Generation of Streamwise Vortical Structures in Bluff Body Wakes

Direct numerical simulation is used to study the periodic generation and evolution of streamwise vortical structures in the near wake of a circular cylinder. It is observed that streamwise structures are formed due to stretching of core vorticity which escapes out of the core and due to stretching of small scale streamwise vorticity already present outside the core. Distinct hairpin structures are observed which play a central role in transferring vorticity out of the core. Furthermore, the hairpin structures are associated with a spanwise subharmonic mode and are thus manifestations of a period-doubling mechanism.