The structures of the flow induced by a circular cylinder performing sinusoidal oscillations in a fluid at rest are investigated by means of flow visualization. The experiments are carried out at Keulegan–Carpenter numbers between 1.6 and 15 and at Stokes numbers between 5 and 160. Above a certain value of Keulegan–Carpenter number, depending on the Stokes number, some asymmetry appears in the flow separation and the associated vortex development behind the cylinder. The two vortices that are developed in a half cycle differ in strength and may be convected in different directions. This results in a fascinating set of flow patterns. Eight different regimes of flow can be identified within the ranges of Keulegan–Carpenter number and Stokes number studied. Furthermore, most of the resulting flows show a three-dimensional instability along the axis of the cylinder. Measurements of the wavelength of these disturbances are presented.
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