Secondary vortex streets in two-dimensional cylinder wakes

Spatially growing wakes behind a circular cylinder at 140 ≤ Re ≤ 1000 are simulated numerically. Two-dimensional, incompressible Navier–Stokes equations are solved using a splitting finite difference method. Special attention is paid to the development of the vortical structures in far wakes. Effects of subharmonic forcing on the vortical structures are also examined by perturbing the uniform velocity of a free-stream. In both unforced and forced wakes, the transition from the primary Karman vortex street in near wakes to the secondary vortex street in far wakes is observed for all of the Reynolds number cases examined. In unforced cases, rapid decay of the Karman vortices is observed, but clear evidence of merging of the Karman vortices in the transition process was not found. On the other hand, in forced wakes, merging of the Karman vortices is found to play a central role in the transition process. The effects of the Reynolds number as well as the forcing parameters on the vortical structures in far wakes are also examined.