a stream of air flows past an airfoil, there are local changes in velocity round the airfoil, and consequently changes in static pressure, in accordance with Bernoulli’s Theorem. The distribution of pressure determines the lift, pitching moment and form drag of the airfoil, and the position of its centre of pressure.” Clancy, L.J., Aerodynamics , Section 5.5
This is a very, very powerful reference in support of the notion that the lift on a wing is a good example of Bernoulli's principle in action. In contrast, none of the Bernoulli Sceptics who have deleted this text or others like it, have ever provided a reference of any kind to support their scepticism about the applicability of Bernoulli to the lift on a wing.

The threshold for inclusion in Wikipedia is verifiability, not truth. See WP:Verifiability. Why is it that the Bernoulli Sceptics feel they are exempt from the general principle in Wikipedia that sources must be quoted? Why is it that the example of lift on a wing, comprehensively supported by a reference to L.J. Clancy's excellent book Aerodynamics, is deleted but the explanation that accompanies that deletion is conspicuously devoid of any reference or source, except perhaps a suggestion that editors should look at the Talk page to see all the debate there. (WP Talk pages are not sources!)

Can we agree that the spirit of WP:Verifiability is that if an inclusion cannot be supported by an adequate reference it can be deleted; and if deletion of an adequately sourced statement cannot be supported by a reference, that deletion should be reverted? Dolphin51 (talk) 12:10, 25 March 2008 (UTC)

Dolphin, first off I am not the editor in question, when I edit this page, unless I forget, I log in. Secondly stop calling people "bernoulli sceptics". Its a with us/against us type label the use of which in unacceptable, and dances around WP:NPA - they are editors and their thoughts are more complex than such classifications allow. I am abstaining from commenting on this paragraph in the text. Apologies for the slightly rant like remark, and if there exist concerns about this comment, lets move it to my talk page. Kind regards User A1 (talk) 22:49, 25 March 2008 (UTC)

I wasn't involved either, but I'll offer a critique that is almost relevant. I notice the section contains phrases like "very much faster", "much lower" and "very fast". In technical material, adverbs are poor substitute for quantitative information. In my experience, the words "much" and "very" are correlated with with bogus ideas and myths. 75.16.46.75 (talk) 23:00, 26 March 2008 (UTC)

• It would be nice to be able to write something like "the air flows around one side of a wing 37% faster than around the other side", but unfortunately that wouldn't be true. The velocity distribution around a given wing section varies with airspeed, lift coefficient, Reynolds Number and Mach number. It is possible to grasp the situation by seeing Joukowsky transform. At the end of the day, Bernoulli's principle is not about the velocity distribution around solid bodies. It can be used to bridge the gap between the velocity distribution and the pressure distribution. As the article says, "Bernoulli's principle does not explain WHY the air flows faster past the top of the wing and slower past the under-side." Dolphin51 (talk) 00:30, 27 March 2008 (UTC)

• Although formulas based on Bernoulli's principle can be used to calculate aproximate lift generated by wings, it doesn't mean that Bernoulli's princple is a good explanation for how wings work. The key phrase in this article is local changes in velocity round the airfoil. It's the changes in velocity (accelerations), and not diffrences in velocity that are responsible for creating lift. It would be nice if more emphasis was placed on this with repeated references to changes in velocity both here and in the Lift (force) page. Static ports in aircraft are a good example where air stream velocities differ (zero inside, airspeed of aircraft outside), but there's no effect on the pressure in the chamber connected to the static port. Rcgldr (talk) 21:08, 1 April 2008 (UTC)

• Cause and effect - The rate at which the information about pressure differentials propagates through the air is the speed of sound, it's not instant, so pressure differential is the cause (since it occurs first), and the affected air's acceleration is the effect (since it occurs afterwards). Rcgldr (talk) 21:08, 1 April 2008 (UTC)

• Wings don't rely on venturi effect, it's an open system, with nothing to directly narrow the air stream. From the wings frame of reference, the airstream over a wing narrows because its speed is increasing as it accelerates towards the low pressure area, and the airstream below a wing widens because its speed is decreasing as it decelerates away from the high pressure area. It may look similar to venturi effect, but the cause is effective angle of attack and air speed, not a narrowing of flow through a closed system. Venturi effect doesn't require any work or change in total energy, but generating lift with wings does require work and a change in total energy. Rcgldr (talk) 17:22, 31 March 2008 (UTC)

• Regarding verifiability, I can only cite references like Encarta, which once explained lift via Bernoulli principle, but later changed to it's current statement that wings deflect air downwards, without any reference to Bernoulli, so someone at Encarta must have verified this in order to make such a change. In terms of books and/or web sites, there doesn't seem to be any consistency, so it's like a court case, whose "expert" witnesses are you going to believe? Rcgldr (talk) 17:37, 31 March 2008 (UTC)

• Looking at the archived discussion, I thought I'd include the link to the picture of the unusual lifting body aircraft, since it may help dispell the belief that all airfoils need positive camber, and because it just looks cool: m2-f2.jpg. Rcgldr (talk) 22:07, 31 March 2008 (UTC)

The threshold for inclusion in Wikipedia is verifiability, not truth. See WP:Verifiability. If you were to include your ideas in Bernoulli's principle, what reference(s) would you quote in the citations to support your inclusions?

