Talk:Bernoulli's principle/Archive 5

This is an archive of past discussions about Bernoulli's principle. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

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Condensation in Flight

The A340 wing image is misleading and poorly described. Aircraft wings cannot decrease the temperature of air flowing over one side simply by having slower-moving air on the other. The pressure would only decrease directly behind the part with the largest area, as that's the region the air is being deflected away from. Acceleration is just a by-product of the vaccuum it creates. 142.222.170.252 (talk) 15:14, 9 November 2016 (UTC)

Application of Bernoulli Principle on airfoil

In the section of misconceptions of application of Bernoulli's principle, the author stated that Bernoulli's principle cannot be used to explain the "paper blowing experiment" because it is comprised of two airflows, which does not adhere to the single airflow that is within the constraints of the principle. If this is the case, then this principle does not apply to airfoil as well, because the airflow above and under the wing is split into two airflows.

--Beckham tan (talk) 00:53, 11 January 2017 (UTC)

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Inconsistencies in article

Reviewing this after some time I'm pleased to see much real science filtering into the article, but find some contradictions in the article.

Please focus first on Note [b] and paragraph 4 referring to it:

b. " Acceleration of air is caused by pressure gradients. Air is accelerated in direction of the velocity if the pressure goes down. Thus the decrease of pressure is the cause of a higher velocity.[8] (From Weltner's "Misconceptions...")

and

Para 4: "This gives a net force on the volume, accelerating it along the streamline.[a][b][c]"

This is the single most important concept: "Pressure gradients accelerate fluid." This is Newton for fluids. This is also the most common misunderstanding and, therefore, the article MUST be clear and explicit on this fundamental principle.

HOWEVER...

Note [d] contradicts that by saying: "there are local changes in flow speed round the airfoil, and consequently changes in static pressure"

The phrase "and consequently" strongly implies that the pressure change is caused by the speed change.

Note [d] should be changed TO: "...changes in flow speed round the airfoil are due to changes in static pressure." ...OR... "...changes in flow speed round the airfoil are caused by changes in static pressure."

...........................

Then, in the Application section for lift, it says: "if the air flowing past the top surface of an aircraft wing is moving faster than the air flowing past the bottom surface, then Bernoulli's principle implies that the pressure on the surfaces of the wing will be lower above than below. "

There is both truth and fallacy there. It is not that the upper flow is faster, but that it has been accelerated more (rearward, totally reversing its direction of movement). This is not the same thing because the air above a real flying wing gets reversed in direction as the moving wing passes by. This is a high acceleration from the steep pressure gradient from ahead of the wing to above the wing. The air only appears faster because of the wind tunnel frame of reference. It is still accelerated more, but from one rearward speed to a greater rearward speed.

If you think I'm just blowing ignorant smoke, See this diagram for air motions in the otherwise still air as a wing passes by. I had wondered about viewing the flows from the still air's perspective and it turned out to be very helpful in solidifying the "pressure accelerates fluid" Fundamental principle.

https://drive.google.com/file/d/1lBeU5I6xWYifLwaPmc439E7dQOwWvAZS/view?usp=sharing

Closely related to that is: In the Misunderstandings about lift section it incorrectly says: "(the airflow above the wing is faster, the question is why it is faster)" should read "(the airflow above the wing is accelerated more, the question is why it is it accelerated more)" It is the cause of this acceleration that most misconceived explanations get wrong.

In the Talk: Applications of Bernoulli...Airfoil section, Beckham tan says: "If this is the case, then this principle does not apply to airfoil as well, because the airflow above and under the wing is split into two air-flows."

This turns out to be false because in my discussions with Embry-Riddle's professor, Charles Eastlake, he pointed out that is indeed valid to use Bernoulli's Equation to equate upper and lower differences because they both have the common conditions before the wing arrived and are therefore closely related and fall within the constraints of Bernoulli's Equation. Since this likely to spark controversy, I'll hold off on the corrections for a while.

I remind readers that I am fully versed in all the misconceptions and have discussed all of this with both Boeing's Aerodynamicist fellow Doug Mclean and Charles Eastlake.

-- Steve -- (talk) 06:24, 4 April 2019 (UTC)

Note (d) precisely quotes "Aerodynamics" by L.J. Clancy, section 5.5. In an attempt to avoid any challenge over plagiarism, I have inserted quotation marks. So Note (d) is a quotation from a reliable, published source. It is not text that can be changed to match a User's preferred wording.

In the past we have had discussions about whether the change in pressure causes the change in airspeed, or vice versa. It is a child-like approach to the subject so let's not go down that path again. Bernoulli's equation says that the static pressure plus the dynamic pressure adds to the total pressure. Static pressure and dynamic pressure both change simultaneously so that the boundary of the flow matches the solid surface of an airfoil, inside of a pipe etc. Bernoulli correctly avoided any attempt to specify which comes first and which comes second.

On Wikipedia, we don't place much weight on which scientist a User has been talking to. We place great weight on which reliable published sources a User is able to cite to allow independent verification of the new material added to the encyclopaedia. Dolphin (t) 12:23, 4 April 2019 (UTC)

.............................. . .

