WING VORTEX INSTABILITIES

Vortex pair instabilities are important to the break-up of aircraft trailing vortices, which are a hazard to other maneuvering aircraft. This is a problem of great practical significance. One approach to the understanding of this flow, the effect of a ground plane, and techniques to control such a flow, is to generate pairs of vortices using computer-controlled flaps. Enclosed below is a brief paper from a Conference in Italy, which really serves to give a visual appreciation of the different kinds of instabilities in vortex pairs which may arise, and it also serves to show how beautiful fluid motion can be! This work comes from Thomas Leweke, with whom I had the privilege to work at Cornell as a part of the Fluids Team, and who subsequently went to Marseille, France.

"Three-dimensional dynamics of a
counter-rotating vortex pair."
Leweke and Williamson (1998)
8th International Symp. on Flow Visualisation, Sorrento, Italy.

The second paper shown below appeared in Journal of Fluid Mechanics, in 1998, regarding our discoveries for short wave instabilities. The work takes, as its "springboard", the high-quality flow visualisations, which in both cases were selected as winners in the Gallery of Fluid Motion competition at the annual American Physical Society meetings, 1994 and 1995. Such instabilities had never been observed clearly in the laboratory before.

"Cooperative elliptic instability."
Leweke and Williamson (1998)
Journal of Fluid Mechanics.

The paper outlines some new discoveries; for example the small-scale instability waves on the vortices have a definite symmetry-breaking phase relationship between the two primary vortices, previously not observed experimentally, or even predicted analytically by classical works (Crow, 1970; Widnall, 1971). We find that the internal deformations of a vortex develop a structure which is beautifully predicted by a theory, hitherto not linked to this problem, namely "elliptic instability". This is an instability which comes from the three-dimensional unstable nature of a flow which has elliptic streamlines in the cross-sectional cores of the vortices. The actual structure of the internal instability is quite fascinating !

Click on the photo to view a movie on the development of the Crow instability.

Some connections between the trailing vortices of a delta wing (which evolve in space), and the vortex pairs from vortex generator studies (which evolve in time), are given in the following invited Keynote seminar paper:

"Instabilities in spatially developing wing wakes and temporally-developing vortex pairs".
Williamson, Leweke & Miller (1998)
ASME Summer Annual Meeting, Washington.

Click on the photo to view a movie on the long wave instability.

We are very excited at the prospect of this continuing program of original research, described in the example papers above, on a problem of practical interest, since it relates to aircraft trailing vortex instability ! We have been extending this work to co-rotating vortices, studying initially the problem of vortex merging, and to vortex pairs which approach a ground plane, where we are interested in how vortices undergoing three-dimensional instability, at different stages of its development, are then influenced by the ground. It is very interesting and sometimes surprising, but a lot of fun!