A rapidly growing instability is observed to develop between unequal-strength counter-
rotating vortex pairs. The vortex pairs are generated in a towing tank in the wakes of
wings with outboard triangular flaps. The vortices from the wing tip and the inboard
tip of the flap form the counter-rotating vortex pair on each side of the wing. The flow
fields are studied using flow visualization and particle image velocimetry. Both chord-
based and circulation-based Reynolds numbers are of O(105). The circulation strength
ratios of the flap- to tip-vortex pairs range from
−0.4 to
−0.7. The initial sinuous stage
of the instability of the weaker flap vortex has a wavelength of order one wing span
and becomes observable in about 15 wing spans downstream of the wing. The nearly
straight vortex filaments first form loops around the stronger wing-tip vortices. The
loops soon detach and form rings and move in the wake under self-induction. These
vortex rings can move to the other side of the wake. The subsequent development
of the instability makes the nearly quasi-steady and two-dimensional wakes unsteady
and three-dimensional over a distance of 50 to 100 wing spans. A rectangular wing
is also used to generate the classical wake vortex pair with the circulation ratio
of −1.0, which serves as a reference flow. This counter-rotating vortex pair, under
similar experimental conditions, takes over 200 spans to develop visible deformations.
Velocity, vorticity and enstrophy measurements in a fixed plane, in conjuction with the
flow observations, are used to quantify the behaviour of the vortex pairs. The vortices
in a pair initially orbit around their vorticity centroid, which takes the pair out of the
path of the wing. Once the three-dimensional interactions develop, two-dimensional
kinetic energy and enstrophy drop, and enstrophy dispersion radius increases sharply.
This rapid transformation of the wake into a highly three-dimensional one offers a
possible way of alleviating the hazard posed by the vortex wake of transport aircraft.