Trailing vortex formation
The physical mechanism by which the trailing vortices are formed may be understood by reference to Fig. 2.6. On the underside of a wing, the pressure
Fig. 2.5 Trailing vortex formation
Flow visualisation using helium-filled microscopic soap bubbles. The flow spirals around a stable core originating from just inboard of the wing tip (Photo courtesy of ENSAM, Paris)
Fig. 2.6 Spanwise flow on a wing
(a) The air flows inwards on the upper surface and outwards on the lower. This is the source of the trailing vortices
(b) View from just downstream of the trailing edge
is higher than the surrounding atmosphere, so the air flows outwards towards the tips. On the upper surface, the pressure is low, and the air flows inwards. This results in a twisting motion in the air as it leaves the trailing edge. Thus, if we look at the air flow leaving the trailing edge from a viewpoint just downstream, as in Fig. 2.6(b), it will appear to rotate. Near each wing tip, the air forms into a well defined concentrated vortex, but a rotational tendency or vorticity occurs all along the trailing edge. Further downstream, all of the vorticity collects into the pair of concentrated trailing vortices (as shown in Fig. 2.10).
If the wing is completely constrained between the walls of a wind-tunnel, the outflow will not occur, and trailing vortices will not form. This ties up with the theory of vortex behaviour mentioned above: the vortices must either form a closed loop, or terminate in a wall. It also points to one of the problems of wind-tunnel testing; the fact that the presence of the tunnel walls influences the flow behaviour.