Dolphin,

I'll go slowly and be more specific since you appear to misinterpret me. As an AE seriously interested in getting this stuff right, I must assume that you'll understand this. It's not nonsense.

I'm not one of the "it's only AoA" or "It's only Newton-reaction" crowd. I think that is what you may mean by 'child-like'. The fact is; lift is a longer story that some of these over simplifications are part of. However, I stick to Bernoulli here.

I'm also, NOT trying to say the article is wrong. It does, however have some contradicting items in it that harm the article's credibility.

I'll present the facts here, then I can provide references later to save space here.

...so...

The article starts out well by correctly stating the common Bernoulli's Principle: "increase in the speed...simultaneously with a decrease in pressure..."

Then, the conservation stuff...

Then, in Para #4, affirms the agreement with Newton: "derived directly from Isaac Newton's Second..."

This gives a net force on the volume, accelerating it along the streamline.[a][b][c]"

Notes a, b, and c all explaining this more specific examples.

Then Para #4 continues restating this more.

OK , BUT, for further down, I must emphasize that this describes ACCELERATION and that it is along a streamline (SPEED changes, not velocity). There is also radial or centripetal acceleration (direction changes) causing a curved path due to a perpendicular force. That is also consistent with Newton even though the classical Bernoulli words don't explicitly include it. It is clearly explained in Prof Babinbsky's video and equivalent paper as well as Weltner and others.

Looking at the Clancy quote again, I have to back down some because it is at best only a mild implication of cause and effect that could contradict Newton (cause and effect). Reading various authors, I found that a choice of words can seem to imply a different explanation, but when comparing them carefully it is easier to see that the author's word choices can more easily be understood as the same explanation. The main takeaway I'll emphasize here, is that a single out-of-context quote may not always be the best one.

First the Demonstrations section, I'm pleased to see that these are covered as Coanda and not demonstrations of Bernoulli, but I still have some issues.

This has a problem: "Bernoulli's principle predicts that the decrease in pressure is associated with an increase in speed, i.e. that as the air passes over the paper it speeds up and moves faster than it was moving when it left the demonstrator's mouth. But this is not apparent from the demonstration.[50][51][52]"

Those references don't support that full statement. 50, Anderson & Eberhardt, only says blowing over paper is not Bernoulli's Principle. 52 Pim Geurts only that it is not Bernoulli, but the Coanda Effect with a counter example.

Now, 51, Max Feil, correctly says blowing over paper is not Bernoulli with the common flat-paper counter example. However, that quote also makes more errors.

First, Max Feil: "but the air on the top came from your mouth where you actually increased its speed without decreasing its pressure by forcing it out of your mouth."

This is very confused because it is the HIGHER pressure INSIDE the mouth that creates the gradient, thus 'automatically' making the OUTSIDE ambient pressure the lower one and accelerating the air into the ambient. The higher pressure in the mouth and lower ambient _is_ consistent with both Newton and Bernoulli as the article explains it.

Second, Max Feil. "As a result the air on both sides of the flat paper actually has the same pressure, even though the air on the top is moving faster."

This is correct and well proven that the moving air does not by itself have, nor cause the lower pressure.

Here, he directly contradicts himself twice:

RE: "The reason that a curved piece of paper does rise is that the air from your mouth speeds up even more as it follows the curve of the paper, which in turn lowers the pressure according to Bernoulli."

This is two unfounded assertions that, in several ways, contradict common measurements in aerodynamics as well as Newton and Bernoulli as explained in the article.

First, that the SPEED increases along the path which is tangential acceleration and implies a lower pressure downstream, per the accepted Bernoulli stated above. We know the pressure at the blower exit is atmospheric pressure, then this says the air exits the curve below atmospheric pressure. There is no justification for that except the false assertion that speed causes the lower pressure. A perpendicular force does not cause linear acceleration.

We do know that a perpendicular force/ pressure causes a curved path (centripetal) NOT a tangential acceleration as he implies.

Measurements have long shown the pressure over a wing camber _increases_ along the curve as the curve flattens, which supports that the lowered pressure is associated with the radius of curvature, not distance along the curve.

Second, the logical conclusion of Feils assertions would have moving air causing a lower pressure downstream which would cause further increase in speed leading lower yet downstream pressure to a runaway situation with "fast air speeding up forever."

SO... For these reasons, Feils is a bad reference.

On to the need to mention lift.

Calculating lift with Bernouli. Eastlake's physics paper describes this process, but is not used for reference. Since I originally had problems with this concept and didn't see the basis explained in his paper, this is one of the reasons I talked to him. The method is valid due to the common point ahead of the wing.

This statement: "established by Bernoulli over a century before the first man-made wings were used for the purpose of flight. Bernoulli's principle does not explain why the air flows faster past the top of the wing and slower past the underside."

Seems to imply that Bernoulli commented on the speeds over a wing. I have seen amateur claims that he did.