Would I be correct in assuming you might quote Anderson and Eberhardt's book Understanding Flight? This book is very much in the minority in the field of aerodynamics. It makes some claims that are at odds with the majority of authors and aerodynamicists. You will find some quotations from this book here. Regards. Dolphin51 (talk) 13:12, 31 March 2008 (UTC)

• No, I've visited that web site, but had no plans to quote anything from that website, although that site does include text about static ports, a real world example that differences in air stream velocity don't produce lift. Rcgldr (talk) 21:18, 1 April 2008 (UTC)

Is this an "April Fool's" entry, or do you really not understand that the pressure on both sides of the fuselage is the same? ComputerGeezer (talk) 22:42, 1 April 2008 (UTC)

Static ports are only connected to one side of the fuselage, the other side is connected to a chamber inside the aircraft. A static port hole is flush mounted to the fuselage, so that the air flows across the opening. Air speed inside the hole is zero, airspeed outside the hole is the same as the airspeed of the aircraft, the ratio is aircraft's airspeed / zero, yet the pressure in the chamber is the same as ambient. The point here, once again, it's not the differences in velocities, but the changes in velocities that cause lift. Rcgldr (talk) 05:42, 2 April 2008 (UTC)

Hi Rcgldr. In a number of contributions you have emphasised that lift is caused by changes in velocity, not differences in velocity. My response is a mixture of "I don't agree with you" and "I don't know what you are talking about." Lift on a wing arises when there is a difference in the average pressure on one side of the wing and the other. Bernoulli's principle tells us that fluid speeds and pressures are related. Consequently when there is a difference in the average pressure on the two sides of the wing, there must simultaneously be a difference in the average fluid speed on the two sides. My explanation is based on Chapter 3 of Aerodynamics by L.J. Clancy. There are numerous citations to Clancy's book in Bernoulli's principle. Can you quote your source of information?

I'm not disputing that pressure differentials cause acceleration of air, resulting in differening velocities. What I'm disputing is that what counts is how those differences in velocity were created. As I stated before, Venturi effect doesn't require a change in total energy, but in order for a wing to produce lift, work and change in total energy are required, (work = induced drag times distance relative to the air, by definition). Rcgldr (talk) 16:00, 2 April 2008 (UTC)

You have also made numerous comments about static ports and altimeters, and their relevance to Bernoulli's principle. In its most basic form, Bernoulli's principle says total pressure is constant along a streamline. The air in the static system is stationary so there are no streamlines to be found therein. Consequently your comments about the static port of an aircraft showing that Bernoulli's principle does not explain lift makes no sense to me. What source can you quote to support your comments?

In the case of the static port system, the air outside the port isn't stationary, it's just not being accelerated. Wouldn't the airflow across the static port hole opening be considered a streamline, and the non moving air inside the static port another streamline with zero speed and therefore also constant total pressure? But, ignoring the static port example, since total pressure is constant along a streamline, and wings need to peform work and change the energy of the air (acclerate the air downwards at the cost of also accelerating air forwards) in order to produce lift, then these constant total energy streamlines must not be related to the lift, which requires producing changes in energy (from the work done by a wing). So wouldn't this effectively show that Bernoulli's principle does not explain lift? Rcgldr (talk) 15:51, 2 April 2008 (UTC)

I do not understand what statement you try to pose, and on what arguments Bernoulli's principle (BP) is not explaining lift. BP never can explain lift on it's own, it requires knowledge of the flow field to apply it. But, as stated in the first application in the article, once it has been established that the air is flowing much faster over the upper- than the lower-side of the wing, BP explains why these speed differences result in lift. And there are more explanations for lift, see Lift (force), but they are outside the scope of this article.

BP never can explain lift on it's own. Then why include lift in the real world applications section of a BP article? Once it has been established that the air is flowing much faster over the upper- than the lower-side of the wing, BP explains why these speed differences result in lift. My understanding of BP explanations of lift is that total energy is conserved along streamlines, but wings peform work on the air changing the total energy. From a BP perspective, how is this change in total energy when producing lift described? Once it has been established that the air is flowing much faster... faster flow doesn't always mean lower pressure, it depends how much change, if any, in total energy occurred during the process of accelerating a fluid. Rcgldr (talk) 22:53, 2 April 2008 (UTC)

Your statement that the air outside the static port is not accelerated, is wrong. Fluid particles, when moving along with the flow, are accelerated and decelerated as their flow velocity changes. And what do you mean with stationary, since stationarity depends on a specific choice of the coordinate system (while Newton's laws and the BP are valid in any inertial frame of reference)? For the static port to work, the pressure there should be equal to the ambient pressure, requiring that the flow speed is approximately equal to the incoming air speed (for a frame of reference fixed to the plane).

Finally, about the static port: if I understand correctly, but I may not, you state that:

1. for a static port in the form of a small hole connected to some chamber inside the plane,
2. the fluid in the chamber is stagnant and
3. BP will erroneously predict that the pressure inside the chamber is the stagnation pressure (as also measured by the pitot port), irrespective of the pressure at the static port hole.

