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

Wikipedia:Be bold, if you know it's wrong, correct it! :) Dysprosia 02:38, 16 Oct 2003 (UTC)

(THIS IS NOT TRUE) is not a good way of correcting. :) -- Jake 02:45, 16 Oct 2003 (UTC)

In other words anon, could you provide the correct information? Thanks Dysprosia 02:46, 16 Oct 2003 (UTC)

Contents 1 Wow I can tell a bunch of ME's wrote this stuff bc they use the simplified versions, call them selves college grads too 2 Why Bernoulli's Principle should not be merged with Lift Force 3 Proposed merge of Bernoulli's principle and Bernoulli's equation 4 New Source 5 Disagreement with the rating of this article 6 Radiative shocks? 7 Bernoulli's Equation is crap?

[edit] Wow I can tell a bunch of ME's wrote this stuff bc they use the simplified versions, call them selves college grads too

Everybody relax. Both explanations are right. They are equivalent. I'll fix this to make that a little more clear.

What about Bernoulli's Theorem in discrete mathematics? I think that either Bernoulli's Theorem should not be redirected to here or that there should be a disambiguation page. (Sorry, don't have time to make an article.)

Removed "There are other ways of understanding aerodynamic lift that many novices find more intuitive (see Coanda Effect)." The Coanda effect is not a valid explanation of airfoil lift - it is completely explained by the existence of a bound vortex, and can be fully explained in the absence of viscosity (i.e., potential flow); Bernoulli's equation is a convenient way of relating the pressure and velocity at one point to another point as an inviscid fluid moves from one place to another (i.e from upstream to above/below a wing). Fluid-dynamic lift is independent of the completely viscous corner-turning Coanda effect, which is actually a type of vectored thrust. --Knotnic 00:37, 8 August 2005 (UTC)

see [1] for a well-written discussion by a physicist and flight-instructor

"Bernoulli's principle...is named for the Dutch/Swiss mathematician/scientist Daniel Bernoulli, though it was previously understood by Leonhard Euler and others." Could someone please clarify what was understood by others before Bernoulli, and what (if anything) Bernoulli added? --Judah

[edit] Why Bernoulli's Principle should not be merged with Lift Force

There are several different lift forces, notably the magnus force, the force from the coanda effect, and (the most common explanation for aerodynamic lift) Bernoulli's principle. Furthemore, there are many applications of Bernoulli's Principle that do not have to do with aerodynamic lift -- notably the venturi effect and choked flow. (I also mentioned this in the discussion linked-to by ther "merge" block). If you are going to merge Bernoulli's Principle with Lift Force please adress this concern first. zowie 23:21, 22 October 2005 (UTC)

Since nobody seems to have spoken out for merging Bernoulli Principle and Lift Force in quite a while, I took the liberty of removing the merge tag. zowie 23:28, 22 October 2005 (UTC)

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Some corrections... 1) the coanda effect has nothing to do with viscosity. A gas will follow a curved surface, because if it doesnt, then it leaves the surface... and as it flows past the surface, it drags out some of the stationary gas between the moving gas stream and the fixed surface. This continues until there is no stationary gas between the two. In reality, we dont have this detached-then-reatached behaviour. Any tendency for the gas flow to leave the surface immediately results in the gas in the stationary pocket being dragged out. Coanda effect only explains why the gas (or liquid) WILL follow a convex surface. That gas diversion and the gas diversion under a wing explains ALL of the effect of wing lift. No other explanation is necessary. Several other mathematical relationships exist, but they do not explain how a wing works. The Bernoulli equation will give a precise relationship between the gas velocity and pressure etc. and this is often assumed to show that it this explanation for wing lift is correct. It is not correct because the assumed cause and effect relationship is invalid. It is the pressure differences that cause the velocity profile, not visa vers.

2) the Bernoulli equation is an energy balance equation. It states that the energy at all points in a steady state flow will have the same energy. It does not say any one item causes the other.

3) No mass EVER accelerates without an applied net force. In gasses, this means that the acceleration (Change in velocity) of any particle of gas is ALWAYS the result of the pressure gradient along the flow path. Common sense (as well as complex physics proof)shows that where the velocity distribution is entirely the result of the pressure gradient, the pressure gradient cannot be the result of the velocity gradient.

The confusion relatated to venturi effect and Bernoulli that has reinforced this plethora of incorrect derived associated theory, is because it is difficult to understand how the lowest pressure in a venturi can be lower than any externally applied pressure. The function is easier to understand if you consider air as a bunch of lumps each having mass, and each being pushed uppon by other surrounding air acting like springs in all directions. Now simplify the model further... a row of masses with springs between them. All springs in compression. If we push one end toward the middle, we would expect the whole assembly to start moving. If we apply a restriction (some drag force) to one of the mass's in the middle, and continue pushing the mass at one end, then we get a build up in the spring compression on one side (increase in prssure). Now allow the restricted mass to escape, grabbing the next one as it gets to your restriction. The released one, jumps away.. and if we kept everything else still, that released one would then jump back and vibrate back and forth. BUT if we were to allow the next mass to go just as the first one was as far away from the second as possible, then there is less spring force difference to make the second one spring back. If we continue this rapidly and continuously we will have each succesive mass moving faster, and having less spring force on both sides of it, than all of the springs started with. This model would gradually reach a state of dynamic equilibreum. If we then added a controlled movement rate to the exit side of this model, we would similarly find that the fast moving middle masses with lightly compressed springs between them would slow and join the exit flow, and the springs between them would compress back to the initial level because it took a certain level of force, and absorbed energy to slow each of the moving masses.