Question: Is there any indication that he did, or is this another unfounded assumption?

....

Lastly, the biggest item. I know this is lift, but it is mentioned here as well.

Data supplied to me by JPL's Kim Aaron (linked previously) clearly shows that when viewed from the still air frame of reference, the standard description of the air above a wing flowing faster than below is false. In short, what it shows is completely consistent with the correct description of Bernoulli's Principle. Namely, that the Upper air IS ACCELERATED more than the lower. In fact, this acceleration completely reverses the direction of travel of this bit if air. In the wind tunnel reference it is still accelerated more, but to a speed higher due to the frame of reference. In the stationary frame of reference we see the same acceleration, but different speeds making this word "faster" meaningless. Speeds are relative to the inertial frame of reference but accelerations are not.

-- Steve -- (talk) 03:05, 8 April 2019 (UTC)

..........

Conservation of energy

The present article states that the Bernoulli Principle can be derived from the Law of Conservation of Energy (LCE). Could someone clarify this claim? As I understand it (and I hasten to say I am not a physicist) the LCE states that the total energy in a closed system is constant. So for example, if total kinetic energy in a closed system is decreased, some other form of energy must have increased. But so far as I know the LCE in itself says nothing about the distribution of energy within a closed system. For example, if a liquid is circulating by its own momentum in a closed toroidal tube of varying cross section, the LCE does not tell us that total energy in any particular 'packet' of the liquid is constant. Nor does it say that total energy in different parts of the liquid, or in parts of the tube with different cross sections, is equal. But some such propositions seem necessary to derive the Bernoulli Principle as commonly defined. I am not suggesting that no such propositions would be true, just that I do not see how they can be derived from the LCE on its own.81.146.36.34 (talk) 19:29, 24 July 2019 (UTC)

I agree that it is rather bold to say Bernoulli’s Principle can be derived from the LCE. I would prefer it said BP is consistent with the LCE, or is an expression of the LCE. In the past, our article said something like “BP can be derived from conservation of mechanical energy”, but this was changed back to the LCE. If you re-examine BP in terms of the mechanical energy of a packet of fluid in a frictionless fluid flow I’m sure you will find it makes more sense.

Basic physics text books derive BP using the work-energy theorem and this is easy to comprehend. The work-energy theorem says that the change in kinetic energy of a body is equal to the work done on the body.

Your example of a fluid flowing in a closed toroidal tube is unlikely to be helpful. It will be better to focus on the static pressure and dynamic pressure at different points on the same streamline. If the streamline is horizontal, BP says that the total pressure (static plus dynamic pressure) is constant along the streamline. Moreover, in any region that is free of vorticity the total pressure is constant along ALL (horizontal) streamlines. Dolphin (t) 22:29, 24 July 2019 (UTC)

Besides classical conservation of energy, additional assumptions are needed which restrict the types of energy transformations that can occur. For instance, one assumes there is no dissipation as would otherwise occur in the presence of viscous forces or heat fluxes. The absence of these dissipative forces means there is no energy exchange between parcels of fluid on different streamlines, such that (speaking imprecisely) the distribution of energy among streamlines, once specified, is fixed everywhere and for all time. An alternate way to think about it is that the Bernoulli Principle is an experimental fact in certain systems which, in combination with classical conservation of energy, necessitates that certain kinds of energy transformation are negligible in those systems.

Anyway, I think the current article is fine. Even though it may be a little imprecise in the introduction, the body of the article does state the additional assumptions that must be in place in order to derive Bernoulli's principle from conservation of energy.MaxwellMolecule (talk) 00:25, 25 July 2019 (UTC)

Also the absence of dissipative forces means no mechanical energy is lost to heat, so only conservation of mechanical energy has to be considered.MaxwellMolecule (talk) 00:28, 25 July 2019 (UTC)

Or, idk, maybe it would be better to reword in the introduction. Replace

"Bernoulli's principle can be derived from the principle of conservation of energy."

with something like

"Bernoulli's principle is an expression of the principle of conservation of energy in isentropic flows." MaxwellMolecule (talk) 00:38, 25 July 2019 (UTC)

@MaxwellMolecule: Thanks for suggesting an improved alternative. However, I caution against your wording on one point - it suggests that an understanding of Bernoulli requires a prior understanding of “isentropic flows”. Most people will be introduced to Bernoulli long before they are introduced to the concept of entropy. Dolphin (t) 06:46, 25 July 2019 (UTC)

36.34 & Maxwel,

I also don't like the idea of starting the explanation of BE being derived LCE because even though the constant energy idea is valid and allows you to simply use the sum as the standard, it implies that the LCE is the ultimate explanation of the science. I prefer to think of it as a consequence of the science, if not a given requirement. If you don't add, nor remove energy from the system, energy is constant by definition (or a condition for applying Bernoulli's Equation.