You would be right if BP is valid all the way from outside the airplane until inside the chamber. But it is not, since it is only valid if viscous effects can be neglected. Near the airfoil there is a boundary layer with strong velocity gradients and strong viscous effects, where BP is not valid. In this boundary layer the flow velocity drops from its value at the outer edge of the layer until zero at the surface of the plane. And in such a boundary layers, the pressure is approximately constant over the thickness of the layer. So at the static port hole, the pressure is the ambient pressure just outside the boundary layer, and the velocity is about zero. So just inside the hole and inside the chamber, the pressure will be constant and equal to the ambient pressure.

Crowsnest (talk) 21:43, 2 April 2008 (UTC)

Don't wings also have a similar boundary layer issue? Yet they produce lift, what is the difference? Rcgldr (talk) 22:53, 2 April 2008 (UTC)

The boundary layer does affect the pressure as measured by your considered pressure-measuring device connected to the static port hole, but it hardly changes the lift force. Just because the pressure just outside the boundary layer is nearly the same as the pressure on the airfoil surface. But that is about boundary layers, and has nothing to do with Bernoulli's principle. Crowsnest (talk) 07:31, 3 April 2008 (UTC)

I recommend you have a look at position error. It will assist your understanding of static ports. Dolphin51 (talk) 11:17, 2 April 2008 (UTC)

• If people with PhD's can't agree on BP and lift (my impression based on web sites and book references), then how is the average person supposed to? Rcgldr (talk) 00:03, 3 April 2008 (UTC)

People with an education in fluid dynamics do not disagree about Bernoulli. They disagree about how to explain Bernoulli and lift to novices. The "Real World Application" entry does a reasonable job of explaining the basics. Anything more is probably beyond the scope of Wikipedia. (You might consider a course in Aerodynamics if you want a deeper understanding.) ComputerGeezer (talk) 03:23, 3 April 2008 (UTC)

I would be happy with some simple explantion how the BP perspective on lift describes where in the process that work is being done on the affected fluid, changing it's total energy, and why the fluid flow ends up being curved, resulting in lift. Rcgldr (talk) 06:04, 3 April 2008 (UTC)

All these questions are about lift, not about Bernoulli's principle. So in that respect you pose your questions at the wrong place. And, within the framework of inviscid flow for which Bernoulli's principle is valid, no work is being done in this dissipationless approximation. This is because, in a coordinate system fixed to the airfoil at uniform flight, the airfoil does not move while work is proportional to force times the velocity of the airfoil (not the air flow). You also do not ask this question when you use a garden hose, with water flowing out of the hose in a different direction than it comes from the water tap. Crowsnest (talk) 07:31, 3 April 2008 (UTC)

All these questions are about lift, not about Bernoulli's principle. So in that respect you pose your questions at the wrong place.Which is why I was asking why lift was included in this article. Again, since information about the pressure differentials travels through the air at the speed of sound, resulting in accelertion of air, there's a slight delay between a change in pressure differentials, and the air's acceleration response, which leads me to believe that the pressure differentials are the cause (they occur first), and accelerations are the result (they occur later). In addition, for a normal wing, most of the acceleration (not velocity) of air is downwards, regardless of the frame of reference. This is why I feel that including Bernoulli effects regarding the air's response to pressure differentials would be appropriate in the lift (force) article, but not here. However, I appear to be in the minority opinion here, and the experts disagree on this, so I'm giving up on this. Rcgldr (talk) 15:54, 3 April 2008 (UTC)

The first example in Bernoulli's principle#Real world application is clear, in my opinion, stating that "Bernoulli's principle does not explain WHY the air flows faster past the top of the wing and slower past the under-side".

Now you start making statements about sound waves, also quite off-topic. However, there is no first and later, with respect to pressure and acceleration: Newton's 2nd law of motion states that the time rate of change of momentum (mass density times velocity for a fluid) is equal to the sum of the applied forces. Nothing is said about forces being first and acceleration being later. Especially not in a fluid with a direct interaction of pressure and fluid motion changes in neighboring fluid elements. Crowsnest (talk) 21:09, 3 April 2008 (UTC)

statements about sound waves - not sound waves, but the rate at which information about pressure differentials travels through the air. As a wing travels through the air, there's a lag between the time the wing introduces pressure differentials into the air, and before all of the affected air responds, I'd call that cause and effect. This is more noticable at higher sub-sonic speeds, like mach .8 or higher (note a 747 traves at mach .85 to mach .90). Rcgldr (talk) 17:15, 4 April 2008 (UTC)

Information travels with the speed of sound, but it is not only pressure traveling with the speed of sound. Pressure is not an independent quantity which can propagate through vacuum, like light. It needs a medium, the fluid. And, for adiabatic processes like you describe, pressure changes mean changes in the fluid mass density, which can only exist if there is an associated momentum (so also velocity) field variation balancing the mass density changes. It is oversimplistic to talk about pressure being first, and the air responding. Pressure is part of the state of the air, as is velocity. They always change together. Crowsnest (talk) 18:21, 4 April 2008 (UTC)

• I've found this web site to have the best "simple" explanation when Bernoulli principles do and don't apply, and clarifies the relationship between pressure and velocity: Misinterpretations of Bernoulli's Law Rcgldr (talk) 01:12, 17 April 2008 (UTC)

