What causes low pressure on a wing?

[Change in pressure]

Even today there are lot of misconception about how and WHY static pressure reduce above wing.

1)Faster air velocity cause low static pressure

One involves holding a piece of paper horizontally so that it droops downward and then blowing over the top of it. As the demonstrator blows over the paper, the paper rises. It is then asserted that this is because "faster moving air has lower pressure"

One problem with this explanation can be seen by blowing along the bottom of the paper: were the deflection due simply to faster moving air one would expect the paper to deflect downward, but the paper deflects upward regardless of whether the faster moving air is on the top or the bottom.Another problem is that when the air leaves the demonstrator's mouth it has the same pressure as the surrounding air; the air does not have lower pressure just because it is moving; in the demonstration, the static pressure of the air leaving the demonstrator's mouth is equal to the pressure of the surrounding air.A third problem is that it is false to make a connection between the flow on the two sides of the paper using Bernoulli’s equation since the air above and below are different flow fields and Bernoulli's principle only applies within a flow field.

2) Curved airflow above wing cause low static pressure

Why curved airflow cause low in pressure?
Because of centrifugal force make air molecules far appart so pressure drop or?

3) Why air follow curved wing surface insted goes in straight line?
Is reason for that visocsity which "stick" air to the surface or high ambinet pressure above wing push low pressure into upper wing surface?

 

[Mohankpvk]

This is a link to a video which explains the reason due to which the fluid follows the curvature of the surface.
In case of wings, the fluid follows the curvature and the inclination of the wings.So we can apply Newtons second law(in the vertical direction) to the stream of fluid to find the vertical force acting on it and third law can be used to find the reaction acting on the wings.This reaction is the lift force.
I think there is a different explanation using Bernoulli's theorem.

 

[Change in pressure]

This test with cup of tea is not coanda effect it is more about surface tension... (McLean in "Understanding Aerodynamics" states that the water deflection "actually demonstrates molecular attraction and surface tension.")
here is 50min talking only on lift misconception,even scientis can not arggue what cause lift.
this men write book only on lift misconception



one more just about what cause lift

 

[Mohankpvk]

This test with cup of tea is not coanda effect it is more about surface tension... (McLean in "Understanding Aerodynamics" states that the water deflection "actually demonstrates molecular attraction and surface tension.")
here is 50min talking only on lift misconception,even scientis can not arggue what cause lift.
this men write book only on lift misconception



one more just about what cause lift

I don't know about the cup experiment.But I learned in a forum that the sufficient condition for lift force in a wing is that the air stream should be redirected downwards(by any mechanism).Even the aerofoil cross section is not necessarily needed.The aerofoil cross section just increases the efficiency.Just an inclined plane can produce lift.(here we are not considering the case where beyond a certain inclination stalling occurs)
This is true even in case of turbines (can be analysed by using the velocity triangles).

 

[rcgldr]

The simplest explanation I've read is that a wing, even a flat plane at an effective angle of attack, sweeps out a volume of air as it travels through the air, leaving what would otherwise be a void behind the peak upper surface of the wing if the air didn't accelerate into the the volume swept out of what would otherwise be a void. If the flow doesn't detach, then it will tend to accelerate mostly downwards (lift) and somewhat forwards (drag). If the flow detaches, then what would otherwise be a void is filled with vortices, greatly reducing lift and increasing drag.

 

[Klystron]

Consider lifting body such as the XM-1 or the space shuttle fuselage ignoring wing surfaces. α angle of attack of the moving body generates lift as it travels through the fluid. In fixed wing aircraft increasing alpha also generates lift from wing surfaces while also inducing drag.

Rotating wing aircraft such as helicopters generate lift in similar fashion by varying rotor angle relative to the plane of rotation. The rotating "disk" operates as a "wing" at sufficient angular velocity relative to fluid flow.

Perhaps @boneh3ad can comment on fluid pressures and relevant equations that answer the original post.

Example: hold your hand flat in the wind generated by a moving car. At some modest forward speed rotate your hand at an angle (α) to the flow and feel your hand lift normal to the vehicle motion. I attached a picture of the M-1 lifter from the air museum mounted at high α; and video of an XM-2-F1 flight and an M-2 drop at Edwards.