The (Bernoulli is derived from LCE) claim is simply that as you follow some fluid around the system and see the kinetic energy change there will be an accompanying opposite potential energy change. Knowing those two values at one point, but only one at a second point allows you to calculate the second value at the second point. It precisely DOES tell us that the total energy in a packet remains constant along its journey. That is the real essence of the LCE and why the Bernoulli Equation is correct.

Framing it the way you did by decreasing the total kinetic energy in your toroid is an indication that energy has been removed and the velocities all around the system have decreased. In this case you can't slow the fluid down and expect the static pressure to increase. You have changed the flow, so the Bernoulli constant (and total energy) has changed. You must leave the system alone and let it do what it is doing.

[This seems to indicate that you may have the common misunderstanding that "Fast air always has a lower pressure than any unrelated slower or still air" because that also is dealing with two different flows which is not covered by Bernoulli's Equation]

The "is an expression of LCE" is fine.

This may not be the place to discuss/learn the science except to make the article more clear.

On another, but possibly related item, I would like to see the article make a very clear distinction between Bernoulli's Principle and Bernoulli's Equation. They are not the same thing. BP is a general statement of the inverse relationship of V and P without making clear the required conditions (and appears to be the very cause for the widespread misconception about BP. BE is the precise mathematical relationship between the values and must be applied to a single uniform flow and you must know that to apply it properly.

Regards -- Steve -- (talk) 03:00, 25 July 2019 (UTC)

Energy conservation is certainly more fundamental than Bernoulli's principle, and is true even when Bernoulli's principle is not. That's why I wouldn't mind writing "Bernoulli's principle is an expression of (or consequence of...) energy conservation … in certain kinds of flows …" Something like that.

@EngineerSteve It might indeed be appropriate for the article to make a clear distinction between Bernoulli's principle and Bernoulli's equation, since I've myself become slightly confused about what's meant by what. Right now I'm going off of my general physics background … a couple solid fluid dynamics sources would be helpful for the discussion. MaxwellMolecule (talk)

I think the venturi meter diagram is misleading

It has a direction arrow, and the word "flow" - however - the effect is the same regardless of the flow direction, isn't it ?

I think the diagram arrow should be amended to be double-headed, so everyone understands that this is not dependant on direction. — Preceding unsigned comment added by 110.143.72.26 (talk) 05:03, 9 February 2020 (UTC)

When using potential flow theory to model the flow as frictionless, you are right, and changing the flow direction (keeping the flow speed the same) will give the same pressure difference. However, for a real fluid the results will be different: for an accelerating flow (as shown in the diagram) "Bernoulli" applies well – over short distances, when friction along the pipe wall has a minor effect as compared to the pressure drop according to the Bernoulli equation – while for a decelerating flow it (often) does not. This is due to (near) flow separation in case the pipe cross-sectional area expands in the downstream direction, leading to an additional pressure drop in the flow direction due to frictional losses. This pressure drop is larger when the flow expansion is more sudden. So for measurement, Venturi meters are used with the flow direction as shown in the diagram. -- Crowsnest (talk) 21:29, 28 February 2020 (UTC)

I agree with Crowsnest. Some relevant information is available at Adverse pressure gradient. Dolphin (t) 04:02, 29 February 2020 (UTC)

I think someone messed with the 'normalized' bernoulli equation

The "3rd part" there shouldn't be, I'm erasing it and it can be reviewed by someone later. --181.169.118.48 (talk) 04:22, 10 August 2020 (UTC)

Three lines above the equation in question, piezometric head h (or hydraulic head) is defined as:

h = z + .mw-parser-output .sfrac{white-space:nowrap}.mw-parser-output .sfrac.tion,.mw-parser-output .sfrac .tion{display:inline-block;vertical-align:-0.5em;font-size:85%;text-align:center}.mw-parser-output .sfrac .num{display:block;line-height:1em;margin:0.0em 0.1em;border-bottom:1px solid}.mw-parser-output .sfrac .den{display:block;line-height:1em;margin:0.1em 0.1em}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}⁠p/ρg⁠

Total head H is defined as:

H=z+{\frac {p}{\rho g}}+{\frac {v^{2}}{2g}}

Seeing

h = z + ⁠ p / ρg ⁠

then H can be written as:

H=h+{\frac {v^{2}}{2g}},

The equations are mathematically correct so your deletion was unjustified. I suggest you restore the original version. Dolphin (t) 13:42, 10 August 2020 (UTC)

Applications: black-tailed prairy dog

The mechanism proposed in the cited article on the wind-induced ventilation in the burrows of prairy dogs seems very implausible to me. As is well known, the pressure in a boundary layer, such as in the wind flow near the Earth surface, remains constant at different elevations at the same cross section (normal to the surface). Bernoulli's principle for such a flow with vorticity is applicable along a streamline, and not for different streamlines at different elevations. Although I do think (my "original research") that Bernoulli's principle – on different grounds as used in the cited paper – may to a certain extend be used to explain the natural ventilation, as induced by the different shaped mounds at the different entrances of the burrow. But due to lacking secondary sources on the proposed mechanism in established fluid-dynamics journals and books I suggest to remove this example. -- Crowsnest (talk) 20:33, 20 April 2021 (UTC)