• From footnote 8: When a stream of air flows past an airfoil, there are local changes in velocity round the airfoil, and consequently changes in static pressure, in accordance with Bernoulli’s Theorem. - This statement implies that acceleration of air (changes in velocity) are the cause of changes in static pressure. It would be more accurately stated that acceleration of air and changes in static pressure coexist, with neither being the cause or effect. Wings are bad examples for Bernoulli because for most of the chord length, the air flow is turbulent (small eddies). Laminar flow only occurs for the first 30% or less from the leading edge on a normal (non-laminar) wing, both top and bottom. The flow transitions into turbulent flow, detaching during the transition, then reattaching as turbulent flow. It is common for glider magazines to do oil flow testing on a wing to observe the detachment zones on wings (do a web search for oil flow test glider) as part of a review. Jeffareid (talk) 05:40, 29 May 2008 (UTC)

You are right that the use of the word "consequently" may give rise to the idea that pressure changes are caused by velocity changes. But this use of "consequently" by L.J. Clancy may also point to Bernoulli's theorem directly mentioned thereafter: consequently, according to Bernoulli's theorem, these velocity changes are associated with pressure changes. Language is difficult to use in a precise manner for simultaneous interactions, take for instance Newton's 3rd law in shorthand: "To every action, there is an equal and opposite reaction", which also may give the impression that the action is first, and the reaction follows, while they are inseparable, simulaneously and cannot exist independently of each other.

Regarding turbulence and oil strikes for flow visualization on wings: both are related to the thin boundary layer near the airfoil, where shear stresses and dissipation are important. Inside the boundary layer, Bernoulli's principle is not applicable, but outside it is. Because the boundary layers are thin, and have negligible pressure differences over their thickness, Bernoulli's principle is often used in numerical codes to predict the lift on airfoils — with good results, when comparing the numerical predictions with measurements, and provided the range of validity of Bernoulli's principle is respected. -- Crowsnest (talk) 19:13, 29 May 2008 (UTC)

[edit] Intended replacement "A common misconception about wings"

I intent to remove A common misconception about wings, and replace it by a section on how to compute lift using given flow kinematics and an appropriate form of the Bernoulli equation. The reason is that the section "A common misconception about wings" is not about whether Bernoulli's principle is right or wrong, not even about misusing Bernoulli's principle, but about ways to calculate lift. The idea is to replace it with a new "Integral force" or "lift force section", giving some equations how to calculate force from pressure, plus:

• refer to the Lift (force) page, and
• say that there are misconceptions about how to calculate lift, giving links to e.g. lift (force)#Equal_transit-time,
• that there are other ways to calculate lift than by the Bernoulli equation (again link to lift article).

I intent to copy the present "Misconception" section to this talk page, and say something about it on the talk page of lift (force). Crowsnest (talk) 20:41, 27 March 2008 (UTC)

Hi Crowsnest. Thanks very much for raising your intentions on this Talk page, and giving other editors the opportunity to contribute. So many editors simply delete an area of text, leaving other editors to rectify the damage.

You say you intend to remove A common misconception about wings. The reason you give is that this section “is not about whether Bernoulli's principle is right or wrong, not even about misusing Bernoulli's principle, but about ways to calculate lift.” Your meaning is not clear, but I assume you are saying this section should be about ways to calculate lift. I disagree. The article is about Bernoulli’s principle, not about lift, and certainly not about calculating lift. There is an abundance of information about calculating lift here. The section “A common misconception about wings” is not about calculating lift, nor should it be. I agree that the title “A common misconception etc” is not an accurate summary of the information that follows, but that is for historical reasons – see this section prior to my comprehensive re-write on 29 January 2008. I wrote the present content of “A common misconception” and I would welcome someone else coming up with a more accurate title. The present content in “A common misconception” gives an accurate and honest coverage of two alternative views of Bernoulli’s principle and its application to lift on wings. These alternative views are entirely legitimate and are accompanied by appropriate citations. Removing these two alternative views of Bernoulli’s principle would be tantamount to censorship and I would object to it. Placing these two views on the Talk page would not be an acceptable alternative.

You propose adding to Bernoulli's principle a new section giving equations on how to calculate force from pressure. I suggest such equations are appropriate to Lift (force) but are not appropriate to Bernoulli’s principle. Lift (force) already has a suitable opening for more such equations here. Such equations are unrelated to the information about “Stick and Rudder” and “Understanding Flight”, so such equations should not be considered a replacement for the information about these two books. I would be very happy to see people considering improving the information about these two books, but I would not be happy to see the information deleted, regardless of whether it is replaced by equations about how to calculate lift.

You propose writing that the equal transit time myth is an example of a misconception of how to calculate lift. The equal transit time myth is not about calculating lift. It is about explaining lift for student pilots and newcomers to aviation for whom Bernoulli’s principle is too advanced.

You also propose making a comment that there are alternatives to Bernoulli when calculating lift. Would this serve any purpose? For example, conservation of momentum can be used as an alternative to Newton’s Second Law of Motion, but do you see any value in amending the WP article on Newton’s Second Law of Motion to point out that there are alternatives to this Law when doing calculations in kinematics? Do you see any value in amending the WP article on momentum to point out that there are alternatives to the principle of conservation of momentum?

