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-   -   Why is air pressure lower on the top of an airfoil ? (http://www.cfd-online.com/Forums/lounge/111978-why-air-pressure-lower-top-airfoil.html)

omaraero January 18, 2013 22:14

Why is air pressure lower on the top of an airfoil ?
 
Hello all,

I have read a lot about this and still haven't found a satisfactory answer...

So what makes the air go faster on the top of an airfoil ?

I want to hear your opinion, and please no need to mention equal transit time :D

There is also this ''new theory of flight'' which refutes the circulation theory, mainly because it requires the wing to have a sharp trailing edge whereas a lot of current airplanes have rounded trailing edges...

http://www.youtube.com/watch?v=t7e_6bkUFzE

so what do you guys think ? any comment is appreciated

Bye,

micro11sl January 21, 2013 18:12

Hi,
Unfortunately, I missed the point in the YouTube link. I'll try later. I can't figure out how much the "New Theory" would change our ways in aircraft design. If such Kutta-Zhukowski and Prandtl's theories were complete wrong, then the tools in aerodynamic prediction would insufficiently accurate or completely mispredicting flow conditions. Well knows tools, such as XFOIL(2D), AVL(3D), wouldn't be able to calculate lift and drag. Because modern CFD approach RANS uses turbulence models, in which the near wall treatment uses some conclusions of boundary layer theory, such as law-of-the-wall, then even the RANS approach suffers the limitation. However, the fact is the aforementioned tools can predict 2D or 3D aerodynamic forces well. Some difficulties could occur when the flow is transitional between laminar and turbulence or the angle of attack is high to have unsteady separations.

"Why flow travels faster on the upper surface": I think this applies only when the angle of attack is positive and the camber is positive? If a cylinder is rotating, the flow speed around is also different on each side. And a positive rotation can have lift, and this is why relating lift to the circulation (we might simply denote circulation as a rotating cylinder for easy understanding). In this way, the cylinder, as we can exaggerate as a round aerofoil, is not necessary a sharp trailing edge shape. As I can remember what I learned, the flow around can be then seen as a superposition of a vortex and an uniform flow, which are the basic solution of Naiver-Stokes equations. However, this paragraph explains why circulation can have lift without explaining how circulation is generated, nor why upper surface flow can travel faster.

When I took my aerodynamic course in undergraduate, the teacher did give some explanation without saying the flows on both side need to join at the same time at the trailing edge, but I can barely remember that. Roughly, he mentioned the rounded trailing edge problem and seemed to explain it associating to "the starting vortex". Hopefully this may give you some idea.

As far as I know, in modern CFD calculations, there's also some treatment on the trailing edge when calculating wall distance. I don't know whether this is related to your problem. If you know, discussion is welcome.

I don't have time now since I've got a paper to write. I'll turn back later for probably more accurate discussion.

Regards,
Sheng

omaraero January 21, 2013 20:36

Thanks for your input micro11sl.

My question isn't related to CFD, which I am not familiar with by the way (I am still an undergrad taking my first fluids course). I am just wondering how exactly does this low pressure region arise above the wing ?

I remember reading something saying that it's because the air is being bent and deflected downward that we have this low pressure. But why exactly ?

As far as circulation is concerned, the way I understand it is that the higher velocity induces the low pressure, whereas some articles state that the low pressure arises ''first'', and higher velocity is just a consequence of lift..

It seems like there isn't any causality chain explaining this...

omaraero June 10, 2013 18:49

Hi it's me again with more questions about this :D

Regarding the circulation theory, correct me if I am wrong but, it appears to say that no circulation = no lift. Now circulation refers to the bound vortex which takes place as a reaction to the starting vortex. That means before the shed vortex occur, there is no circulation therefore there is no lift. What does that say about the flow around the wing ? Are velocities and pressures the same everywhere ? The flow is still deflected down so we should have an upward force...

There's a good article about lift that seems to give a good explanation of it : http://www.allstar.fiu.edu/aero/airflylvl3.htm

"When the air is bent around the top of the wing, it pulls on the air above it accelerating that air down, otherwise there would be voids in the air left above the wing. Air is pulled from above to prevent voids. This pulling causes the pressure to become lower above the wing."

How would pulling the air cause the pressure to become lower ?



Sorry for the multiple questions but I am just trying to make a physical sense of this...

Feel free to post any comment you might have

micro11sl June 11, 2013 05:14

Quote:

Originally Posted by omaraero (Post 433225)
Hi it's me again with more questions about this :D

Regarding the circulation theory, correct me if I am wrong but, it appears to say that no circulation = no lift. Now circulation refers to the bound vortex which takes place as a reaction to the starting vortex. That means before the shed vortex occur, there is no circulation therefore there is no lift. What does that say about the flow around the wing ? Are velocities and pressures the same everywhere ? The flow is still deflected down so we should have an upward force...

There's a good article about lift that seems to give a good explanation of it : http://www.allstar.fiu.edu/aero/airflylvl3.htm

"When the air is bent around the top of the wing, it pulls on the air above it accelerating that air down, otherwise there would be voids in the air left above the wing. Air is pulled from above to prevent voids. This pulling causes the pressure to become lower above the wing."

How would pulling the air cause the pressure to become lower ?



Sorry for the multiple questions but I am just trying to make a physical sense of this...

