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I read many posts and papers that stated that lift and drag forces cannot exist without viscosity (and also posts stating the contrary). (Does that mean that invicid fluids does not have any force interaction with structures...and wouldn't that mean such fluids would pass through any structures if there is no force interaction?).

I have not been able to wrap my head around how lift and drag force cannot exist without viscosity. For example: if there is a flat plate plate placed at an inclination to the flow of incompressible invicid fluid, the plate will change the direction of flow of the fluid and hence will have a force acting on it.

Now i imagine this force can be separated into lift and drag components? If not why is this not possible?

Guess I am missing something fundamental in my understanding, or misunderstanding some terminology? Can you please help me?

Some refs i have used:

i) A Technical Note from Arc: Explicit Role of Viscosity in Generating Lift (https://doi.org/10.2514/1.J055907)

ii) A (newish) open-access paper from Springer: Can lift be generated in a steady inviscid flow? (https://doi.org/10.1186/s42774-023-00143-3)

iii) https://aviation.stackexchange.com/questions/89106/will-air-accelerate-over-a-wing-and-generate-lift-if-the-air-has-zero-viscosity

iv) https://aviation.stackexchange.com/questions/29617/what-is-the-relation-between-the-boundary-layer-and-lift-of-an-aerofoil

v) https://www.physicsforums.com/threads/why-do-air-foils-produce-lift.707155/

vi) Does a wing in a potential flow have lift?

vii) https://www.reddit.com/r/AerospaceEngineering/comments/v3fsuj/if_we_need_viscosity_to_generate_lift_why_do_cfd/

GRANZER
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2 Answers2

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In potential flow, there are no net forces (lift or drag) without vorticity and no way to create vorticity without viscous shear stresses.

This means that the flow around a flat plate at an angle of incidence (for example) would not leave the trailing edge smoothly but rather would go around it and leave somewhere on the upper surface instead. Numerically, a vortex distribution could be assumed to exist on the surface (or inside it, if a body with thickness is considered) and its strength could be adjusted in such a way as to make the flow leave smoothly from the trailing edge. See articles on the Kutta condition for more details. (https://en.wikipedia.org/wiki/Kutta_condition).

When vorticity is included on (or inside) the surfaces of 2D bodies in potential flow, then lift forces can be obtained with no further requirement of viscosity.

In 3D potential flows, both lift and drag can be found, but only with the further addition of trailing vortices behind the bodies.

D. Halsey
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For example: if there is a flat plate plate placed at an inclination to the flow of incompressible invicid fluid, the plate will change the direction of flow of the fluid and hence will have a force acting on it.

Correct. The consequence is that we could have airplanes, even if the air had zero viscosity.

Furthermore, any object (a ball for example) would be braked by the contrary flow of the fluid (at its frame of reference).

What changes without viscosity is the effects around the forced flow. The regions of the fluid close to the forced flow are not dragged together.

For a real viscosity fluid, as showed in this video, the balls are only "attracted" because the air around the (thin) upward flow from the nozzle are also dragged to move upward. And this requires air moving to the centre because there no other source of vertical flow except the nozzle.

Claudio Saspinski
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