Re=5,000

8,000

10,000

Small-scale structures in the near-wake of a cylinder. The onset of
small-scale (Kelvin-Helmholtz) instabilities in the shear layers from
a circular cylinder is indicated by contours of positive (red) and 
negative (yellow) vorticity. Lower level concentrations are detectable
at Re = 5,000. When the Reynolds number is increased to Re = 8,000 
pronounced small-scale concentrations appear downstream of the base of 
the cylinder, and at Re = 10,000, they are evident upstream of the base.
These small-scale vortices feed into the large-scale Kármán vortices
and, in fact, the formation length of Kármán vortices decreases at
larger values of Reynolds number. [1] This change in formation length 
has important consequences for the fluctuating lift and the mean base 
pressure coefficient on the cylinder.

                                          Chyu and Rockwell