In this photograph, we show one of instabilities of a pair of straight counter-rotating
vortices. Although analytical studies of vortex pair instabilities, by
Crow (1970) and Widnall et al. (1974), have suggested the existence of
long and short wavelengths, it is quite surprising that there exists,
in the literature, no clear evidence of such structure in the laboratory.
Here,
the vortex pair is generated at the sharpened edges of two flat plates,
hinged to a common base and moved in a prescribed symmetric fashion. Visualization
is achieved using fluorescent dye. The evolution of the vortex pair was
found to depend strongly on the vortex velocity profiles, which are determined
by the motion history of the plates. We have identified at least three
different length scales from our experiments, one of which we present
in the photograph above. In this image, we demonstrate the development
of a short-wave instability (wavelength less than one vortex separation).
The remarkably clear visualization of the vortex core reveals its complicated
internal structure, and the observed phase relationships show that the
symmetry of the flow with respect to the midplane between the vortices
is lost.
Crow S.C. (1970) Stability theory of a pair of trailing vortices. AIAA
J., 8, 2172.
Widnall,
S.E., Bliss, D.B. & Tsai, C.-Y., The instability of short waves on
a vortex ring, J. Fluid Mech., 66, 35.
Contributed by Prof. C.H.K. Williamson, Cornell University, Ithaca, NY,
U.S.A.
