Why is the top flow faster over an Airfoil?

There is an intriguing phenomenon when you closely examine the science behind airfoils. Why does the air above the airfoil flow much faster than the air below ? How come the two never meet? This video gives a logical explanation to this problem.

[alwayschooseford]

Summary: "An air foil works this way because of the way it is."

[morpheas]

These 2 videos raised even more questions than they answered for me.

[andrewtaco]

I've never been more exhausted from thinking about something that should be obvious.

[hediabdellah]

now answer why the pressure is higher at the outside of a curve

[professionalprocrastinator]

I will explain what happens from a mesoscopical point of view. What happens is that fluid particles tend to travel in a roughly straight line if undisturbed. This is the principle of inertia. However, the presence of the lifting surface disturbs them and they will first deviate off course due to the presence of the leading edge (or whatever they first encounter). Now particles are moving in a roughly straight line away from the lifting surface (upwards or downwards depending on where they were coming from upstream). Because of that, the flow becomes rarefied in the direct vicinity of the lifting surface which induces a pressure decrease close to it. Due to the fact that there exists a pressure gradient force pointing towards the lifting surface, fluid particles that are moving away from it are accelerated towards it. This is why flow remains attached for small values of the angle of incidence and this also explains lift: The action of the lifting surface on the fluid particles is to accelerate them in a certain direction (if the surface is generally cambered or inclined downwards, it will force fluid particles to generally accelerate downwards), hence they give away their momentum to the lifting surface in the opposite direction (principle of reciprocity).

Bernoulli's "principle" kinda forgets this dynamical point of view and rather explains things from an energetical point of view: There is a transfer between kinetic energy (velocity) and potential energy (pressure and head) along streamlines: In the absence of source or sink terms in the Bernoulli equation (production and dissipation of energy), kinetic energy is won where potential energy is lost, and vice versa. This does not explain why kinetic energy is lost (or gained) and why potential energy is gained (or lost) since you lose directional information by projecting the Navier-Stokes equations onto a streamline, but along with the "third law explanation" that I gave in the first paragraph, the Bernoulli principle will help provide you with the full picture: That's because whenever fluid particles travel very quickly towards an obstacle, they contain lots of bulk kinetic energy which is transformed into pressure when they hit the leading edge and the pressure side of the lifting surface. Because fluid particles have become compressed when moving past the leading edge along the suction side, the pressure difference between patches of flow away from the surface compared to that close to the surface will become more important, which yet again reinforces the pressure gradient force caused by this rarefaction. Curvature does not magically act as is suggested in this vid: We should rather consider that the geometry curves away from the flow as flow particles move along a straight-ish line and bump into each other, getting a net momentum that forces them to go towards the rarefaction.

[Lesics]

Dear friends, Here is the 2nd part of airfoil video series. We are working hard to release the 'Transistor video' by the end of this month.

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[Eltaurus]

3:33 "such a sudden drop in pressure will not considerably increase the particle speed"

The drop in pressure will change the speed independent of how sudden it is, as Bernoulli Equation demonstrates. The correct explanation here is that change of speed at the end of the trajectory doesn't significantly change the total time the path takes.

[wbeaty]

More classic version, still logical: if the flow-pattern is actually the sum of a 'circulation' plus a uniform horizontal flow, then, in the region above the airfoil, the circulation always adds to the average velocity. Below the airfoil, the circulation must subtract from the average velocity. (Works fine for rotating cylinders, works for more complicated 2D airfoils.) Result: any parcels which split at the leading edge, will never meet again, as long as circulation is present.

Another result: if the shape and angle of the airfoil gives zero lift, also the circulation becomes exactly zero. In that case, any split parcels at the leading edge will recombine again at the trailing edge.

Rule of thumb: if split parcels recombine at the trailing edge, it means that the lifting force is *exactly zero.* The infamous "transit time fallacy" turns out to be a description of a zero-lift airfoil.

[hamdanalharbi]

first part of the pressure is not consistent! i was lost when he started talking about the pressure in the lower part of the airfoil!

[kudamurapa]

I think the problem is to try and use Bernoulli's theory/equations collectively for both the upper and the lower surface. If for a minute you stop and treat the upper and the bottom part separately and apply Bernoulli's equation you will find out that it's not wrong after all. Let's start with the upper surface: the flow approaches the aerofoil and the area in which it flows converges and then diverges. Bernoulli's theorem states that when the area converges the flow speeds up and the pressure drops and when it diverges the flow slows and pressure starts to increase. This is exactly what happens on the top of an aerofoil. Now considering the bottom surface: when flow approaches the aerofoil the area slightly converges prompting a slight pressure decrease and then the area diverges increasing the pressure and slowing down the flow. Now if you consider the collective contribution of the conditions below and above the aerofoil you will get lift. But I don't think Bernoulli alone is suffic

[justcarcrazy]

Brilliant video! It's good to see the proper conclusions being drawn from observations for a change, rather than finding a convenient but wrong explanation.

[yugsth]

0:29 In the curved flow, why is pressure higher outside ?

[wingnut]

This is like the chicken/egg question. Does the speed difference cause the pressure difference or the other way around?

[majidmahjoob]

It means that the decreased pressure condition is the cause of acceleration of air above the wing, has been there before movement of air

[diamondsintheroughpodcast]

you explained why the top particle goes faster, due to lower pressure gradient, but you didn't explain why the wing curvature CREATES the lower pressure. So this explanation is incomplete.

[shortscreator]

since bernoulli's theorem cannot be applied on two streamlines then how bernoulli's principle is resonsible for the BLOWING OFF of the ROOF DURING STORMS PLS ANSWER

[johnnyllooddte]

your logical conclusions are wrong...first of all the air is not moving..the foil is moving..therefore the air is always at the same point above and below..your model cannot demonstrate this.sorry..close but no cigars

[Alessandro]

Amanzing explanation! : Why does the pressure drop as we get close to the curvature? Because of the curvature! Like saying:
Why is the sky blue? Because it's blue!
Why does fire burn things? Because fire burn things!
Why does the Sun illuminate the Earth? Because the Sun illuminate the Earth!
It would be better to not explain anything at all if you have to explain it like this.

[joegarrison]

Did not like this explanation. The air is stationary, the wing moves through the air. Although it has the same effect it is poorly worded. Some of this is correct but angle of attack is critical but not mentioned. Do not recommend.

[startrekwarsmixguy]

These videos have been really interesting, but I have to admit that I find the word 'airfoil' to be irrationaly irritating for some reason.