Small-scale structures in the near-wake of a cylinder

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.