bernoulli's principle

[xxChrisxx]

What, you mean like the law of Moses or Sharia law?

What an exceptionally odd thing to say.

[Per]

I don't like Bernouilli's principle. It is a correct observation and it helps 'seeing' (or even, to some degree, calculating) the lift of, for example, an aircraft wing. But many people are misled by the theory because they think that the observation that higher speed corresponds to lower pressure, means that higher speed causes lower pressure. That is bogus.

Newton's Laws are a much more sound starting point for understanding aerodynamics. An aircraft wing generates lift because it pushes the air downwards, so it in turn is pushed upwards by the air (3rd Law).

You see, some armchair aerodynamicists use (or rather abuse) Bernouilli's principle and say "the flow over the top of the wing is faster than the flow below the wing, causing lower pressure at the top and hence lift". But they never seem to stop to wonder WHY the air flows faster over the top. The reason is, of course, the lower pressure generated by the object moving through the air. The faster airflow is a side effect of the pressure difference, not the cause of it.

[xpensive]

I don't like Bernouilli's principle. It is a correct observation and it helps 'seeing' (or even, to some degree, calculating) the lift of, for example, an aircraft wing. But many people are misled by the theory because they think that the observation that higher speed corresponds to lower pressure, means that higher speed causes lower pressure. That is bogus.
...

Oh dear, you don't "like it", but perhaps you should read up a bit before you call it "bogus"?

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

[trinidefender]

I don't like Bernouilli's principle. It is a correct observation and it helps 'seeing' (or even, to some degree, calculating) the lift of, for example, an aircraft wing. But many people are misled by the theory because they think that the observation that higher speed corresponds to lower pressure, means that higher speed causes lower pressure. That is bogus.

Newton's Laws are a much more sound starting point for understanding aerodynamics. An aircraft wing generates lift because it pushes the air downwards, so it in turn is pushed upwards by the air (3rd Law).

You see, some armchair aerodynamicists use (or rather abuse) Bernouilli's principle and say "the flow over the top of the wing is faster than the flow below the wing, causing lower pressure at the top and hence lift". But they never seem to stop to wonder WHY the air flows faster over the top. The reason is, of course, the lower pressure generated by the object moving through the air. The faster airflow is a side effect of the pressure difference, not the cause of it.

In terms of lift production airflow as a result of Newton's third law actually doesn't provide a large percentage of lift at low Mach numbers. However at transonic speeds (speeds close to the speed of sound) and speeds above the speed of sound this percentage becomes larger.

According to your working "knowledge" of the decrease in pressure resulting in an increase in velocity I have this to say to you. There were tests conducted with puffs of smoke in a wind tunnel both over and under a wing. According to your logic, the puffs of smoke would reach the trailing edge of the wing at the same time if the speed increase was purely as a result of an increase in velocity over the top of the wing from a decrease in pressure. However it was found that the puffs of smoke above the wing reached the trailing edge of the wing before the puffs of smoke below the wing. Why exactly this happens is not fully understood. I myself am trying to do more research on it.

[hollus]

While this might not be 100% of what is happening, the shape of the wing forces the air past a convex surface (top surface in a plane, bottom in an F1 wing). Past a certain point in that convex shape, the molecules of air, if left to do on their own, would prefer to just go straight and leave a vacuum near that surface. Imagine that vacuum (only gets to be partial), that is what provides the lift. Now, since there are more air molecules next to that one that wanted to go straight, it gets pushed back to the wing. That's why the vacuum is only partial and why we can use the incompressible air approximation. And that same push, unopposed in the other side, is what we see as the air accelerating "over" the wing.
The air rushes to fill a vacuum. At the same time one could say that there is a (partial) vacuum because the air accelerated.
In my mind, below mach 0.3, the whole airflow is a holistic thing, where things behind affect things ahead as much as the opposite (diffusers are proof), and the partial vacuum and the acceleration are cause and effect at the same time. They are the necessary effect for the whole flow to take lowest energy state, or the path of less resistance. I am 100% with Per.

[Per]

I don't like Bernouilli's principle. It is a correct observation and it helps 'seeing' (or even, to some degree, calculating) the lift of, for example, an aircraft wing. But many people are misled by the theory because they think that the observation that higher speed corresponds to lower pressure, means that higher speed causes lower pressure. That is bogus.
...

Oh dear, you don't "like it", but perhaps you should read up a bit before you call it "bogus"?

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

If you read me properly you will see that I do not call the principle bogus, but the conclusion that some people draw from it. The principle relates velocity and pressure, but does not say that the increase of velocity is the cause of the decrease in pressure. However, many people think this is the case.

