Carlos Pimentel's blog

Potential Flow Theory (PFT) has always had a bad reputation among most fluidynamicist, among other things, because it is quite abstract and has only been successfully applied to explain, more or less, basic concepts of fluid dynamics, more specifically in aerodynamics, mainly through low-order numerical methods, such as panel ones, including the "well-known" vortex lattice method (VLM). Most lecturers teach during the fluid dynamics or computational aerodynamics courses that the PFT defines an irrotational (vorticity-free), incompressible (divergence-free) and inviscid (viscous-free) flow, which they call "ideal". However, its supposed inviscid characteristic is not theoretically defined and its current numerical implementation (including its attached circulation) is only a crude assumption! Such a misunderstanding can be demonstrated with a single image, from which it can be concluded that in fact, the current interpretation of the PFT has been used until now to

The Potential Flow Theory (PFT) states that a 'potential flow' must be incompressible (divergence-free) and irrotational (vorticity-free). However, compressible potential flow also exists, but it is not defined within the classical PFT. Then, strictly and in a broader sense, a potential flow must only fulfill the irrotationality condition. At this point, the term 'viscosity' is absent from such a definition, however, for the same reason that such a term does not appear, the fluid-flow viscosity value can be considered zero. Then, an 'inviscid potential flow' is now called an 'ideal flow' in order to be more specific. However, from the current interpretation, a viscous-forced potential flow (Kutta-Zhukovski-type pseudo- real flow) can also be defined, hence, 'potential flow' is not a synonym of ideal flow, as most authors misunderstand. In other words, the ideal flow is a potential flow, but a potential flow is not necessarily the ideal flow

In the scope of Potential Flow Theory, specifically in the Vortex Lattice Method (3D; inviscid flow past a shell-body), it has been widely accepted "by default" that flow detachment (and thus, vorticity generation) occurs only from trailing and lateral (wing tips) edges, which seems to be a crude simplification that limits its application range. Such an approach avoids that partially or massive flow detachment can be precisely solved (through spatially precise shedding vorticity), since current simplified models consider vorticity embedded on the entire plate, considering a zero vorticity assumption just behind it (no flow perturbation), a more than questionable physical behavior. From this approach, even the boundary layer could be treated as a kind of detached flow but "damped" due the fluid viscosity, which exists on the entire surface not only along the external edges! (at low AoAs; viscous regime). Fig. 1 A hurricane is a kind of vortex. Source: https://th.bin

Another physical phenomenon, in addition to the generation of lift force or the effect of viscosity on it, that has not been explained in detail by fluid dynamics (neither theoretically nor computationally), is that related to the stall mechanism that a lifting surface can suffer, for example, on the wing of an aircraft. In general terms, the stall condition is understood as the loss of lift force and an abrupt change in flow regime, going from a relatively stable condition to a completely turbulent one. For reasons of simplification, I will omit to explain the effect on the drag force or on the pitching moment, as well as other details that are assumed to be known by the reader (effects of the thickness and shape of the leading edge in obtaining the aerodynamic forces, nature of the variation or discrepancy of the experimental data, basic knowledge of Potential Flow Theory and/or the vortex lattice method; VLM). Fig. 1 Airfoil during stall. Source: https://i.pinimg.com/originals/d7/3b