In the scope of Potential Flow Theory, specifically in the Vortex Lattice Method (3D; for 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; even the boundary layer could be treated as a kind of detached flow but "damped" due to the fluid viscosity, which exists on the entire surface! (at low AoAs; viscous regime).

Physically, the internal vorticity generation (rectangular plate case, exemplified for simplification purposes) must be explained as a "vorticity interconnection" due to a transport mechanism (chordwise and spanwise) through the external vorticity generated from its edges; it means the vorticity cannot be cutted off suddenly due to a continuity assumption on the (numerically discretized) shell-body*. In a recent private communication, emeritus Prof. Joseph Katz (SDSU) says: "...vorticity is shear - and shear can be generated by the interaction of local streams - that was the argument used in my student dissertation way back then.", which should be a physical justification to include internal wake detachment**, in the understanding that conservation of mass is satisfied due to the distance between all discretized elements being considered infinitesimal (zero flux across the plate). On the other hand, the numerical justification seems to have already been proved through a full multi-wake model (The Full Multi-wake Vortex Lattice Method; Pimentel, 2021).***

As an analogy, from a numerical point of view, in the (unsteady) Full Vortex Cloud Method (2D; Lewis, 1991), the vorticity is shed from each discretized element along the bidimensionally represented surface, obtaining satisfactory results for a massive detached flow condition behind a bluff-body (e.g., a bidimensional cylinder). Since the unsteady case represents a series of steady solutions obtained at a discretized time step, the extension to 3D must be done in a straightforward manner, which means, by proposing a (full non-linear) model that allows to detach vorticity on the entire (tridimensional) surface, not only along some separation lines. Any tridimensional simplified model (by only detaching external wakes, even including LE), could increase the Lagrangian grid distortion (in the scope of the 3D Vortex Particle Method), which most probably (to be determined yet) leads to inaccurate results (aerodynamic loads/coefficients calculation****), despite its lower computational cost. It should be remembered that a "simple" case such as the flow past a quadrangular thin flat plate has not been solved yet in a precise way (by previous research) through vortex methods, even in the pre-stall condition where turbulence effects could be neglected (the easiest condition to solve).

* Same analogy for temperature distribution or structural stresses applied along the plate's edges.

** Main rejection by most reviewers, calling it "unjustifiable" or "nonphysical", however, Prof. Katz faced the same criticism in the 80s while conducting a thesis focused on solving parachute aerodynamics via vortex methods; he supports the detached vorticity generation on surface hypothesis, but he thinks that it must also be proved theoretically. In fact, in a recent publication called "Vorticity generation and conservation on generalised interfaces in three-dimensional flows" (Terrington et al., 2022), it is shown theoretically that vorticity is generated at a vortex sheet (shell-body) due to the difference in flow velocities between its two faces, being a purely inviscid mechanism.

*** From a multi-wake model's viewpoint, no internal detached wakes (or detached wakes with null circulation) means that circulation strengths between neighboring discretized elements on the surface are equal, thus a perfectly constant pressure distribution on the plate!, which does not match with what is physically expected.

**** A straight wakes' model (called "Only External Wakes"; OEW) shows an acceptable fit for lift but an overestimated drag along the analyzed AoA range. In the 2D case, the Kirchhoff-Rayleigh inviscid separated (perpendicular to the plate) flow shows an underestimated drag coefficient. According to (Batchelor, 1967), is solving a "cavity flow" with vacuum behind the plate, or in other words, no vorticity modeled just behind it; thus, by analogy to 3D, the OEW would also model a cavity case!!!).

Links to animations:In the scope of Potential Flow Theory, specifically, in the Vortex Lattice Method (3D; for a shell-body) had 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 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 behaviour; even the boundary layer could be treated as a kind of detached flow but "damped" due to the fluid viscosity, which exists on the entire surface! (at low AoA's; viscous regime).

Physically, the internal vorticity generation (rectangular plate case, exemplified for simplification purpose) must be explained as a "vorticity interconection" due to a transport mechanism (chordwise and spanwise) through the external vorticity generated from its edges; it means, vorticity cannot be cutted off suddenly cause the continuity assumption on the (numerically discretized) shell-body*. About this, in a recent private communication, emeritus Prof. Joseph Katz (SDSU) says: "...vorticity is shear - and shear can be generated by the interaction of local streams - that was the argument used in my student dissertation way back then.", which should be a physical justification to include internal wake detachment**, in the understanding that conservation of mass is satisfied due to the distance between all discretized elements is considered as infinitesimal (zero flux across the plate). On the other hand, the numerical justification seems to be already proved through a full multiwake model (The Full Multi-wake Vortex Lattice Method; Pimentel, 2021).***

As an analogy, from numerical point of view, in the (unsteady) Full Vortex Cloud Method (2D; Lewis, 1991) the vorticity is shed from each discretized element along the bidimensionally represented surface, obtaining satisfactory results for massive detached flow condition behind a bluff-body (e.g. bidimensional cylinder). Since the unsteady case represents a series of steady solutions obtained at a discretized time step, the extension to 3D must be in a straightforward manner, it means, by proposing a (full non-linear) model that allows to detach vorticity on the entire (tridimensional) surface, not only along some separation lines. Any tridimensional simplified model (by only detaching external wakes, even including LE), could increase the Lagrangian grid distortion (in the scope of 3D Vortex Particle Method), which most probably (to be determined yet) leads to inaccurate results (aerodynamic loads/coefficients calculation****), despite its lower computational cost. It should be remembered that a "simple" case as the flow past a quadrangular thin flat plate has not been solved yet in a precise way (by a previous research) through vortex methods, even in pre-stall condition where turbulence effects could be neglected (the easiest condition to solve).

* Same analogy for temperature distribution or structural stresses applied along the plate's edges.

** Main rejection by most reviewers, calling it "unjustifiable" or "unphysical", however, Prof. Katz faced the same criticism in the 80's while conducting a thesis, focused to solve parachute aerodynamics via vortex methods; he supports the detached vorticity generation on surface hypothesis, but he thinks that it must be proved also theoretically. In fact, in a recent publication called "Vorticity generation and conservation on generalised interfaces in three-dimensional flows" (Terrington et al., 2022) is shown theoretically that vorticity is generated at a vortex sheet (shell-body) due to the difference in flow velocities between its two faces, being a purely inviscid mechanism.

*** From multiwake model's viewpoint, no internal detached wakes (or detached wakes with null circulation) means that circulation strengths between neighboring discretized elements on surface are equal, thus, a perfectly constant pressure distribution on the plate!, which does not match with physically expected.

**** A straight wakes' model (called "Only External Wakes"; OEW) shows an acceptable fit for lift but an overestimated drag along the analyzed AoA range. In 2D case, the Kirchhoff-Rayleigh inviscid separated (perpendicular to plate) flow shows an underestimated drag coefficient. According to (Batchelor, 1967), is solving a "cavity flow" with vacuum behind the plate, or in other words, no vorticity modeled just behind it; thus, by analogy to 3D, the OEW would also modeling a cavity case!!!).

Links to animations:

The Full Vortex Cloud Method: https://www.youtube.com/watch?v=t9UlMXD7EzA

The Full Non-linear Vortex Tube-Vorton Method: https://www.youtube.com/watch?v=vrQET2cSroY