Could there be lift without viscosity?

One of the classic debates in aerodynamics (or fluid dynamics, in general) is whether the existence of viscosity in a fluid is an essential condition for lift generation in an immersed body. First, the concept of viscosity must be explained, which is a property that is closely related to the physical constitution of the fluid at a micro level, that is, at the molecular scale. Thus, this can be understood as the level of resistance or friction that its "particles" oppose to the displacement of their neighbors in the tangential direction (with respect to the flow), and that depends on the time it takes for the particles of an 'upper layer' to fill the spaces left in a 'bottom layer' (relative to a 'solid wall') as the fluid flows.

Fig. 1 CASA-295 aircraft CFD simulation (just to get your attention!).

To put it in context, a typical example of a fluid with a high viscosity is honey, which moves more slowly than water, which has a much lower viscosity value. Now indeed, a fluid that is strictly without viscosity (inviscid flow) does not exist naturally, there are substances such as helium that have a viscosity very close to zero (called superfluids), and that is why they have been used to try to find the answer to the main question from an experimental point of view, although up to now it seems that the results have not been conclusive as the debate persists.

Fig. 2 Viscosity comparison for different fluids. Source: https://civilmint.com...

Some 'sophisticated' theories try to solve the problem in the sense that lift cannot exist without viscosity, however, from my particular point of view, they fail because their main hypothesis is based on a false assumption, such as that an 'ideal flow' (in the scope of the Potential Flow Theory; PFT), by definition inviscid, remains attached to the entire surface, perfectly surrounding the object in question. In fact, the previously described physical phenomenon would only exist in a hypothetical case where the fluid viscosity tends to an infinite value (or Reynolds numbers close to zero) but not to a zero value (see Fig. 3), which would explain by itself that there is no drag 'force', as the PFT currently does! According to such a theory, this phenomenon would even be present in the case of an infinitely thin plate with a perfectly sharp leading edge! which, in a condition of a 'normal' value of viscosity, would prevent the flow attaching, regardless of the recirculation that may exist just behind such an edge.

Fig. 3 Left: Current PFT with "zero viscosity"; Right: a hypothetical case with viscosity tending to infinity. Do you see any difference in the shape of the streamlines?

At this point, I prefer not to delve into the subject from a theoretical perspective since I do not have sufficient knowledge to understand the theory behind these developments (and, to tell the truth, it is not of my current interest since my approach has always been focused on application); however, I will try to follow the thread from an engineering point of view through the obtained results of a recent computational research, where the lift, the drag, and even the pitching moment in infinitely thin bodies can be obtained numerically under a strictly inviscid (or true ideal flow) assumption.

In such a computational method, the development is compiled from a circulation (and vorticity) approach, detached from the entire surface of a three-dimensional shell-body [1] based on a reinterpretation of the PFT, and implemented through the steady vortex lattice method (VLM) concepts. Such work has been extended to its unsteady version of vortex rings [2] and finally to that of vortex 'particles' (hybrid of tubes and vortons) [3] (see Fig. 4), which validates that the direction and magnitude of the resulting fluid-dynamic force, therefore lift, drag, and moment, are generated without the existence of fluid viscosity*, and in any case, its generation is due to the (inviscid) detached vorticity from the entire surface. However this by itself does not mean that viscosity does not contribute to such 'forces' and moments, as in fact it does for certain operating conditions, for example, at very low angles of attack (within the boundary layer), where due to the alignment of the flow and the plate, tangential (shear) stresses are generated, the viscous effects cannot be neglected from the physical viewpoint. However, within the framework of the current research, the fluid viscosity is included as an external element to the PFT and not assumed as part of itself, by arbitrarily attaching the circulation/vorticity, as it has been done since its conception.

Fig. 4 Simulation of an inviscid flow behind a swept flat plate with angle of attack.

And to all of the above, from which approach should the question ideally be answered first: experimentally, theoretically, or numerically? By the way, why is the Eulerian direct numerical simulation (DNS) not capable of solving such a question yet? Is it only a matter of increasing meshing resolution and computational resources, as usual?

*This should also be confirmed for thick plates, where probably viscosity acts on the lower surface maintaining the attached flow, being detached only in the upper one.

[1] Journal article: The Full Multi-wake Vortex Lattice Method: a detached flow model based on Potential Flow Theory | Advances in Aerodynamics | Full Text (springeropen.com)

[2] Pre-print: https://www.researchgate.net/publication/372419147_The_Unsteady_Full_Multi-wake_Vortex_Lattice_Method_a_full_rolled-up_detached_vorticity_approach

[3] Journal article: The Full Non-linear Vortex Tube-Vorton Method: the pre-stall condition | Advances in Aerodynamics | Full Text (springeropen.com)