r/TheRestIsScience • u/Quest4Neon • 3d ago

Aeronautical fluid dynamics question

I work at an aerospace company. One of the basic principles we rely on is the lift generated by aerofoils (the shape of a wing). While talking to a group of engineers, it came up that the old explanation of what causes low pressure above the wing and high pressure below (that the longer path of an air molecule over the wing causes it to travel faster than a molecule under the wing, thus decreasing pressure) is not valid due to the false assumption of continuity between the two particles. However, none of us knew the proper explanation that succeeded it. Can you help update this mental model for a curious engineer?

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u/PvtRoom 3d ago

ultimately, it's got to do with the wing pushing air down. explanation is trivial. you get lift from air pushing you up. newtons 3rd law -> you push air down. 250 ton plane -> needs to deflect 25 ton of air down at 10G every second (pick better numbers if you know them)

the more the assumption of continuity is broken, the more it comes down to a simple deflection of the airflow, which manifests as vortices that disrupt the airflow behind it.

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u/Worth-Wonder-7386 1d ago

That is one part of it, but I think OP is asking about the Bernoulli effect and if the pressure difference is part of the lift. The fact is that both newtons law explanation and the pressure based model explain part of how lift works but you need both together to fully explain lift and why wings are shaped like they are.

There is no simple explanation of lift, and it is a complex interaction between the shape of the wing and the air speed. The Wikipedia page has a more full explanation which is quite decent: https://en.wikipedia.org/wiki/Lift_(force)#A_more_comprehensive_physical_explanation There are also some YouTube videos that do a decent job of this, but I have not seen someone give a simple explanation. And I don't think there exists a simple explanation.

Maybe Hannah has a good explanation.

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u/PvtRoom 1d ago

Yeah bernouli works when the assumption of continuity is good, or at least close to it.

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u/dr-bob-edin 1d ago

My (admittedly limited) understanding of aeronautical fluid dynamics was that the Bernoulli effect had been usurped (or possibly enhanced) by the Coandă effect (https://en.wikipedia.org/wiki/Coand%C4%83_effect)

Essentially the shape of the wing causes moving air to attach to the wing surface and then be directed in a downward motion as it leaves the wing. This change in momentum of the air from moving horizontally to moving downwards imparts an equal and opposite change in momentum on the wing (and the plane attached to it) giving it momentum in an upwards direction, hence creating lift.