The Unsteady Wake of an Oscillating Cylinder at Low Reynolds Number

A great deal of consideration has been given to flow-bluff body interactions because of the practical need to know the fluid forces acting on bodies immersed in a flowing stream. A flow regime exists where forced and fluid-induced transverse vibrations of a cylinder synchronize the vortex shedding and cylinder frequencies, and interact with the flow to control the wake formation process. Measurements have been made with a hot-wire anemometer for Reynolds numbers between 120 and 350, where cylinder vibrations at, and near, the natural shedding frequency both synchronize the wake and suppress the initiation of turbulence. Data are presented for the changes induced in the vortex formation by different conditions of forced excitation, and it is shown that the formation region length is reduced by as much as 50 percent from its value behind the stationary cylinder. A detailed study is made of fluctuating velocities between 1–9 dia downstream at a Reynolds number of 200, and the paths of the early vortices after formation are inferred from the fluctuating velocity profiles. The formation length is shown to be a suitable scale for displaying the velocity distribution in the synchronized wake, and the mechanics of the formation process are discussed in terms of the present experimental results. Wake correlation effects and the onset of turbulence are also discussed in light of the hot-wire measurements.