Bound Vortex

a vortex that is considered to be tightly associated with the body around which a liquid or gas flows, and equivalent with respect to the magnitude of speed circulation to the real vorticity that forms in the boundary layer owing to viscosity.

In calculations of the lift of a wing of infinite span, the wing can be replaced by a bound vortex that has a rectilinear axis and generates in the surrounding medium the same circulation as that generated by the real wing. In the case of a wing of finite span, the bound vortex continues into the surrounding medium in the form of free vortices. Knowledge of the vortex system of a wing permits calculation of the aerodynamic forces acting upon the wing. In particular, the interaction between bound and free vortices gives rise to the induced drag of the wing. The idea of the bound vortex was made use of by N. E. Zhukovskii in the theory of the wing and the screw propeller.