Ex id

Proc. R. Soc. A (2005) 461, 3827–3845through a symmetrical plane sudden expansion preceded by a gradual contraction from a square duct. As is well known, for Newtonian fluid flow above a critical Reynolds number the flowfield downstream of an expansion becomes asymmetric. For the viscoelastic liquid investigated here the asymmetry is greatly reduced, with very similar reattachment lengths for the two recirculation regions. More significantly, the flow unexpectedly develops a strongly three-dimensional jet-like structure, with side-to-side symmetry centred on the ‘vertical ’ symmetry plane of the contraction/expansion geometry. Especially interesting is the flow within the contraction itself, where the nature of the flow field for the viscoelastic liquid is also fundamentally different to that for a comparable Newtonian fluid flow: large velocity overshoots with very strong gradients occur near to the sidewalls that, due to their appearance, we have termed ‘cat’s ears’. The fully developed approach flow in the square duct is unremarkable