You have made a number of additions to Wikipedia articles but I don’t recall any citations accompanying your additions to confirm their verifiability. The threshold for inclusion in Wikipedia is verifiability, not truth - see WP:Verifiability. Please ensure that your substantial additions are adequately supported by citations in the form of references that confirm the verifiability of those additions. Happy editing. Dolphin51 (talk) 23:48, 27 March 2008 (UTC)

Well, there seems to be nothing wrong with the amount of references in this section, but all is wrong with it being here in the first place. Bernoulli's principle is about a constant of motion along streamlines (or for potential flows in the whole fluid domain), which can be used to determine the local flow dynamics (pressure) for given flow kinematics, not on how to determine (directly) an integral lift force on wings. To obtain the lift force, you need to compute the flow kinematics, which is outside the scope of Bernoulli's principle.

Further, the first two examples are more like book reviews of two individual books, than an encyclopedic topical description of common misconceptions. In my opinion, while interesting, and containing many references to other sources, such book reviews contesting the claims of the authors are original research. Some more comments on the three topics in this section:

• "Stick and Rudder" is just giving the POV of Langewiesche on the ways how he likes to explain lift. Not about the validity of Bernoulli's principle. Further it is misleading: the reader may easily get the impression that Bernoulli's principle is different from Newton's 2nd law of motion, while it is just determined as a first integral of the momentum equation.
• "Understanding Flight", is also about different ways to explain lift. Misconceptions on the Coandă effect relation with lift are clearly already explained in Lift (force).
• "Equal transit-time fallacy" doe not even mention Bernoulli's principle, and is already described well on Lift (force). Further there is the List of works with the equal transit-time fallacy.

Bernoulli's principle is a tool, very handy in many applications, but (perhaps) not to "explain" lift. I think the emphasis in this article should be on how to use Bernoulli's principle. The hammer article, for instance, also does not contain a section: how not to use a hammer.

Just removing this section and leaving a copy on the talk page, and linking to Lift (force) is a good solution, in my opinion. I think it better to refrain from adding a lift section here, given all controversies about lift.

Crowsnest (talk) 09:21, 28 March 2008 (UTC)

While reading through the section I ran across a seeming contradiction in the article. There is this quote from Anderson and Eberhardt which the section claims is incorrect:

“The acceleration of air over the top of a wing is the result of the lowered pressure and not the cause of the lowered pressure.”

The article intro says:

"(when) the speed increases it can only be because the fluid on that section has moved from a region of higher pressure to a region of lower pressure"

which sounds completely consistent with the supposedly incorrect quote from Anderson and Eberhardt's -- both say that acceleration is the result of a pressure gradient. How is it that one of the statements is wrong? Spiel496 (talk) 22:40, 28 March 2008 (UTC)

The problem is that Bernoulli does not say whether velocity change causes pressure change, or pressure change causes velocity change; it just relates the two. Whether one "causes" the other should be argued in lift (force), not here.

I think a "misunderstandings about Bernoulli" section might be appropriate if properly referenced. But the existing section is really OR that should not be on the main page. (It is, however, valuable work which we should preserve to prevent the original error from being re-introduced on the main page.) ComputerGeezer (talk) 23:35, 28 March 2008 (UTC)

Bernoulli's principle, for incompressible flow without additional forcing by e.g. gravity, just says that pressure changes are balanced by velocity changes. It does not say anything directly about cause and effect, in that respect.

However, Bernoulli's principle is derived from Newton's 2nd law of motion for a fluid parcel, describing the change of momentum due to forcing. In that respect pressure changes, if considered as external forces, could be described as causing the velocity changes. But, the pressure changes are also the result of the interacting fluid parcels: in incompressible flow the pressure is ensuring that the volume of fluid elements does not change.

So, there is an interaction between pressure and velocity (momentum, kinetic energy), not a one-way action.

In that respect, the first statement is formally incorrect, since it states that velocity changes are caused by pressure changes. The second statement is correct, since it implies a balance between pressure changes and associated velocity (kinetic energy) changes. Crowsnest (talk) 23:52, 28 March 2008 (UTC)

[edit] Move

The section "Misconceptions" is OR and does not belong in the main article. I am moving the section to this talk page. It should remain here so that nobody tries to "correct" the discussion of lift in the main article.

I replaced that section with a short discussion of the foolish debate about whether it is pressure or velocity that "causes" lift with a reference to http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html and a link to Lift (force)

ComputerGeezer (talk) 01:08, 1 April 2008 (UTC)

Good move Geezer. It seems the sensible thing to do.

As has been shown over the past couple of months here in te discussion page, lift is a complex subject. Not even experts on the subject agree what exactly causes lift. Thus, trying to give a full explanation in place of the simplistic ones goes way over the heads of most Wikipedia readers.

I think the current wording is good: there are simplistic explanations and they are much too simple to tell the whole story. The NASA page is very good. It holds a good level of explaining. In my opinion, most text after the link to that page in that section could be removed. --J-Star (talk) 08:28, 1 April 2008 (UTC)

support section move - I think that the section is not aiding the article sufficiently. Whilst it is nice to be able to correct errors in peoples understanding, I don't think that this is the article's goal. It is my opinion that the goal of the article is to explain the phenomena, not its misuses, User A1 (talk) 12:21, 1 April 2008 (UTC)

Thanks for the move. Crowsnest (talk) 12:32, 1 April 2008 (UTC)

The section below was moved here from the main page, for archival purposes. Please do not edit in the part below, until the next yellow tag.