 

[jim hardy]

There's at least one homebuilt lifting body airplane
https://en.wikipedia.org/wiki/Wainfan_Facetmobile

 

[boneh3ad]

https://www.physicsforums.com/insights/airplane-wing-work-primer-lift/

 

[jrmichler]

There's another homebuilt lifting body, the Dyke Delta. It's been around since the 1960's, and Wikipedia says about 50 were built. Owner report: https://www.eaa.org/en/eaa/eaa-chap...2016-07-the-rest-of-the-story-dyke-delta-n1aw

Also, John McPhee wrote an interesting book titled The Deltoid Pumpkin Seed about a not quite lighter than air lifting body aircraft.

 

[Change in pressure]

We turned away from topic,main question is way curved stremline cause low pressure?

 

[Klystron]

We turned away from topic,main question is way curved stremline cause low pressure?

Did not intend my post to cause thread to diverge. This excellent article addresses your post:

https://www.physicsforums.com/insights/airplane-wing-work-primer-lift/

While this article addresses streamlines and provides much useful analysis: https://www.physicsforums.com/insights/demystifying-the-often-misunderstood-bernoullis-equation/

Those of us interested in aerodynamics may have been trying to provide actual (aircraft) examples in place of teacups and paper .

 

[Change in pressure]

Did not intend my post to cause thread to diverge. This excellent article addresses your post:

While this article addresses streamlines and provides much useful analysis: https://www.physicsforums.com/insights/demystifying-the-often-misunderstood-bernoullis-equation/

Those of us interested in aerodynamics may have been trying to provide actual (aircraft) examples in place of teacups and paper .

https://user.uni-frankfurt.de/~weltner/Misinterpretations of Bernoullis Law 2011 internet.pdf

 

[boneh3ad]

https://user.uni-frankfurt.de/~weltner/Misinterpretations of Bernoullis Law 2011 internet.pdf

I don't know what point you are trying to make with this link, but it isn't strictly correct. There is really no way to make a claim that curved streamlines cause a pressure gradient rather than the other way around. You could just as easily argue that the pressure gradient acts as the centripetal force that bends the streamlines and you would still be correct. It's a chicken and egg situation. In reality, they are coupled phenomena and it would be essentially impossible to say that one causes the other.

Additionally, the Coandă effect applies to the behavior of a jet when it interacts with an object. It doesn't strictly apply to an infinite free stream flowing over a surface. That having been said, Coandă generally isn't taught in typical fluid mechanics courses because it's really just a subset of a larger phenomenon, which is that of the boundary layer that "sticks" to a surface as a fluid flows around it. In other words, the behavior is qualitatively similar in a free stream to that in a jet. The streamline curvature that results from viscosity can certainly still be used to describe the flow around a wing, but it would be improper to call this the Coandă effect, in my opinion. There are limits to this, however. Specifically, a sufficiently adverse pressure gradient can cause the boundary layer to separate and a large recirculating bubble to form at the surface, redirecting the free stream away from the surface.

Finally, the idea that energy conservation is not appropriate for the derivation of Bernoulli's equation is pure nonsense. Bernoulli's equation is equally a statement about conservation of energy as it is a statement about conservation of momentum (i.e. force balances). Neither interpretation or derivation contradicts the other. I take issue with the idea the article suggests that this is a source of confusion in the use of the Bernoulli equation. A well-taught unit on Bernoulli's equation and its origins should include all of the caveats on its use, which can be deduced directly from conservation of energy or momentum.

I should also mention that at no point in either of the two articles I wrote for this site did I claim that Bernoulli's equation could explain why low pressure forms on the upper surface and high pressure on the lower surface of a wing. In fact, I specifically mentioned that Bernoulli is not an explanation for how these flow fields form, and is only a tool that can be applied to a flow field that is already known to find lift.

