In this photograph, turbulent large-scale lambda-shaped structures are shown to develop in the downstream wake of a cicular cylinder. The spanwise dye visualization used here shows the wake evolve as the flow travels upward past the horizontal cylinder. The vortical structures, whose appearance downstream resembles that of a turbulent spot in a boundary layer, are caused by a 'point' disturbance, in the form of a small ring placed around the cylinder at a particular spanwise position. The ring as a spanwise length of 0.5 D (D=cylinder diameter) and with an outer diameter of 1.2 D. The lines of fluorescene dye, which are washed off the cylinder surface (and illuminated by a laser floodlight), each mark separate Karman vortices shed from the body as it is translated down the length of our new XY towing tank at GALCIT. The red dye (rhodamine) marks the wake of only the ring. 'Vortex dislocations' appear in the wake at a frequency (f-f_R) when the shedding behind the ring (frequency f_R) is periodically out of phase with the shedding from the cylinder (frequency f). These same dislocations evolve into large-scale structures as one moves downstream. The visualization above shows a close-up view of the rather complex vortex dynamics in a typical dislocation region. Reynolds number = 100. Similar vortex dislocations are found in the wake of a uniform cylinder between spanwise cells of different shedding frequency.

Contributed by Prof. C.H.K. Williamson, Cornell University, Ithaca, NY, U.S.A.