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A great many studies have been directed towards understanding the flow over
wings and, in particular, over a delta wing. Very surprisingly indeed,
in our view, there exists no visualizations, from the laboratory, of the
far-field development of the trailing vortex pair as it travels downstream,
to our knowledge. Our flow visualization, involving novel free-flight
gliding of a delta wing in water, shows for the first time, the exquisitely
beautiful structure of the turbulent wake, in side view. One should note
that these photographs are all to closely the same scale! The fluorescene
dye, illuminated by a laser, shows that the near wake comprises an interaction
between the primary streamwise vortex pair with the 'braid' wake vortices
between the pair. Far downstream (64 chordlengths behind the wing) the
primary vortex pair has reconnected and become large-scale rings (lower
picture), although with a distinctly smaller length scale than predicted
from the analysis of Crow, S.C. (1970) [Stability theory for
a pair of trailing vortices, AIAA J., 8, 2172].
Contributor: Prof. C.H.K.
Williamson, Cornell University, Ithaca, NY, U.S.A.
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