Ironically, your insightful link says, "Bernoulli's principle can also be derived directly from Newton's 2nd law." Which is more or less the point I was making.

[Per]

According to your working "knowledge" of the decrease in pressure resulting in an increase in velocity I have this to say to you. There were tests conducted with puffs of smoke in a wind tunnel both over and under a wing. According to your logic, the puffs of smoke would reach the trailing edge of the wing at the same time if the speed increase was purely as a result of an increase in velocity over the top of the wing from a decrease in pressure. However it was found that the puffs of smoke above the wing reached the trailing edge of the wing before the puffs of smoke below the wing. Why exactly this happens is not fully understood. I myself am trying to do more research on it.

How would my logic say that both puffs reach the trailing edge at the same time? I don't deny that airspeed over the top surface is faster. So the fact that the puff of smoke over the top will reach the trailing edge faster, seems quite trivial to me (yes the distance over the top surface is longer but that difference is small). Or am I misunderstanding your point?

About Newton's 3rd Law not accounting for a large percentage in lift, that's an interesting point and I hope you can provide more info (Google doesn't confirm your point but several websites including NASA confirm that both Bernouilli and Newton's 3rd Law are valid theories to explain lift).

[thepowerofnone]

Per is spot on here, quite how you can be researching this and make a comment like you did trinidefender is beyond me. Whilst normally I have a lot of time for your comments, this one is utterly misleading. Also, congratulations on what sounds like the broadest piece of research I have ever heard of in modern aerodynamics.

Whilst yes, it is true, in the traditional image of Newton's Third Law (momentum down on air = momentum up on wing) real experiments do not show significant enough flow deflection to account for all lift, this is because of viscosity, and vorticity, but make absolutely no mistake that the forces required to make a plane fly or whatever else must be transmitted solely through the air, otherwise physics breaks. I'm sure you know this, but your post wasn't in my opinion clear and could be misleading to the uninformed. It's because of shedding vortices that we don't see this, but if you were to try to bring those vortices to a stop with a wall and a no-slip condition you would experience a force in doing so. Ignoring the real world occurrence of vortex shedding Per is 100% correct, and since vortex shedding is a relatively advanced level of aerodynamics, his explanation was very good.

As he goes on to say, both you and him are actually agreeing about the pressure-velocity relationship: the fact that the puffs don't reach the trailing edge at the same time is precisely why the argument (on a traditional wing) of "flow on upper surface has further to travel in same time; therefore must travel faster; therefore causes low pressure and lift; thus flight" does not work, and that was all Per is on about. Yes if you accelerate a flow, pressure drops, but if you had two different pressures separated by a diaphragm and burst it, the pressure difference would cause an acceleration of flow, therefore the relationship is circular, where many casual observes treat p==f(v), v!=f(p).

[xpensive]

Never mind, for CFD you use Navier-Stokes equations anyways;

http://en.wikipedia.org/wiki/Navier%E2% ... _equations

This was virtually impossible to apply, at least not in 3D, until computers became powerful enough.

[trinidefender]

Per is spot on here, quite how you can be researching this and make a comment like you did trinidefender is beyond me. Whilst normally I have a lot of time for your comments, this one is utterly misleading. Also, congratulations on what sounds like the broadest piece of research I have ever heard of in modern aerodynamics.

Whilst yes, it is true, in the traditional image of Newton's Third Law (momentum down on air = momentum up on wing) real experiments do not show significant enough flow deflection to account for all lift, this is because of viscosity, and vorticity, but make absolutely no mistake that the forces required to make a plane fly or whatever else must be transmitted solely through the air, otherwise physics breaks. I'm sure you know this, but your post wasn't in my opinion clear and could be misleading to the uninformed. It's because of shedding vortices that we don't see this, but if you were to try to bring those vortices to a stop with a wall and a no-slip condition you would experience a force in doing so. Ignoring the real world occurrence of vortex shedding Per is 100% correct, and since vortex shedding is a relatively advanced level of aerodynamics, his explanation was very good.

As he goes on to say, both you and him are actually agreeing about the pressure-velocity relationship: the fact that the puffs don't reach the trailing edge at the same time is precisely why the argument (on a traditional wing) of "flow on upper surface has further to travel in same time; therefore must travel faster; therefore causes low pressure and lift; thus flight" does not work, and that was all Per is on about. Yes if you accelerate a flow, pressure drops, but if you had two different pressures separated by a diaphragm and burst it, the pressure difference would cause an acceleration of flow, therefore the relationship is circular, where many casual observes treat p==f(v), v!=f(p).

I know I was very broad and not very clear in my post it is just these days work has me bogged down and sleeping and studying when I do have time as a result of moving onto a new aircraft type and needing to get my type rating on that aircraft. Company is short on pilots so extra rotations for me as well.

