An 'inviscid' boundary layer! Is this a bug?! (Part 2)

Yes, it is! This can be easily demonstrated by visualizing the velocity field directly in Fluent instead of the Results module, where no kind of boundary layer (BL) appears:

Video: A visualization bug in the Results module for an inviscid simulation (version: 2024 R2).

However, this bug is only present for simulations performed directly by the native inviscid model. Note that not only the lower limit for visualization is affected, but also the maximum (by almost 2 m/s), as if they were two different simulations. At this point, I will not go deeper into this subject, as trying to find the error in the error is nonsense. Even for an AoA case (3 degrees; not converged solution) such a BL evolution remains (see Fig. 4), while in Fluent it does not. I will probably contact someone who can help me understand what is happening with such results...who cares, nobody performs inviscid simulations nowadays!

Fig. 4 Velocity contours for the AoA=3 deg. case; a modeled BL is also present.

As I just said, this problem only appears in the inviscid model, which according to the official documentation assumes a free-slip BC at the walls by default. On the other hand, by using the laminar model, setting the air viscosity to zero and manually selecting the free-slip condition for the airfoil surface, everything seems to work fine, I mean, both visualizations in Fluent and the Results module show consistent results: no BL for the free-slip case and a modeled BL for the non-slip (zero velocity at the walls) one (convergence is also smooth). But here is another point to discuss: which results should I take as valid for an inviscid (Eulerian) simulation? If the BL has a viscous origin by definition, how can one be present in a non-viscous simulation? Or I may be misinterpreting such a velocity gradient as a BL...

From my point of view, the non-slip (zero velocity) BC at the walls forces, among other variables, a vorticity at the surface, the same as in the PFT by the Kutta condition, which is applied only at the trailing edge in such a theory. For both inviscid flow simulations performed here, it is clear that nevertheless of the interpretation that is intended to be given to each case, an inviscid flow near the surface is rotational, regardless of the wall BC (free-slip or non-slip) selected (see Figs. 5, 6, 7 and 8).*

Fig. 5 and 6 Vorticity contours (curl of the velocity around z-direction) for an inviscid flow with free-slip BC at airfoil surface.

Fig. 7 and 8 Vorticity contours (curl of the velocity around z-direction) for an inviscid flow with non-slip BC at airfoil surface.

Furthermore, from the previous figures, it is clear that the vorticity is generated along the whole surface and then naturally advected downstream, this being in agreement with the concepts of the Full Multi-wake Vortex Lattice Method [2], which results in an extension of the classical PFT. As I said in the first part of this article, this short qualitative analysis is made as an effort to elucidate wrong interpretations, most of them coming from an old non-revised theory, which even today has a great impact on beliefs about the fundamentals of fluid dynamics (e.g. fluid detachment due to viscosity [highly counter-intuitive! {absurd}], pressure field as the main justification to obtain the resultant force, among others).

By the way, in the case of the free-slip simulation, where does the drag come from? maybe from the detached vorticity? how is it transferred to the body? Does this simulation describe a "real" inviscid physical behavior? Too many questions today...

* Vorticity should be confined to an infinitely thin region at an infinite Re case.


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