Feel free to post any comment you might have

As you quoted, "it pulls on the air above it accelerating that air down." The key words are "accelerating" and "down". If air tends to accelerate, the pressure will tend to decrease. A simple physical observation is, when you blow air between two hanged piece of paper, the paper will move closer to each other. On the other hand, if air stream is bent down along the aerofoil surface, lift will be a simple result of momentum conservation.

omaraero June 11, 2013 20:32

Yeah, I guess I was reading too much into it and looking at it more in a cause and effect situation and wondering if the fast moving air is the cause or consequence of lift.

This article gives a good intuitive idea about it as well :

http://www.avweb.com/news/airman/183261-1.html

" The inertia of the air that goes over the top of the wing tries to keep it moving in a straight line, while the pressure of the atmosphere tries to push it down towards the wing's surface. The inertia prevents the atmospheric pressure from packing the space as firmly as it would if the wing were standing still. The result is a low-pressure region above the wing"

It pretty much sums it up nicely.

diamondx June 12, 2013 11:12

Quote:

This article gives a good intuitive idea about it as well :

http://www.avweb.com/news/airman/183261-1.html
It is indeed very interesting... the lift is explained very well

Aeronautics El. K. June 19, 2013 07:06

Hi guys!

Well, no offence meant but I believe you should return to the fundamentals and study Chapters 4 and 5 of the "Fundamentals of Aerodynamics" by Anderson, instead of reading something on dubious websites ;)
Again, no offence meant with this comment. I'm just stating my opinion because I noticed for example that in the last link (the one from avweb) the author arbitrarily connects the Circulation (denoted by Γ in aerodynamics) with a circular movement of the air around the airfoil which definitely is NOT the case... I didn't have to continue reading after that...

claire8 June 25, 2013 05:52

In all honesty, I'm not knowledgeable in aerodynamics. So, I'm not sure if these article I found about airfoil is useful. However, allow me to share it.

http://www.av8n.com/how/htm/airfoils.html

omaraero July 6, 2013 15:38

1 Attachment(s)
Hi guys,

claire8 : it looks like a good article, I've read it a long time ago I might look at it again.

Aeronautics El. K. : Like I mentionned, I was just looking for a physical description. I borrowed an aero textbook so I will for sure give it a read and see if it'll make a little bit more sense (or not).

However, back to the articles i've posted. I've just realized that this void argument is rather incomplete. "When the air is bent around the top of the wing, it pulls on the air above it accelerating that air down,"

It make it seems like as if as soon as the air is bent around the top the pressure is continuously decreasing back to the trailing edge and if I follow the logic I am tempted to think that nothing happens at the leading edge part, there is no gap that needs to be avoided per say.
So what happens before the air is bent on top of the wing ? the article does not say anything about it and that is the critical part of the airfoil because the pressure is at its lowest !. Once it makes it over the top, the pressure starts increasing, but still remains lower than atm. (see top half of picture attached).

Am I missing something in the author's explanation ? What do you guys think ?

And doesn't this void argument goes against conservation of mass ? If you were to apply it to a venturi tube situation where the area is getting bigger, the streamlines won't rush in the expanded area, they would not be forced to accelerate down, the streamlines will flow in the larger area but I don't see any fluid pulling on the fluid above it and accelerating it....
Or maybe this does not apply to venturi tubes because the flow is confined ?

Thanks again,

Michael-STM March 22, 2014 16:31

This is a very interesting question but i think nobody can answer that.
Because the question is ill-posed, so it can not be answered, this question is like asking: why when i kick a ball it moves? what are you going to explain? newton`s law? they does not explain anything....maybe you have to explain the behaviour of atoms...but you are still going to miss the point...and etc...
So the best answer to the wing question is this: because the solution of the Navier- Stokes are like this. You can not go deeper than this without getting lost in physics and philosophy.

derekm April 29, 2014 05:40

Warning this gets a bit "fluffy"
I think questions like this arise because of a lack of "play time" i.e. just experimenting for the love of it ( btw I really love experiments ) . And perhaps the over use of electronic measurement devices. This all removes the "feel" for a subject.
For me A wing is an unrolled venturi so it must have lift. The bernoulli principle although predicted by conservation of energy, I have experienced a reality seen in manometer gauges on a venturi pipe, its not just a set of equations in a book.
The Physics and the Maths build a model to predict the future, they are not the causality, they are not reality.

Joachim September 1, 2014 00:45

Haha pretty cool topic! We talked a lot about that with some friends when studying for the qualifying exam. :D

The best explanation I could find at the time was given by a professor from Cambridge (Holger Babinsky) in his "funny" article "how do wings work?". You can find it on the web for free, it is really interesting and relatively short: http://iopscience.iop.org/0031-9120/.../pe3_6_001.pdf

for a quick taste, here is the abstract:
The popular explanation of lift is common, quick, sounds logical and gives
the correct answer, yet also introduces misconceptions, uses a nonsensical
physical argument and misleadingly invokes Bernoulliís equation. A simple
analysis of pressure gradients and the curvature of streamlines is presented
here to give a more correct explanation of lift.


To summarize, everything can be explained relatively simply using Newton second's law (of course...Some people will say we just moved the problem, but at some point, we'll need to start from somewhere I guess! These are laws and not theorems, but that should do it for now!).

Basically, since the airfoil is curved, the air particles will have to follow its contours. The only way for this happen (to bend the flow) is if a pressure gradient is present. If we did not have any pressure gradient, the particles would just go straight through the airfoil (why would a particle turn if it moved through a field of uniform pressure? The forces acting on it would balance out, so that the particle would maintain its initial speed), which does not make much sense. You can look at nice examples in the paper to see more precisely what I am trying to explain.

I hope that helps!

Joachim


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