Thank you for your concern and explanation. This is beyond my realm of expertise, so I will remove the example for now until I have a better understanding of the topic or another party weighs in. Thanks again Viséan (talk) 20:40, 20 April 2021 (UTC)

Thanks. Sorry I could not support the proposed mechanism in your interesting example of natural ventilation. -- Crowsnest (talk) 20:46, 20 April 2021 (UTC)

Compressible flow

Our article contains a section titled Bernoulli's principle#Compressible flow equation. This section explains that:

Bernoulli developed his principle from observations on liquids. His equation is applicable only to incompressible fluids but it can be used with minimal error at speeds up to approximately Mach number 0.3 in compressible fluids. It is possible to use the fundamental principles of physics to develop similar equations applicable to compressible fluids. There are numerous equations, each tailored for a particular application, but all are analogous to Bernoulli's equation and all rely on nothing more than the fundamental principles of physics such as Newton's laws of motion or the first law of thermodynamics.

This is a summary of compressible flow that is most appropriate in the article about Bernoulli's principle. However, what follows are three sub-sections with the following titles:

Compressible flow in fluid dynamics
Compressible flow in thermodynamics
Unsteady potential flow

I think the most appropriate place for this information is our article on Compressible flow rather than our article on Bernoulli's principle. I suggest these three sub-sections should be cut from our article on Bernoulli's principle and pasted into Compressible flow.

My reasoning for proposing the relocation of these three sub-sections includes:

The quotation above explains that Bernoulli’s equation is strictly applicable only to incompressible fluids. It is contradictory to then present detailed information about compressible fluids; especially when we have an article dedicated to compressible fluids.
These three sub-sections contain high-level physics and math. In particular, the math is more advanced even than the math presently found in our article Compressible flow. The math is out of place in Bernoulli's principle.
The great strength of Bernoulli's principle is that in regions of irrotational flow the Bernoulli constant is the same along all streamlines and so can be applied broadly to large regions of steady flow around streamlined bodies. This is stated explicitly in the section Simplified form. However, as explained by Crowsnest in this edit, in compressible flows the equivalent of Bernoulli's constant is only applicable along a streamline and so does not display the same broad applicability that exists for Bernoulli's constant.

What do others think? Dolphin (t) 12:40, 7 April 2022 (UTC)

Moving this material to the compressible flow article makes sense to me. Mr. Swordfish (talk) 20:47, 7 April 2022 (UTC)

To my opinion these general forms of the Bernoulli equation should be here. Both Batchelor (1967, §3.5) and Landau & Lifshitz (1987, §6) start with deriving the Bernoulli equation for the general case, i.e. as given in the sections valid for compressible flow. They mention that Daniel Bernoulli derived them first for incompressible flow. Also "Prandtl's Essentials of Fluid Mechanics" (2004, §4.1.1) clearly states that the compressible flow formulation is "... the general form of the Bernoulli equation ...", the subject of this article. Crowsnest (talk) 20:11, 8 April 2022 (UTC)

If?

If we cannot explain the lifting effect of an airfoil, or the reasoning of why a ball can remain suspended in an airstream using the Bernoulli Principle, how can we explain these phenomenon? Science teachers need a reasonable explanation so as not to confuse young'un's. Flight Risk (talk) 21:21, 12 September 2022 (UTC)

We can explain the lift force that acts on an airfoil. There are several ways to explain lift, all valid, and Bernoulli’s principle is is one of them. We can explain a ball remaining suspended in an airstream.

These phenomena are relatively complex and not entry-level topics for young’uns. A more appropriate topic for newcomers to the physical sciences is Bernoulli’s principle in general, using the venturi and the airfoil merely as examples of phenomena for which we make use of Bernoulli when explaining their principal characteristics. Dolphin (t) 22:18, 12 September 2022 (UTC)

There are people who can. Amateurs need to stay out of it because they are just repeating / spreading misconceptions. This includes Ed Regis 'technical author' who wrote the Feb 2020 Scientific American article after ignoring the advice he got.

Science teachers do need it, badly, but there is so much bad information out there that it is virtually impossible to get anything good.

Bernoulli's Principle does not 'explain' why or how lift occurs, nor the ball-centering. Most people stating how it works, while well meaning, misunderstand it - and what Euler said in the mid 1700s while following up on Bernoulli's work.

THE FUNDAMENTALS ARE SIMPLE, but there are quite a few that apply and few people understand them and know that they apply.

I know they want to be helpful because they were told / taught something by someone who spoke with authority, but it was wrong, so well meaning amateurs need to stay out of it.

