The physical mechanism of transition in bluff body wakes

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.