Page "Lift (force)" Paragraph 52

The effects of viscosity are contained within a thin layer of fluid called the boundary layer, close to the body.

As flow over the airfoil commences, the flow along the lower surface turns at the sharp trailing edge and flows along the upper surface towards the upper stagnation point.

The flow in the vicinity of the sharp trailing edge is very fast and the resulting viscous forces cause the boundary layer to accumulate into a vortex on the upper side of the airfoil between the trailing edge and the upper stagnation point.

This is called the starting vortex.

The starting vortex and the bound vortex around the surface of the wing are two halves of a closed loop.

As the starting vortex increases in strength the bound vortex also strengthens, causing the flow over the upper surface of the airfoil to accelerate and drive the upper stagnation point towards the sharp trailing edge.

As this happens, the starting vortex is shed into the wake, and is a necessary condition to produce lift on an airfoil.

If the flow were stopped, there would be a corresponding " stopping vortex ".

Despite being an idealization of the real world, the “ vortex system ” set up around a wing is both real and observable ; the trailing vortex sheet most noticeably rolls up into wing-tip vortices.