3-DIMENSIONAL VORTEX STRUCTURES IN A CYLINDER WAKE

The three-dimensionality of the velocity held in the wake of a circula r cylinder has excited considerable interest and debate over the past decade. Presented here are experimental results that characterize the underlying vorticity field of such wakes. Using particle image velocim etry (PIV), instantaneous velocity fields were measured and from these the vorticity of the longitudinal vortices lying in the region betwee n Karman vortices was found. Near the saddle point, induced by the str etching of the Karman vortices, the vorticity of the longitudinal vort ices was found to be greater than the Karman vortices themselves. Thei r circulation was of the order of 10% of the Karman vortices. The high levels of vorticity result from the stretching of the longitudinal vo rtices, as evident in the topology of the vortices. It is shown that t he longitudinal vortices are locked in phase to the Karman vortices, e ffectively riding on their backs in the braid region. While only one m ode of longitudinal vortex formation was observed, evidence was found of a step change in the vorticity levels at a Reynolds number of appro ximately 200. This is consistent with the transition point between the two modes of vortex shedding shown to exist by Williamson (1988). It had previously been proposed that the observed vortex patterns were co nsistent with the evolution of the longitudinal vortices from perturba tions of vortex lines in the separating shear layer which experience s elf-induction and stretching from the Karman vortices. Evidence is pre sented that supports this model.