Let me see if I can dig up some NASA articles and whatever else I have saved and get back to you with a more detailed and thorough approach to the whole question of lift being attributed to Bernoulli's theory and newtons laws relating speed to the whole thing.

Well food for thought. You take a piece of paper. Hold it at one end horizontally to your mouth. You will notice that the piece of paper hangs down progressively more the further away from where you are holding it. Blow along the top of the flat side of the paper and watch the piece of paper lift up. To me this would appear as the increase in velocity of you blowing air over the top of the piece of paper is associated with a reduction in pressure and henceforth lift on the piece of paper being created.

[trinidefender]

I do apologise as my replies before were apparently quite unclear and misinforming. Just have been spending a lot of time these days studying for a new helicopter type that I am moving onto so don't have to much time on my hands for thorough answers.

When I stated before that Newton's third law doesn't have as much to do with lift created I was more referring to the pure reaction force created by a flat wing placed at an angle to airflow.

When I said Bernoulli's principle accounts for more lift it should be applied to a wing with a curved upper surface. Re-reading my posts I can see how what I wrote sounds slightly odd and doesn't make much sense to others.

I do concede that above a wing the increase in velocity is created by airflow having to speed up to fill the "void" created by the wing moving through the air. Hence the lower pressure of the "void"

Here is some reading if others want it. Haven't read any of the other sections in the link though.
http://www.av8n.com/how/htm/airfoils.html#sec-bernoulli

I just have one issue to raise here, you stated that the distance over the top of a wing is not much longer than over the bottom of the wig. This an be true, it generally only applies to relatively thin symmetrical airfoils at fairly low angles of attack. If you increase the AoA of said wing the air separation point does not stay at the nose tip of the wing. The higher pressure below results in the separation between upper and lower flows to actually be slightly on the underside of the wing surface. Making the wing thicker exacerbates this situation.

Now take the situation of an asymmetrical wing. Because the lower side is relatively flat and the upper surface is curved the upper surface is greatly lengthened compared to the lower. I hope I am still making sense here. That is why in most applications and most speed ranges asymmetrical wings are much more efficient because they rely more on the conventional thinking of Bernoulli's principle rather than simply "deflecting the airflow"

Look at this image


Finally, at transonic and supersonic speeds things start to get really messy, sound pressure waves start to build up on the front of wings etc. This would need its own thread to even scratch the surface on that topic, however supersonic military jets tend to use symmetrical airfoils because a. The centre of pressure of lift is generally more stable and b. they, as far as my understanding goes, work much better above the speed of sound. As such these jets more rely on the simple deflection of airflow downwards, hence large delta wing configurations.

Any confusion and I'll try to re-re-explain myself.

[thepowerofnone]

Per is spot on here, quite how you can be researching this and make a comment like you did trinidefender is beyond me. Whilst normally I have a lot of time for your comments, this one is utterly misleading. Also, congratulations on what sounds like the broadest piece of research I have ever heard of in modern aerodynamics.

Whilst yes, it is true, in the traditional image of Newton's Third Law (momentum down on air = momentum up on wing) real experiments do not show significant enough flow deflection to account for all lift, this is because of viscosity, and vorticity, but make absolutely no mistake that the forces required to make a plane fly or whatever else must be transmitted solely through the air, otherwise physics breaks. I'm sure you know this, but your post wasn't in my opinion clear and could be misleading to the uninformed. It's because of shedding vortices that we don't see this, but if you were to try to bring those vortices to a stop with a wall and a no-slip condition you would experience a force in doing so. Ignoring the real world occurrence of vortex shedding Per is 100% correct, and since vortex shedding is a relatively advanced level of aerodynamics, his explanation was very good.

As he goes on to say, both you and him are actually agreeing about the pressure-velocity relationship: the fact that the puffs don't reach the trailing edge at the same time is precisely why the argument (on a traditional wing) of "flow on upper surface has further to travel in same time; therefore must travel faster; therefore causes low pressure and lift; thus flight" does not work, and that was all Per is on about. Yes if you accelerate a flow, pressure drops, but if you had two different pressures separated by a diaphragm and burst it, the pressure difference would cause an acceleration of flow, therefore the relationship is circular, where many casual observes treat p==f(v), v!=f(p).

I know I was very broad and not very clear in my post it is just these days work has me bogged down and sleeping and studying when I do have time as a result of moving onto a new aircraft type and needing to get my type rating on that aircraft. Company is short on pilots so extra rotations for me as well.

Let me see if I can dig up some NASA articles and whatever else I have saved and get back to you with a more detailed and thorough approach to the whole question of lift being attributed to Bernoulli's theory and newtons laws relating speed to the whole thing.