Really, the biggest problem with how Bernoulli's equation is typically taught is not the way it is derived, but instead the fact that many teachers/instructors themselves do not know how it works or its limitations. In fact, I've never seen a university course make any of the various erroneous claims. That seems to be a problem that lives primarily in the realm of primary school science teachers and YouTube.

 

[Change in pressure]

Here is nice old school video where test curved stremline pressure change and also talk where bernoulli can not be applied.

 

[boneh3ad]

Here is nice old school video where test curved stremline pressure change and also talk where bernoulli can not be applied.

Nothing in that video contradicts anything I've said.

 

[Change in pressure]

Nothing in that video contradicts anything I've said.

I didnot say that..

What is your explanation what cause low pressure on top of wing, faster velocity ,curved streamline or maybe faster velocity is consequnece of low pressure ?

 

[boneh3ad]

I didnot say that..

What is your explanation what cause low pressure on top of wing, faster velocity ,curved streamline or maybe faster velocity is consequnece of low pressure ?

All of the above. There is no way to meaningfully decouple those phenomena. The fundamental truths here are that conservation of mass and momentum must hold. Conservation of momentum is a direct result of Newton's second law, and implies that a change in velocity is effected by some corresponding force. In fluid mechanics, that force is usually provided by the pressure gradient. So, the general, not-very-helpful answer is that the flow velocity and pressure gradients arise together in a way that satisfy both of the above conservation laws. (Note: for compressible flows, conservation of energy is also important as it cannot be decoupled from the other two conservation laws as it can in an incompressible flow.)

Generally, the distinction of which effect caused the other is not important. One is necessary and sufficient for the other. The only example I can think of that might have a clear-cut "answer" is that of a flow through a pipe with a constriction. Conservation of mass implies that the velocity must increase through the area reduction no matter what, and then you could separately apply conservation of momentum to argue that the pressure must be decreasing in that region as well to provide a force for that acceleration. Still, this is not a very useful or insightful exercise.

 

[hmmm27]

Momentum.

Air blowing across the bottom of the wing pushes the wing up and itself down. Straightforward.

Air blowing across the top of the wing keeps going for awhile, even after the shape of the wing pulls away underneath it, thus pulling the wing up, and itself down. Call it "centrifugal" for your own visualization purposes, if you like.

 

[Change in pressure]

All of the above. There is no way to meaningfully decouple those phenomena. The fundamental truths here are that conservation of mass and momentum must hold. Conservation of momentum is a direct result of Newton's second law, and implies that a change in velocity is effected by some corresponding force. In fluid mechanics, that force is usually provided by the pressure gradient. So, the general, not-very-helpful answer is that the flow velocity and pressure gradients arise together in a way that satisfy both of the above conservation laws. (Note: for compressible flows, conservation of energy is also important as it cannot be decoupled from the other two conservation laws as it can in an incompressible flow.)

Generally, the distinction of which effect caused the other is not important. One is necessary and sufficient for the other. The only example I can think of that might have a clear-cut "answer" is that of a flow through a pipe with a constriction. Conservation of mass implies that the velocity must increase through the area reduction no matter what, and then you could separately apply conservation of momentum to argue that the pressure must be decreasing in that region as well to provide a force for that acceleration. Still, this is not a very useful or insightful exercise.

Do you agree with me that air jet from hair dryer,blower ,blowing with mouth have identical pressure as ambient pressure ?
So the air does not have lower pressure just because it is moving.

If air velocity will lower pressure with speed than altimeter in plane will allways show increase in altitude when plane speed increase even plane do not climb.Because static ports on fuselage allways "feel" airflow(800km/h) which aircraft flys...

Classic example of missaplied bernouli theorem..

 

[boneh3ad]

I do agree with that. A jet of that source will be at constant pressure throughout and that pressure will be atmospheric.

Altimeters, however, are not an example of misapplied Bernoulli. They apply Bernoulli just fine and have for years.

 

[jim hardy]

Conservation of mass implies that the velocity must increase through the area reduction no matter what, and then you could separately apply conservation of momentum to argue that the pressure must be decreasing in that region as well to provide a force for that acceleration. Still, this is not a very useful or insightful exercise.

