Flow structure from an oscillating cylinder Part 1. Mechanisms of phase shift and recovery in the near wake

Cylinders of various cross-section were subjected to controlled oscillations in a direction transverse to the incident flow. Excitation was at frequency f e , relative to the formation frequency f * 0 of large-scale vortices from the corresponding stationary cylinder, and at Reynolds numbers in the range 584 les Re les 1300. Modifications of the near wake were characterized by visualization of the instantaneous flow structure in conjunction with body displacement-flow velocity correlations. At f e / f * 0 = ½, corresponding to subharmonic excitation, as well as at f e / f * 0 = 1, the near wake structure is phase-locked (synchronized) to the cylinder motion. However, the synchronization mechanism is distinctly different in these two regimes. Near or at f e / f * 0 = 1, the phase of the shed vortex with respect to the cylinder displacement switches by approximately π. Characteristics of this phase switch are related to cylinder geometry. It does not occur if the cylinder has significant afterbody. Over a wide range of f e / f * 0 , the perturbed near wake rapidly recovers to a largescale antisymmetrical mode similar in form to the well-known Kármán vortex street. The mechanisms of small-scale ( f e ) vortex interaction leading to recovery of the large-scale ( f 0 ) vortices are highly ordered and repeatable, though distinctly different, for superharmonic excitation ( f e / f * 0 = n = 2, 3, 4) and non-harmonic excitation (non-integer values of f e / f * 0 ). The frequency f 0 of the recovered vortex street downstream of the body shows substantial departure from the shedding frequency f * 0 from the corresponding stationary body. It locks-on to resonant modes corresponding to f 0 / f e = 1/ n . This wake response involves strictly hydrodynamic phenomena. It shows, however, a resonant behaviour analogous to that of coupled flow-acoustic systems where the shear layer is convectively unstable