Partial roll-up of a viscoelastic Karman street

The instantaneous velocity field of a circular cylinder wake is built using a PIV technique when a small amount of viscoelastic liquid is introduced t hrough the cylinder. It is shown that the viscoelastic fluid slows down the vorticity concentration and produces a street of partially rolled-up vorti ces. The underlying mechanism appears very analogous to that of a surface t ension in the Kelvin-Helmholtz instability. The partial roll-up is studied in terms of the Weiss determinant. This quantity is a local measurement of the spatial separation between the strain and vorticity. In the viscoelasti c wake, the Weiss determinant reaches much lower values than in the Newtoni an wake. This result shows that the elasticity prevents the clear separatio n between vorticity and strain during the roll-up process. Since the Weiss determinant is directly related to the pressure field, it suggests that ela sticity can drastically modify the pressure levels even when vorticity and strain levels are unaffected. (C) 2001 Editions scientifiques et medicales Elsevier SAS.