The Talk page is not supposed to be to learn the topic. If you want good information for yourself, please go here: https://rxesywwbdscllwpn.quora.com/

-- Steve -- (talk) 18:47, 14 September 2022 (UTC)

Misunderstandings about the generation of lift

This section should be closely reviewed, and perhaps removed. For one thing, some of the references are IMO not of sufficient grade to serve as reference for an encyclopedia entry on fluid dynamics. E.g. an article in a pilots magazine is hardly a good reference for a fluid dynamics topic. Often such articles are themselves based on Wikipedia information and therefore create circular references, rather than reliable ones. But the main reason this section needs work is that it feeds the very misconception it claims to dispel. It clearly assumes a connection between the flow above and below the wing via Bernoulli's law, which is incorrect. Just read this very article. Bernoulli's principle establishes a relation between different points upstream/downstream of one another within the same flow. There is no relationship that this principle establishes between different flows. The flows above and below a wing are different flows, physically separated by the wing. Thus Bernoulli's principle does not establish any relationship between the respective velocities or pressures. — Preceding unsigned comment added by 73.189.225.197 (talk) 17:55, 23 March 2022 (UTC)

Actually, as explained in one of Charles Eastlake's papers, Because the energy ahead of the wing is the same for both the soon to be upper and lower flows (streamlines), Bernoulli's Equation does indeed provide a reasonably good *calculation* of the lift force, but it in no way explains the physics. I spoke with Eastlake myself to clarify his paper. It is easily searchable. The Physics Teacher Vol 40 March 2001 : "An Aerodynamicist’s View of Lift, Bernoulli, and Newton" 19:07, 14 September 2022 (UTC)

-- Steve -- (talk) 19:07, 14 September 2022 (UTC)

Just to make sure this does not simply remain a comment. This https://www.youtube.com/watch?v=XWdNEGr53Gw youtube video shows a lecture about the basics of lift. From minute 29 through minute 31 the lecturer addresses this very topic. The flows above and below the wing have no relationship that relates through Bernoulli's Principle. — Preceding unsigned comment added by 73.189.225.197 (talk) 01:10, 27 March 2022 (UTC)

I disagree. At its most basic level, Bernoulli’s principle applies only to points along one streamline. But in a region of irrotational flow, the Bernoulli constant is the same along every streamline. (I think our cited source for this is Victor Streeter’s textbook - see reference No 6 which applies to the third para in the lead.) The flow outside the boundary layer is irrotational so streamlines above and below are part of the one region of irrotational flow, and they all share the same Bernoulli constant. Dolphin (t) 01:18, 27 March 2022 (UTC)

I have watched the YouTube video you identified above. Prof Babinsky is talking about blowing with his mouth across the top of a piece of paper. He is also talking about using a hair dryer to blow across the top of an airfoil. (He is not talking about an airfoil moving through the atmosphere.) He correctly states that “in general, the Bernoulli constant along one streamline is different to the constant along any other streamline.” That is true in general, where the generality includes the flow in boundary layers and other regions of rotational flow. But explaining lift by using Bernoulli’s principle uses a much simpler model of the flowfield where we ignore the presence of the boundary layer.

Prof Babinsky explicitly mentions streamlines flowing out of a reservoir and says all those streamlines might share a common Bernoulli constant, and he is correct. The atmosphere is a large region of uniform energy and therefore streamlines in the atmosphere around a wing share a common Bernoulli constant, just like the streamlines flowing out of a reservoir.

In summary, when Prof Babinsky focuses on the fact that the Bernoulli constant above is different to that below, he is not talking about a wing moving through the atmosphere; he is talking about blowing over a piece of paper, and other classroom experiments. Dolphin (t) 06:56, 27 March 2022 (UTC)

Well, number one, this article is about Bernoulli's principle and not about lift. As such, it is important to be clear about the fact, that the principle only applies along a streamline. You can't argue that point. As such, any statement that pulls this into question is problematic and reduces the quality of the article.

Second, it does not matter if there is a piece of paper between two random streamlines, a wing, some other object or nothing at all. The same constant does not apply. In that it also does not matter how a flow is generated, from a reservoir, from one's mouth, a hair drier or some more sophisticated method. Therefore arguing that there is an example with a piece of paper, but a wing is something very different, is just word play. It has no practical meaning.

Third, Prof. Babinsky is very clear that while man might not make too big a numeric error by assuming the same constant in a narrow field of parallel flow, the values are very different between the top and bottom of a wing indeed.

The obvious intent here is to further a misconception, in that Bernoulli's principle somehow is the cause of the different pressures above and below a wing. This is wrong. Besides, this sort of discussion is off topic for this article as it does not concern itself with lift, but with Bernoulli's principle as such. 73.189.225.197 (talk) 18:05, 27 March 2022 (UTC)

I would suggest reading Holger Babinsky's article on the subject: http://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-turbine/howwingswork.pdf Paying particular attention to the following passage:

However, the fact is often overlooked that Bernoulli’s equation applies only along a stream-line. There is no explicit relationship between the pressure and velocity of neighbouring streamlines. Sometimes, all streamlines in a flow originate from a region where there is uniform velocity and pressure (such as a reservoir or a uniform free-stream) and in such a case it is possible to apply Bernoulli’s equation throughout the flow.

Perhaps it would help clear up some of the confusion.

