Near-wake structure of an oscillating cylinder: effect of controlled shear-layer vortices

The instantaneous structure of the near wake of a cylinder subjected to small-amplitude perturbations is characterized using high-image-density particle image velocimetry. Emphasis is on control of the small-scale shear-layer vortices, which feed into the Karman vortices. Modifications of the Karman vortex formation are classified according to patterns of modulated and locked-on shear-layer vortices. The formation length of the Karman vortices can be dramatically shortened and, in the limiting case, occur adjacent to the base of the cylinder when it is perturbed at the inherent instability frequency of the shear layer and its subharmonics. Moreover, the induced shear-layer vortices can lead to large-amplitude transverse undulations of the entire near-wake region during formation of the Karman vortices. These variations of the near-wake structure are further elucidated by considering the transient response of the wake, induced by abrupt cessation and onset of periodic motion of the cylinder. Distinctive intermediate states of the wake arise during relaxation to its asymptotic state; such relaxation requires a very large number of periods of the inherent instability of the shear layer.