Vortex shedding behind cylinders with spanwise linear nonuniformity
Résumé
Three-dimensional direct numerical simulations of vortex shedding behind cylinders have been performed when the body diameter and the incoming flow involved spanwise linear nonuniformity. Four configurations were considered: the shear flow, the tapered cylinder and their combination which gave rise to namely the adverse and aiding cases. In contrast with the observations of other investigators, these computations highlighted distinct vortical features between the shear case and the tapered case. In addition, it was observed that the shear case and the adverse case (respec. tapered case and aiding case), yielded similarities in flow topology. This phenomenon was explained by the spanwise variations of U/D which seemed to govern these flows. Indeed, it was observed that large spanwise variations of U/D seemed to enhance three-dimensionality, through the appearance of vortex-adhesions and dislocations. Spanwise cellular pattern of vortex shedding were identified. Their modifications in cell size, junction position and number were correlated with the variation of $U/D$. In the lee side of the obstacle a wavy secondary motion was identified. Induced secondary flow due to the bending of Karman vortices in the vicinity of vortex-adhesion and dislocations was suggested to explain this result.