As for the section under consideration, I do think it could be improved and I'll have some suggestions in a day or so. Mr. Swordfish (talk) 19:18, 27 March 2022 (UTC)

In Fundamentals of Aerodynamics by John D. Anderson Jr (1984, McGraw Hill) on page 117 it states

For a general, rotational flow, the value of the constant in Eq. 3.14 will change from one streamline to the next. However, if the flow is irrotational, then Bernoulli’s equation holds between any two points in the flow, not necessarily just on the same streamline. For an irrotational flow, the constant in Eq. 3.14 is the same for all streamlines, and:
$p+{\frac {1}{2}}\rho v^{2}=const$ throughout the flow.

This quote shows the importance for aerodynamicists of always clarifying that they are talking about irrotational flow. The editor who initiated this thread does not mention the word irrotational so I assume he is unaware of its significance. Dolphin (t) 04:59, 28 March 2022 (UTC)

My takeaway at this point is that this section may be confusing, at least to some readers. The statement:

Several of these explanations use the Bernoulli principle to connect the flow kinematics to the flow-induced pressures. In cases of incorrect (or partially correct) explanations relying on the Bernoulli principle, the errors generally occur in the assumptions on the flow kinematics and how these are produced.

is certainly correct and well sourced, but seems overly general and difficult to follow for someone who is not already familiar with the material. The terms "flow kinematics" and "flow-induced pressures" are likely unfamiliar to the majority of our readers. Rather than trying to craft a general statement that applies to the many different incorrect applications of BP to lift I think it would better serve the reader to highlight the most common one as a specific example and link to the article on Lift for further exposition. Seems to me that the section should make the following points

A very common explanation of lift mis-applies BP (with a very brief summary of equal transit time and why it's incorrect)
The fact that BP is commonly misused in this circumstance does not imply anything is wrong with BP
BP is commonly used correctly as part of a mathematical treatment of lift.

I'm not convinced that this is the best place to delve into the Bernoulli v Newton "controversy", but I'm ok with it remaining as long as we keep it short. Looking for feedback on this approach from other editors; if received positively I'll take a crack at crafting a draft. Mr. Swordfish (talk) 18:14, 31 March 2022 (UTC)

I agree that the sentences you quote are inappropriate in this article. Removing them will be an improvement. Your proposal looks good - I encourage you to go ahead with developing the three points you have given us. Dolphin (t) 23:24, 31 March 2022 (UTC)

A quick comment about your points 1 and 2: The equal transit-time fallacy is based on false reasoning for the essential kinematics of the flow field; subsequent application of Bernoulli’s principle is entirely separate from speculation about the kinematics; application of BP shouldn’t be characterised as mis-application or misuse. Application of BP to any region of irrotational flow is appropriate and correct but if the assumed kinematics are inaccurate or incorrect, the resulting pressures will be equally inaccurate or incorrect, but that doesn’t constitute misuse or mis-application of BP. Dolphin (t) 13:34, 1 April 2022 (UTC)

Yes, you are correct. The Equal Transit Time Fallacy (ETT) does not misapply Bernoulli's equation; it starts with a "nonsensical" physical assumption about why the air is faster over the top of the wing and proceeds to correctly apply the equation to infer a lower pressure due to the increased speed. I'll be careful about the wording in the draft. Mr. Swordfish (talk) 23:45, 1 April 2022 (UTC)

I have composed a draft revision of this section in my sandbox https://en.wikipedia.org/wiki/User:Mr_swordfish/sandbox. Comments cheerfully accepted. It does not contain any references yet. If it receives positive responses I will add them. Thanks. Mr. Swordfish (talk) 20:50, 7 April 2022 (UTC)

I have made some suggestions on Mr Swordfish's sandbox. See my diff. Dolphin (t) 12:59, 8 April 2022 (UTC)

There is now a release candidate draft in my sandbox. I'll release it in a few days unless there is further comment. Mr. Swordfish (talk) 11:57, 9 April 2022 (UTC)

A single word was recently added to this section:

One of the most common erroneous explanations of aerodynamic lift ...

And this is certainly a correct and supportable statement.

However, it is also correct and supportable without the qualifier "erroneous", which is a stronger statement. i.e. it's not just one of the most common erroneous statements, it's one of the most common explanations, full stop. My preference is to remove the qualification, but let's try to come to a consensus before making that change. Further discussion? Mr. Swordfish (talk) 02:24, 13 April 2022 (UTC)

The word “erroneous” has recently been added to the new text constituting this sub-section: see the diff.

My preference is to retain the word erroneous because I think it more accurately reflects the situation described in the cited sources. The previous statement, that the Equal Transit Time explanation was “one of the most common explanations of aerodynamic lift” appears to me to overstate the situation:

How many different explanations are commonly used is unknown, or at least uncited,
The number of times each explanation is used is also unknown, and unknowable,
The number of times the ETT is used is also unknown and unknowable.

