The physical mechanism of transition in bluff body wakes

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 the wake immediately behind the cylinder shows distinct signs of a cooperative elliptic instability as found previously for interacting counter-rotating vortices. Further downstream, after the vortices have been shed into the wake, the instability again grows in the cores. Three dimensional simulations provide a semi-quantitative estimate of the 'elliptic content of instability, and confirm that elliptic instability seems to be dominant in the initiation and maintenance of the 3-D perturbation. (C) 2001 Academic Press.