[edit] A common misconception about wings

Some authors of introductory books about aviation and flying, when describing how wings generate lift, have advocated an explanation that avoids any reliance on Bernoulli’s principle. Two examples are given below.

[edit] Stick and Rudder

For example, in 1944 Wolfgang Langewiesche ^[1] wrote Stick and Rudder, an introductory text for aviation enthusiasts and student pilots. Langewiesche aimed to explain the operation of aircraft in simple terms that would be factual but easily understood by newcomers to aviation. He particularly strove to avoid concepts that were outside most people’s day-to-day experiences. “Forget Bernoulli’s Theorem” he wrote. ^[2] Langewiesche was a skilled aircraft pilot and instructor. He did not doubt the validity of Bernoulli’s theorem (principle), nor its applicability to aircraft, but he did recognize that any aspiring pilot need not struggle to grasp the Theorem in order to understand the basics of operation of an aircraft. Langewiesche wrote ^[2] “Bernoulli’s Theorem doesn’t help you the least bit in flying. While it is no doubt true, it usually merely serves to obscure to the pilot certain simpler, much more important, much more helpful facts.”

Langewiesche recognized it is important for aspiring pilots to accept that the wing of an aircraft is capable of generating lift. He also recognised that aspiring pilots need to have an understanding of Newton’s laws of motion. In writing Stick and Rudder he made it unnecessary to look further than Newton’s laws of motion to explain the forces on an aircraft. “That’s what keeps an airplane up. Newton’s Law says that if the wing pushes the air down, the air must push the wing up." ^[3] In particular, he made it unnecessary for the reader to resort to Bernoulli’s principle to explain lift.

[edit] Understanding Flight

As a second example, in 2001 David F. Anderson and Scott Eberhardt wrote Understanding Flight, ^[4] another introductory book about aviation and flying. In their Introduction, Anderson and Eberhardt acknowledge Langewiesche’s injunction “Forget Bernoulli’s Theorem”. They also say “The object of this book is to provide a clear, physical description of lift and basic aeronautical principles.” Anderson and Eberhardt provide readers with an understanding of Newton’s laws of motion, and avoid other more complex principles. In particular, they avoid any emphasis on mathematics. In their Introduction they say “It is our belief that all fundamental concepts in aeronautics can be presented in simple, physical terms, without the use of complicated mathematics. In fact, we believe that if something can only be described in complex mathematics it is not really understood. To be able to calculate something is not the same as understanding it.”

Like Langewiesche, Anderson and Eberhardt do not dispute the validity of Bernoulli’s principle. They say “This reduced pressure causes the acceleration of the air via the Bernoulli effect”. ^[5]

In their efforts to use simple concepts to explain the lift generated by a wing, Anderson and Eberhardt make some statements that are not consistent with other higher principles in physics and fluid dynamics. For example, they say “The acceleration of air over the top of a wing is the result of the lowered pressure and not the cause of the lowered pressure.” Further on, they say “But the lowering of the pressure above the wing is the result of the production of the downwash.” ^[5] Anderson and Eberhardt attempt to explain the lift on a wing as a sequence of events, one causing a second, and the second causing a third. The acceleration of air around a wing, the lowering of air pressure, and the production of downwash all occur simultaneously. One does not cause the other. They are caused by the shape of the airfoil, its speed relative to the air, its orientation to the passing air, and even the viscosity of air. At this point, Anderson and Eberhardt may have added unnecessary complexity by implying that aviation enthusiasts and aspiring pilots need to understand which comes first, acceleration of the air, lowering of its pressure, or the production of downwash. Bernoulli’s principle says only that a change in pressure, a change in speed and a change in elevation all occur simultaneously. Bernoulli correctly avoided saying one was the cause and the others were the effect.

Like Langewiesche, Anderson and Eberhardt use the concept of downwash to explain why a wing generates lift. Anderson and Eberhardt explain downwash by referring to the Coanda effect. “This downward-traveling air is the downwash and as we will see is the source of lift on a wing. Why does the air bend around the wing? The answer is in an interesting phenomenon called the Coanda effect. The Coanda effect has to do with the bending of fluids around an object.” ^[6] In Understanding Flight there is the tacit implication that if lift can be explained by the Coanda effect there is no room for any other explanation, and certainly no room for Bernoulli’s principle.

Anderson and Eberhardt also make some statements about Bernoulli’s equation and its applicability to flight. For example, in the Appendix titled Misapplications of Bernoulli’s principle Anderson and Eberhardt begin by saying “Bernoulli’s equation has mistakenly become linked to the concept of flight.”

Introductory books like Stick and Rudder, Understanding Flight and others occupy a valid place in the field of aviation because they provide newcomers with simple, easy to understand explanations that are sufficient for the newcomers to gain a basic understanding of flight and then move forward to new topics. These introductory books cater for readers for whom Bernoulli’s principle is unnecessarily complex.

