RESPONSE OF THE SHEAR LAYERS SEPARATING FROM A CIRCULAR-CYLINDER TO SMALL-AMPLITUDE ROTATIONAL OSCILLATIONS

The frequency response of the shear layers separating from a circular cylinder subject to small-amplitude rotational oscillations has been investigated experimentally in water for the Reynolds number (Re) range 250 to 1200. A hot-film anemometer was placed in the separated shear layers from 1 to 1.5 diameters downstream of the cylinder, and connected to a lock-in analyser. By referencing the lock-in analyser to the cylinder oscillations, the amplitude and phase of the response to different frequency oscillations were measured directly. It is shown that rotational oscillations corresponding to cylinder peripheral speeds between 0.5 and 3% of the free stream can be used to influence the primary (Karman) mode of vortex generation. For Re greater than almost-equal-to 500, such oscillations can also force the shear-layer vortices associated with the instability of the separating shear layers. Corresponding to the primary and shear-layer modes are two distinct peaks in response amplitude versus frequency curves, and two very different phase versus frequency curves. The response of the shear layers (and near wake) in the range of Karman frequency suggests qualitative similarities with the response of an oscillator near resonance. Forced oscillations in the higher-frequency shear-layer mode range are simply convected by the shear layers. Close to the cylinder, the shear-layer response is shown to be comparable to that of generic free shear layers studied by others.