The physical nature of the initial transition to three-dimensionality of fl
ow past a circular cylinder has been the subject of considerable debate in
the literature. Of several proposed mechanisms, the possibility of classifi
cation as an elliptical instability is re-examined in this article. Detaile
d Floquet analysis of the transition shows clear evidence of the growth of
an elliptic instability in the forming vortex cores followed by amplificati
on by the strong strain field in the hyperbolic region between the forming
and shed vortices. In fact, it appears that the wake immediately behind the
cylinder shows distinct signs of a cooperative elliptic instability as fou
nd previously for interacting counter-rotating vortices. Further downstream
, after the vortices have been shed into the wake, the instability again gr
ows in the cores. Three dimensional simulations provide a semi-quantitative
estimate of the "'elliptic content" of instability, and confirm that ellip
tic instability seems to be dominant in the initiation and maintenance of t
he 3-D perturbation. (C) 2001 Academic Press.