Well food for thought. You take a piece of paper. Hold it at one end horizontally to your mouth. You will notice that the piece of paper hangs down progressively more the further away from where you are holding it. Blow along the top of the flat side of the paper and watch the piece of paper lift up. To me this would appear as the increase in velocity of you blowing air over the top of the piece of paper is associated with a reduction in pressure and henceforth lift on the piece of paper being created.

I feel like we are all agreeing on the details, just emphasising different areas of the theory. On the matter of your example, absolutely agree, and I have heard another explanation which makes sense to me: if you have a line of cars, all bumper to bumper, travelling at 10 km/h, then as the cars pass some line on the road, they all speed up to 20km/h, the gaps between the cars have to increase to maintain the flow of cars (effectively your mass flow). This affect occurs because the size of the car is not affected by its speed. These gaps effectively represent vacuums in the air, so the mean "gap length:total length" has increased, which if you think about it corresponds to a decline in pressure. All agreed on that front.

The area of the physics that Per was emphasising was that for traditional physics to hold, as you blow over the top of the wing, downstream the flow must be deflected downwards to balance forces, which is true if you ignore the shedding of vortices off of the wing; the area that I was emphasising is that in reality the viscosity causes a vortex to be shed at a regular frequency from the trailing edge of the wing, so instead of a deflection of the flow downwards you have this area of rotation in the air. Both Per and my explanations would require a force at a wall to arrest the flow, so both make physical sense.

The reason all three of us are arguing is because aerodynamics so overwhelming interlinked between so many things, which is what makes it one of the most challenging areas of study in academics (in my opinion). Per already made the point that pressure and velocity are interlinked.

[mcdenife]

Per Wrote:

But they never seem to stop to wonder WHY the air flows faster over the top. The reason is, of course, the lower pressure generated by the object moving through the air. The faster airflow is a side effect of the pressure difference, not the cause of it.

I think this is an over complication, the wings are shaped they way the are for a reason. The air flows faster over the top simply because of the shape of the wing and its not a side effect of the pressure difference but rather the cause of the pressure difference. That the faster airflow causes a pressure drop (or differential) is easily demonstrated by observing a fluid flow through a diffuser (or via dynamic pressure eqn (1/2 ρv^2)).

[Per]

My take on the sheet of paper is that due to viscosity, the airflow staying attached to the sheet is the lowest energy state (lower than letting the air separate and travel in a straight horizontal line causing a highly energetic wake behind the sheet). And then Newton comes in once again, saying "deflecting the air downwards requires a force, which can only be provided by the sheet, hence the sheet experiences an upward force".

Also, since your lungs are a source of higher pressure and behind the sheet there is only atmospheric pressure, again you see air accelerating because it flows from an area of high pressure (your lungs) to an area of low pressure (anywhere outside your lungs).

My aero knowledge is clearly smaller than that of thepowerofnone as I know only the very basics of vortex flows, the influence of compressibility and viscosity etc. So I am sure that my views are overly simplistic and therefore not very useful in real life. For that I apologize. I think this is also the reason why we are still discussing here, just as thepowerofnone is saying. But I am still speaking up loudly and proudly because whatever the theory and principle, you can never defy the basic laws of physics. Which is what some laymen are doing when falling into the trap of simplifying Bernouilli's principle. That's all I'm trying to fight.

Trinidefender, one final comment: in the link you posted, please check section 3.5 and then section 3.6. What you will find is that the approach through Newton and through Bernouilli are both correct for finding the lift of an airfoil. But, Newton is more widely applicable, and Bernouilli is mathematically a lot more elegant (and hence a lot easier). So again, Newton and Bernouilli are not in conflict. One is just a simplification of the other.

[Per]

Per Wrote:

But they never seem to stop to wonder WHY the air flows faster over the top. The reason is, of course, the lower pressure generated by the object moving through the air. The faster airflow is a side effect of the pressure difference, not the cause of it.

I think this is an over complication, the wings are shaped they way the are for a reason. The air flows faster over the top simply because of the shape of the wing and its not a side effect of the pressure difference but rather the cause of the pressure difference. That the faster airflow causes a pressure drop (or differential) is easily demonstrated by observing a fluid flow through a diffuser (or via dynamic pressure eqn (1/2 ρv^2)).

I will ask you one question in response: what is the condition for a particle of air to accelerate? (note - "simply because of the shape of the wing" is not considered a correct answer)

Answer this question, and you will see that your explanation is incorrect, whether you refer to an airfoil or a diffuser or whatever (I was thinking about the weather system in our atmosphere consisting of depressions and high-pressure areas and winds blowing between them, but that takes us a bit too far off-topic ).