So we can’t honestly say the ETT is one of the most common explanations of aerodynamic lift. However, we have a better idea of the small number of incorrect explanations of aerodynamic lift, and we can be confident that the ETT is prominent among them. Therefore I don’t have any objection to saying the ETT is one of the most common erroneous explanations …

The first of the cited sources, Physics that Works by Kendall Hunt Pub Co., says “One of the most widely circulated, but incorrect, explanations …” This citation uses the word “incorrect” so doesn’t support our original statement that the ETT is one of the most common explanations of lift.

The second of the cited source, Norman F Smith in The Physics Teacher, doesn’t use the word “erroneous” or any synonym, but it is dated November 1972, almost 50 years ago. I am biased against this sentiment because my first serious physics book, Physics by Resnick and Halliday, was widely used in Universities and Colleges and first copyrighted in 1960. In Chapter 18, Fluid Dynamics, it contains an accurate description of aerodynamic lift. It contains an accurate diagram of the streamlines around an airfoil. There is not the slightest hint of the ETT explanation of the kinematics. I don’t doubt that the ETT was widely used in literature aimed at student pilots and newcomers to aviation but serious literature such as that by Resnick and Halliday, aimed at millions of students of science and engineering, presented a description of aerodynamic lift that must be considered scrupulously correct even today. Dolphin (t) 12:46, 13 April 2022 (UTC)

Our role here is to summarize the information provided by reliable sources and present it to our audience in a readable form. To answer your points 1,2, and 3 above, we don't need to (and shouldn't) research it ourselves; we simply look to the reliable sources and see what they have to say.

Smith refers to ETT as "...the textbook explanation that is more or less standard in the United States..."

The NASA website describes it as "... one of the most widely circulated, incorrect explanations." Note the use of the comma, which implies that it is both widely circulated and incorrect. If they were trying to say it was the one of the most widely circulated incorrect explanations, they would have omitted the comma. (https://web.archive.org/web/20140427084226/http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html)

Holger Babinsky describes it simply as "...the most widely used explanation of lift..." (http://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-turbine/howwingswork.pdf)

Anderson & Eberhart describe it as "...the popular explanation that most of us were taught..." indicating that it is perhaps the most widely circulated explanation, but surely is one of the most.

Your reading of the excerpt from Physics that Works is very different than mine. The author seems to be saying that ETT is one of the most widely circulated explanations, and that it is also incorrect. Not that it is just one of the most widely circulated incorrect explanations.

The cites above show that we would have a very solid basis for omitting the word erroneous. The fact that ETT is one of the most widely circulated explanations should be uncontroversial. Is there anyone claiming that it is not one of the most widely circulated explanations?

And if you're still not convinced that it is widespread, have a look at https://en.wikipedia.org/wiki/User:Mr_swordfish/List_of_works_with_the_equal_transit-time_fallacy

My takeaway here is that whether to include erroneous or not is an editorial decision about what to emphasize, rather than a disagreement on the facts as supported by reliable sources. We do need to be clear that ETT is incorrect and including erroneous in the first sentence emphasizes that fact, but the title of the section and the second paragraph seem to be sufficiently clear to make the insertion of the word erroneous superfluous. Mr. Swordfish (talk) 22:19, 13 April 2022 (UTC)

A further comment on the Norman Smith article: I think you are missing the context, since he goes on to say

"Unfortunately, this explanation [fails] on three counts. First, an airfoil need not have more curvature on its top than on its bottom. Airplanes can and do fly with perfectly symmetrical airfoils; that is with airfoils that have the same curvature top and bottom. Second, even if a humped-up (cambered) shape is used, the claim that the air must traverse the curved top surface in the same time as it does the flat bottom surface...is fictional. We can quote no physical law that tells us this. Third—and this is the most serious—the common textbook explanation, and the diagrams that accompany it, describe a force on the wing with no net disturbance to the airstream. This constitutes a violation of Newton's third law."

This paper is to the best of my knowledge the first clear refutation of ETT in peer-reviewed literature. Yes, it's 50 years old, but it still stands up. I think you would enjoy reading it. Mr. Swordfish (talk) 22:21, 13 April 2022 (UTC)

This discussion thread shows that some readers believe Bernoulli's equation is only of limited value because it is only valid along a streamline. This misunderstanding comes as no surprise because our article repeatedly mentions Bernoulli's principle in the context of the streamline. All reliable published sources describe the application of the Bernoulli constant along a streamline simply as a learning aid; these sources then proceed to clarify that where the energy per unit of mass of fluid, or the energy per unit of volume, is uniform the Bernoulli constant does not vary among streamlines. In the flow of an inviscid fluid, in regions of frictionless flow, and regions of irrotational flow, the Bernoulli constant is uniform throughout the region. The energy per unit of mass of a fluid is uniform throughout a reservoir; therefore where a region of flow is driven by a fluid leaving a reservoir, the energy per unit of mass is uniform and the Bernoulli constant is the same on all streamlines. Bernoulli's equation can then be applied throughout the region.

I have erased the multiple mentions of the streamline from the article. See my diffs. Hopefully the article now makes it easier for readers to see that Bernoulli's principle has much broader applicability than just along a streamline. Dolphin (t)