If you want to prove or disprove any of this model of the venturi. Go back to first principles. Consider one atom, or one cubic millimetre of air. Then as WHY will it change speed. Why will it change pressure. If you really think about the cause and effect relationships, your answers will not include the word Bernoulli. F=ma adequately, simply, and completely describes all such gas flows without exception. It also describes all sub sonic wing lift without circulation theory, without using the word Coanda, and without the need for maths. Dave Fowler B.E.(Mech) UNSW M APESMA

[edit] Proposed merge of Bernoulli's principle and Bernoulli's equation

There seems to be redundancy here and it would be better to have a single page.--NHSavage 22:13, 28 April 2006 (UTC)

I'd support that. moink 22:23, 28 April 2006 (UTC)

I support it too - moreover, I support the original idea: to merge the Bernoulli effect with the coverage of the equation or principle. Eric Deeson UK 29 Apr

A draft for the merge is at: User:NHSavage/sandbox. Please feel free to comment and edit. I will leave it up there for a whole to see what comments emerge. I need to sort the interwikis.--NHSavage 08:48, 30 April 2006 (UTC)

As it all seems quite and comments here are positive I have finished the merge.--NHSavage 21:37, 2 May 2006 (UTC)

[edit] New Source

This theorem is explained in the following (excellent) book. Hydrodynamics Horace Lamb, 1932 Chapter 2, Art (21) Pg. 20

I'm not sure how to add it as a source in wikipia, if someone could that would be great. thanks, -Jonathan

[edit] Disagreement with the rating of this article

Hello,

The article has been listed as A class, however I would consider myself to have a reasonably good grasp of Bernoulli's principle and how it applies to general fluid flow, however I must admit the layout and content of the article seemed unclear to me. I was going to mark this with a cleanup tag, then i spotted the A class rating! I was somewhat surprised by this, especially when a reasonably non intuitive effect such as that of a Venturi nozzle was not given a diagram in its section (for example). Also uses/examples appear before an explanation of what it Bernoulli's principle is.

It is also not made clear that Bernoulli's is a way of accounting for the energy components of a fluid, perhaps something (very roughly) along the lines of

Bernoulli's principle is an important principle in fluid mechanics, it arises from the consideration of the different forms that energy can take in a fluid. There are several key forms for which a fluid can take, kinetic, pressure and gravitational potential. Bernoulli's principle implies that all these forms of energy (in an ideal fluid) must be conserved. This means that if there is an increase in velocity (kinetic energy, say over the top surface of a wing when compared to the lower surface) the energy must have been obtained from somewhere, and if no work is performed on the fluid, the fluid itself must have converted another form of energy into that increase in velocity, in this case pressure. Furthermore it explains why fluids can flow downhill (but not why, that requires the use of entropy) and so on.. HI72.10.127.110 17:50, 21 March 2007 (UTC)

Thanks User A1 12:21, 12 December 2006 (UTC)

[edit] Radiative shocks?

Could this bit please be explained further?

An exception to this rule is radiative shocks

Thanks --Chriswaterguy talk 13:38, 5 March 2007 (UTC)

[edit] Bernoulli's Equation is crap?

I am a physics student and my egineering/physics instructor was talking about how bernoulli's equation is pretty much crap and most of the stuff you learn about it doesn't apply to most cases. He was explaining how just about the only case that it applies to would be something like mercury flowing through a glass pipe because in that situation mercury has very low friction with itself and it doesn't compress easily and the adhesive forces and friction forces with the glass are extremely low. Otherwise you have adhesive forces and friction which completely change how the liquid flows and completely destroy how bernoulli's equation works.

He also explained how it can't be used for airfoil lift at all because the air compresses and is of course viscous and it has adhesive forces with the wing. Rather instead what causes the lift is a high pressure center above the wing toward the front and a lower pressure center toward the back and lower and that pressure difference causes a downward vector force which thus has an equal and oppsite compression on the wing moving the wing upwards. This seemed to explain lift much better than the age old, "The air splits and goes over the wing and because the distance across the top is longer the air has to speed up and this causes a lower pressure at the top and it sucks the wing up." For what reason if any does the air have to speed up? There is no reason what so ever why the same air that splits at the beginning of the wing has to meet up at the end of the wing.

Someone correct me if I am wrong on all this but this seems seriously messed up here. Ergzay 18:17, 6 April 2007 (UTC)

As far as i understand, the air over the top speeds up argument has been clearly proven incorrect. This is due to the fact that there is no need for a given set of air to "meet up" at the back of the wing, thus there is no need to speed up. So you are correct. There is a more complex analysis, of which I have not got to understanding sufficiently. However what I can say is that when solving the Navier stokes equation for a 2D wing, then calculating the integral of pressure over the wing, you do end up with an upwards force on the wing. The exact reason for this to me is somewhat unclear, but is definitely an effect predicted by Navier-Stokes in a steady state turbulent flow scenario (and is clearly a function of angle of attack).

Furthermore Bernoulli's equation is not "crap", but it definitely has assumptions built into it. You can actually take into account viscous losses as a "friction work" factor, which can then be calculated theoretically in the case of laminar flow, or you can work it out using correlations based around Reynold's number. Just remember that many calculations we do are simply a useful model of what is really going on, and this is also true here. Bernoulli's equation is a great way to figure out how much shaft work you need to drive low velocity newtonian fluids up a hill for example, and also predicts the siphoning effect User A1 14:28, 7 April 2007 (UTC)