The physical nature of the initial transition to three-dimensionality of flow past a circular
cylinder has been the subject of considerable debate in the literature. Of several proposed
mechanisms, the possibility of classification as an elliptical instability is re-examined in this
article. Detailed Floquet analysis of the transition shows clear evidence of the growth of an
elliptic instability in the forming vortex cores followed by amplification by the strong strain
field in the hyperbolic region between the forming and shed vortices. In fact, it appears that …