Not useful ?
That's the little piece of information that made Bernoulli "Click" for me when figuring out how my feedwater flow nozzle works..
Consider the free body diagram of a thin slice of fluid in horizontal pipe, accelerating in positive X direction, say to the right.
∑Force = ma
Force on left side = P_upstream X area
Force on right side = P_downstream X area
without a pressure gradient it won't accelerate

just as you said
but even this old electrical type could see it with that mental model..

old jim

 

[Change in pressure]

I do agree with that. A jet of that source will be at constant pressure throughout and that pressure will be atmospheric.

Altimeters, however, are not an example of misapplied Bernoulli. They apply Bernoulli just fine and have for years.

Here is example where wrong static port postion and desing of hole cause increase in altitude when speed increase even plane flys at same level all the time.
But some peope thinks that altimeter will allways act like this because static port is allways feel airflow because of aircraft speed,because they remeber what bernoulli tell, "faster velocity=drop in static pressure".
If that will be allways case on every points of aircraft fuselage, than neither altimeter will works correct..

 

[boneh3ad]

Not useful ?
That's the little piece of information that made Bernoulli "Click" for me when figuring out how my feedwater flow nozzle works..
Consider the free body diagram of a thin slice of fluid in horizontal pipe, accelerating in positive X direction, say to the right.
∑Force = ma
Force on left side = P_upstream X area
Force on right side = P_downstream X area
without a pressure gradient it won't accelerate

just as you said
but even this old electrical type could see it with that mental model..

old jim

I meant the exercise of trying to say that one of the phenomena caused the other. It's not a useful distinction.

Here is example where wrong static port postion and desing of hole cause increase in altitude when speed increase even plane flys at same level all the time.
But some peope thinks that altimeter will allways act like this because static port is allways feel airflow because of aircraft speed,because they remeber what bernoulli tell, "faster velocity=drop in static pressure".
If that will be allways case on every points of aircraft fuselage, than neither altimeter will works correct..

A properly designed static port on a plane will not have this problem. It will still measure static pressure, which doesn't change in the atmosphere even when the plane is moving or changing speeds. The change in speeds increases the total pressure, not the static pressure. As long as the port is placed in a region where the free stream flow velocity is the same as the planes, then this will work just fine.

In the video, there are several possible explanations at play. The simplest one is that the video was faked. However, even if it wasn't, there is a fluid Dynamics reason why this behavior was observed: the flow is compressible, so the pressure in the jet need not be the same initially as that of the atmosphere. It will tend to try to reach atmospheric pressure, but may take some distance downstream if the flow is choked.

 

[jim hardy]

I meant the exercise of trying to say that one of the phenomena caused the other. It's not a useful distinction.

I get that since m= f/a they are reciprocal.

I was just heading toward "if air bends around the wing to fill the void on its backside there must be a pressure gradient"
There are usually multiple thought paths that'll lead to an answer
and i don't trust any of mine until they agree.
That's what makes it useful for this plodder.

Not challenging your science at all.

old jim

 

[rcgldr]

Here is nice old school video where test curved stremline pressure change and also talk where bernoulli can not be applied.

At the start of the video, a ball is suspended in an angled jet of air, and is spinning. This is an example of Magnus effect, which is different than the case where a non-detached air flow will tend to follow a convex surface. In the case of the spinning ball, the air flow remains attached longer on the side of the ball spinning away from the jet than it does on the side spinning towards the jet. The net result is that the wake is diverted away from the stream, and a Newton third law pair of forces are involved, the spinning ball with Magnus effect exerts an outward force onto the air, coexisting with the air exerting an inwards force on the ball.

Getting back to air pressure being reduced over a convex surface, or when curving over a flat plate at some angle of attack, in a steady flow, the curvature of the air flow coexists with the pressure gradient. Again as in my earlier post, the air fills in what would otherwise be a void behind the trailing surfaces (the surfaces behind the peak of the upper surface) of a wing. As long as the flow remains attached, the curved flow follows the boundary layer just above the upper surface of a wing. The lower pressure due to the curved flow and coexisting pressure gradient also coexists with the velocity in the direction of flow being greater where the pressure is lower.

