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,

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

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...