The velocity will be doubled and the kinetic energy will be increased fourfold. Therefore the model predicts a significant increase in induced drag for this formation. Describing a real airplane in terms of real vortices is trouble-free. Two counter-rotating vortices have less energy when they are near each other. Yes, there is a region between the two vortices where the velocity is higher, but there are larger regions to the left of the pair, to the right of the pair, above the pair, and below the pair, where the velocity induced by one vortex cancels part of the velocity induced by the other vortex, reducing the energy.
Similar considerations apply when considering ground effect .
The disk-balloon theory makes predictions which do not correspond with reality. Violation of the Laws of Motion A vortex is a well-known solution to the laws of motion. It involves a vortex line running down the core of the vortex, with no vortex lines elsewhere. The trailing vortices of a real wing are joined to the bound vortex that runs along the wing, satisfying this theorem. To the extent it can be described at all, it requires vortex lines penetrating the disk everywhere, including places far from the wing.