Frame of reference is important. Bernoulli is based on using the wing as a frame of reference, where the total energy of the affected air remains nearly constant (there is some loss of energy due to drag components). If the air is used as a frame of reference, then a wing passing through a volume of air results in the air being accelerated downwards, with a pressure jump as the air flows downwards across the plane swept out by the wing. The affected air's pressure is increased, and eventually that pressure returns to ambient, but at that moment the air has a mostly downwards (lift) and somewhat forwards (drag) velocity, and this is called the "exit velocity" (where the affected air's pressure returns to ambient). If enough is known about the flow, then either the pressure jump or the exit velocity can be used to calculate the work performed on the air. Bernoulli applies to the flow above or below the pressure jump, but is violated as the air flows across the pressure jump because energy is being added to the air as it flows across the pressure jump. Normally this view point is used when considering propellers or helicopter rotors, with some difference, since the blades have a cumulative effect since each blade flows through air already affected by other blades.

Another way to get an idea of the energy added to the air by a wing is to consider a glider in a steady descent (no acceleration of the glider) in still air. The decrease in the gliders gravitational potential energy corresponds the increase of energy of the air (some of which will be thermal, but most of which will be mechanical).

 

[Change in pressure]

rcgdlr and boneh3ad, do you agree with this theory?

 

[boneh3ad]

rcgdlr and boneh3ad, do you agree with this theory?

No, because it cites the Coanda effect, which, as I said earlier, only applies to a jet passing by a surface.

 

[Change in pressure]

No, because it cites the Coanda effect, which, as I said earlier, only applies to a jet passing by a surface.

What about constation in above video , that pressure distribution make particles moving with diffrent speed but reverse argument does not hold?

More about coanda..

It is sad that main engineer in Ferrari explain conada with water and spoon.At same time McLean in "Understanding Aerodynamics" states that the water deflection "actually demonstrates molecular attraction and surface tension ,not coanda".
It is funny that neither the scientists can arrange between themselves what is wrong and what is correct.
This is unbelievable!



I think water jet will follow spoon surface even in vacuum but air jet will not.
What do you think?

 

[boneh3ad]

What about constation in above video , that pressure distribution make particles moving with diffrent speed but reverse argument does not hold?

I don't know why you feel the need to continue asking the same question. You could use the airfoil and make a weak argument in favor of causality in one direction, as the video did. Then you could turn around and use the example of a pipe with a constriction to make an argument in exactly the other direction. There is no meaningful causality. The two effects simply must coexist.

It sad that main engineer in Ferrari explain conada with water and spoon.At same time McLean in "Understanding Aerodynamics" states that the water deflection "actually demonstrates molecular attraction and surface tension ,not coanda".
It is funny that neither the scientists can arrange between themselves what is wrong and what is correct.
This is unbelievable!



I think water jet will follow spoon surface even in vacuum but air jet will not.
What do you think?

Welcome to science. Phenomena are often more complicated than simple kitchen experiments imply, and practitioners often disagree.

I think water and a spoon are perfectly reasonable examples of the Coanda effect. Surface tension undoubtedly also plays a role since there is Andrew surface between two dissimilar fluids involved. In science, and especially fluid mechanics, it is exceedingly rare that only one single effect is involved in causing an observed phenomenon. We often try to simplify our explanations of the situation, especially when communicating with a non-expert, so it is more clear. That does mean that some details get left out at times.

There is no reason for a gas like air not to obey the same rules, here. In fact, when he first described the phenomenon, Coanda was working with an air jet moving over a surface, not water. The only difference in a vacuum is that surface tension would tend to hold a stream of water together (if you ignore the fact that it will tend to flash boil) while an air stream will rapidly expand to try to fill the vacuum. Still, the Jets, such that either of them continue to exist, will tend to follow a surface.

 

[rcgldr]

Still, the Jets, such that either of them continue to exist, will tend to follow a surface.

