The purpose of this paper is to study the structure and dynamic development of a pair of co-rotating trailing vortices, during their formation, interaction and merging, using detailed experimental measurements of the velocity and vorticity fields.

The vortices were generated using two half wings (NACA0030) positioned at equal and opposite angles of attack at the entrance of the test section of an open-circuit, subsonic, wind tunnel. Velocity vector measurements were obtained at Re_c = 133,000, on cross-plane grids at several distances from the trailing edges of the wings, using an in-house developed four-sensor hot wire anemometer probe.

The results include cross-plane contour plots of the mean and fluctuating velocity as well as mean vorticity fields. Each of these variables is affected in a different way, providing complementary information on the development of the flow field. After shedding, the two vortices are swept along the stream-wise direction and spiral around each other, thereby developing a braid of two vortices, which then deforms the external flow field. Gradually, the interaction with the external flow field links both vortices together until the final merging and the formation of a new stable linear vortex emerges.

Trailing vortices have been rendered particularly important during the past decades, because of increasing traffic density of very heavy aircrafts and several plane “incidents”, which were attributed to the action of the vortex wake.

The presented results provide information on the evolution and merging of a pair of vortices formed by a closely spaced differential wing configuration. The vortices interact almost immediately after shedding as expected in flap–flap or flap–wing vortices interaction.