[edit] Equal transit-time fallacy

Main article: Lift (force)#Equal transit-time

There is a genuine fallacy inherent in one popular explanation of the lift generated by a wing. This fallacy has become known as the "equal transit-time theory". It is well known that, when a wing is generating lift, the air travels much faster around one side of the wing than the other. To fully understand why the air travels faster around one side than the other it is helpful to understand the Kutta condition, the notion of circulation and the Kutta-Joukowski theorem but these are not simple concepts. Many authors have attempted to provide a simple explanation as to why air travels faster around one side. Some authors have pointed to the camber on most wings and suggested the air has further to travel around the cambered side of the wing than around the flat side, and to do so in equal time requires the air to move faster around the cambered side. This is not an accurate explanation of why the air moves faster around one side than the other, and it has been exposed as a fallacy.

[edit] Notes

The section above was moved here from the main page, for archival purposes. Please do not edit in the part above, below the yellow tag.

[edit] Misunderstandings about the generation of lift

A new level 2 headline has appeared today, called "Misunderstandings about Bernoulli's principle". It was not about misunderstandings of Bernoulli's principle, but about misunderstandings of the generation of lift, so I have altered the headline. The paragraph added by Crowsnest begins "Some of these explanations use Bernoulli's principle" and ends "(e.g. incompressible, compressible etc.)" This paragraph is not supported by any citation or reference, and it lacks any conciseness or clear purpose. It sounds like the author's personal view on things. (The threshold for inclusion in Wikipedia is verifiability, not truth. See WP:Verifiability.) I have tagged the paragraph with an appropriate tag in the hope that it will be cleaned up smartly. Dolphin51 (talk) 12:41, 1 April 2008 (UTC)

I added a reference to Phillips, for which I will add the page number later. Further I am interested whether other people also think this 2nd paragraph to be POV. If so, it has to be improved or removed.

Further note, that WP is an encyclopedia, not a citation list. Not every line or statement needs to be referenced, one of the main rules in this respect is (WP:REF): "All material that is challenged or likely to be challenged needs a reliable, published source.".

The purpose of adding this paragraph was, that many people seem to feel that they should be here, at Bernoulli's principle (BP), when reacting to explanations on lift. Either while the explanation says: "BP is unneeded and unnecessary in explaining lift", in which case they should be at the lift (force) article since that is about lift, not this article. Or, in case of explanations of lift that are misleading or wrong and claim to be correct because they are based on BP (forgetting that correct flow kinematics are needed for BP to give reliable results), in which case they also should be at lift (force) since there several of these misconceptions are treated.

Crowsnest (talk) 08:09, 2 April 2008 (UTC)

Hi Crowsnest. I see you have added a citation and deleted the "unreferencedsection" tag from the text accompanying Misunderstandings about the generation of lift. In particular, your citation is embedded in your paragraph about incorrect explanations of lift. The reference you have cited is a book titled The Dynamics of the Upper Ocean. This is not a credible citation, especially as you have not quoted a page number, section number or chapter number. Your paragraph about incorrect explanations of lift, if it is to be credible, should cite a reference that at least looks authoritative. For example, there are many books with titles like Aerodynamics, The Aerodynamics of the Airplane, Airplane Aerodynamics etc. You have not cited any of these. Instead, you have quoted a book that appears to be on the subject of oceanography. Aircraft do not generate lift in the ocean so it is unlikely that O.M. Phillips is an authoritative commentator on the subject of explanations of lift, correct or incorrect. Please try again to locate the source on which your information about incorrect explanations is based. If you cannot find one it is likely that your paragraph is really based on your personal view so you should re-instate the "unreferencedsection" tag. Best regards Dolphin51 (talk) 10:47, 2 April 2008 (UTC)

Hi Dolphin51. It seems you were editing this talk page, while I was working on the main article. The reference to the page(s) in Phillips will follow later, until I am at the location where I keep the book. I further added the references to Batchelor and Lamb. For some funny reason all three are from Cambridge Univ. Press (although Lamb has also been published between 1945 and 1994 by Dover, New York). All three are very good books. Batchelor and Lamb are classics. Although Lamb called his book "Hydrodynamics", it is about fluid dynamics in general, incorporating also compressible flow and sound waves (maybe the term hydrodynamics was synonym to fluid dynamics in those days, 1879). Crowsnest (talk) 12:14, 2 April 2008 (UTC)

Don't forget hydrofoils. Crowsnest (talk) 12:17, 2 April 2008 (UTC)

Dolphin, you may want to <s> that. The use of water for aerodynamical calculations is not uncommon. With the appropriate use of theorems such as Buckingham's-Pi scale up water tanks can be used to study all sorts of lift & aerodynamic properties. Examples 1 2 and 3. To bring those up, all I did was hop on google scholar and type water tank scale up wing. Books that talk about lift do not have to have "aircraft" or similar in their titles. If I recall correctly "Computational fluid dynamics" by Roache has stuff about wings and lift, but obviously does not have an aeronautical title, nor is it a book on aeronautical engineering - it's just got fluid dynamics. On the other hand, page numbers from books, (with edition) is probably a good idea, if they can be provided. User A1 (talk) 23:19, 3 April 2008 (UTC)

