VORTEX DISLOCATIONS IN A PLANAR WAKE
C. H. K. Williamson
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 0.5D (D = cylinder
diameter) and with an outer diameter of 1.2D. 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.