The tendency to follow a convex surface is reduced in the case of a low viscosity flame jet in air across an airfoil. Note how soon the jet detaches from the surface of the airfoil in the video below. In this case, the low viscosity of the jet entrains very little of the surrounding air, mostly pushing the air directly in front of the jet.

 

[boneh3ad]

The tendency to follow a convex surface is reduced in the case of a low viscosity flame jet in air across an airfoil. Note how soon the jet detaches from the surface of the airfoil in the video below. In this case, the low viscosity of the jet entrains very little of the surrounding air, mostly pushing the air directly in front of the jet.

I'll admit I haven't given that problem much thought, but this seems unrelated to the classical Coanda effect to me at first glance. Yours is a phenomenon that's ultimately related to boundary-layer separation. A hot gas actually has a higher viscosity, so my first inclination would be that this might suggest a lower Reynolds number and reduced resistance to separation.

 

[rcgldr]

Note how soon the jet detaches from the surface of the airfoil in the video ...

I'll admit I haven't given that problem much thought, but this seems unrelated to the classical Coanda effect to me at first glance. Yours is a phenomenon that's ultimately related to boundary-layer separation. A hot gas actually has a higher viscosity, so my first inclination would be that this might suggest a lower Reynolds number and reduced resistance to separation.

I struck through most of my prior post. You're correct that the Reynolds number is low, it's a small wing, and the jet speed is not that fast. I don't know what gas is being used to produce the flame, could this affect viscosity?

 

[boneh3ad]

I struck through most of my prior post. You're correct that the Reynolds number is low, it's a small wing, and the jet speed is not that fast. I don't know what gas is being used to produce the flame, could this affect viscosity?

It most certainly would. It's hard to glean much from that video without more information, but there could be any number of important factors when you have a flow such as that: high temperature gradients across the width of the flame/jet (meaning high density and viscosity gradients), chemical reactions between the combustion byproducts (and the resulting species concentration gradients), potentially ablation from the surface of the object if it isn't protected, compressibility, size... it's potentially a mess, and every one of those factors into Reynolds number and/or other aspects of the behavior of the boundary layer.

 

[Change in pressure]

I think water and a spoon are perfectly reasonable examples of the Coanda effects.

A common misconception is that Coandă effect is demonstrated when a stream of tap water flows over the back of a spoon held lightly in the stream and the spoon is pulled into the stream . While the flow looks very similar to the air flow over the ping pong ball above (if one could see the air flow), the cause is not really the Coandă effect. Here, because it is a flow of water into air, there is little entrainment of the surrounding fluid (the air) into the jet (the stream of water). This particular demonstration is dominated by surface tension. (McLean in "Understanding Aerodynamics" states that the water deflection "actually demonstrates molecular attraction and surface tension.

Air jet follow curved surface even in inviscid flow,indeed air viscosity is so small that is irrelevant...

Listen at 17:50


Listen at 21:10

 

[boneh3ad]

A common misconception is that Coandă effect is demonstrated when a stream of tap water flows over the back of a spoon held lightly in the stream and the spoon is pulled into the stream . While the flow looks very similar to the air flow over the ping pong ball above (if one could see the air flow), the cause is not really the Coandă effect. Here, because it is a flow of water into air, there is little entrainment of the surrounding fluid (the air) into the jet (the stream of water). This particular demonstration is dominated by surface tension. (McLean in "Understanding Aerodynamics" states that the water deflection "actually demonstrates molecular attraction and surface tension.

Air jet follow curved surface even in inviscid flow,indeed air viscosity is so small that is irrelevant...

Listen at 17:50


Listen at 21:10

You can repost the same things over and over again, but clearly I simply disagree with Dr. McLean's characterization of that problem. I would counter that the problem has contributionsfrom both phenomena contributing to the behavior. Arguing about whether that problem is the Coandă effect is essentially arguing about whether a problem featuring a large contribution from surface tension is still covered by a phenomena described only qualitatively by Coandă. This is why Coandă is rarely mentioned in any formal course of fluid mechanics: the effect isn't a mathematically- or quantitatively-rigorous phenomenon. It's just a broad description of a behavior.

So basically, we are now arguing about something that actual fluid mechanicians don't even care about.