Hi Crowsnest and User A1. I am aware of the existence of hydrofoils and the use of water tanks to assess flow patterns around airfoils. My point remains that we are talking about the topic misunderstandings about the generation of lift, and in particular, citations to support what is written on the topic. Readers of Wikipedia are entitled to be highly sceptical that authoritative comments about this topic are to be found in a book titled Dynamics of the Upper Ocean. Similarly, they are entitled to be sceptical about Lamb's book Hydrodynamics written in 1879 - was the Equal-transit time debate raging in 1879 to the extent that Lamb addressed the subject - 23 years before the Wright brothers took to the air? I doubt it. I think Crowsnest's citations of Phillips and Lamb are citations to the last sentence in his paragraph which is about integration of Newton's 2nd Law along a streamline; and consideration of incompressible or compressible flow. I don't believe astute readers will imagine that Phillips or Lamb are plausible references for the core of the paragraph which is about incorrect explanations of lift, nothwithstanding the existence of hydrofoils and the use of water tanks in aerodynamics

Batchelor's book An Introduction to Fluid Dynamics is a plausible citation on the subject of aerodynamics, but personally I doubt that he addresses the subject of incorrect explanations of lift, which is what the paragraph is about. Dolphin51 (talk) 23:42, 3 April 2008 (UTC)

Hi Dolphin51. I hope the edit I just made goes into a direction you also like. I try to leave out as much as possible (e.g. statements on the derivation of Bernoulli's principle). Crowsnest (talk) 20:33, 4 April 2008 (UTC)

[edit] Proposal regarding the article structure

It is becoming increasingly apparent to me that the introduction to this topic is inadequate. While it is fine for a student of fluid dynamics, it does not serve the curious amateur.

I propose that we develop a simpler introductory paragraph, perhaps with a simple Bernoulli demonstration and photo. This would be followed by a History paragraph placing Bernoulli in its context of physics and fluid dynamic fundamentals.

Next we could introduce the real world applications, then we should clarify the (obviously common) misunderstandings about the principle and its application.

The remainder of the article could address all the derivations and applications which are of less than common interest, but provide more comprehensive information to the specialized audience.

Does this make more sense?

ComputerGeezer (talk) 16:18, 4 April 2008 (UTC)

I like this proposal, and ComputerGeezer's reasons for suggesting it. I think Wikipedia should be based on a principle of "begin with the simple, and progress towards the complex". ComputerGeezer's proposal matches this principle exactly. Go for it ComputerGeezer! Dolphin51 (talk) 04:01, 5 April 2008 (UTC)

The introduction is, in it's present form, not adequate for the average visitor. For me, a step-by-step approach like the one proposed by ComputerGeezer sounds fine, starting with the introduction. Further I think that a section is needed on the conditions under which Bernoulli's principle is applicable (e.g. dissipation, entropy changes, heat exchange). Also, some more forms of Bernoulli's equation can be added, e.g. for unsteady potential flows, both incompressible and compressible.

A history section sounds fine also, but I think it should not be a copy of the [[Daniel Bernoulli] article, but go specifically about Bernoulli's principle. Crowsnest (talk) 19:10, 6 April 2008 (UTC)

Comments?

I'm still drafting, but if anyone wants to look in my sandbox I would like some feedback on the introductory paragraph there. ComputerGeezer (talk) 02:22, 7 April 2008 (UTC)

I created Talk:Bernoulli's_principle/Temp with a copy of the lead section of ComputerGeezer's sandox, in order to make it easier to discuss the proposed changes here. I very much like the beautiful and illustrative image. But I do not like the first paragraph, which is factual incorrect and contradictory to Bernoulli's principle, stating that "the velocity of flow in a tube varies inversely from the pressure against the side of the tube". It also suggests that Bernoulli found (or his principle is about) mass conservation. Also the last paragraph is incorrect in stating that Bernoulli's principle is not applicable to compressible flows. Crowsnest (talk) 08:07, 8 April 2008 (UTC)

I like the picture of the venturi meter. Here's a leading question: Does it matter which way the flow is going? With the flow pointing as shown, viscosity could explain the pressure difference. If the flow were instead going from narrow to wide, then the upstream portion has the lower pressure, forcing the reader to accept the somewhat counterintuitive nature of Bernouilli's principle. Spiel496 (talk) 17:30, 8 April 2008 (UTC)

In the shown image, it would matter. When the flow would be from narrow to wider cross section, and the cross section expands abruptly, the flow will separate. And this flow separation will result in viscous or turbulent dissipation, invalidating the applicability of Bernoulli's equation. The slopes of the wall in the pictured transition part, of about 1:2, are certainly steep enough to produce flow separation. But for the shown flow direction, i.e. accelerated flow, the shown transition geometry is very probably smooth enough to have an attached flow. Crowsnest (talk) 22:41, 8 April 2008 (UTC)

If you can find a free image of a three-tap venturi similar to the one at http://hyperphysics.phy-astr.gsu.edu/hbase/pber2.html#pl that would be ideal. But I have not found one. (I could draw it, but it would not have the same impact...) I had also planned to address the expansion (pun, get it?) of Bernoulli's work to cover compressible flows in the (not yet drafted) "History" section. ComputerGeezer (talk) 00:42, 9 April 2008 (UTC)

I added a changed lead section to Talk:Bernoulli's_principle/Temp#New lead-section text (10 April 2008). Please comment and improve. Crowsnest (talk) 23:02, 10 April 2008 (UTC)

[edit] Request for help at Siphon

There is a debate at Talk:Siphon that would benefit from more attention. I figured that some of the contributors here may have the needed expertise. Thanks for your help!--Yannick (talk) 16:18, 7 June 2008 (UTC)