Oscillations of a vertical elastically mounted cylinder in a wave: imaging of vortex patterns

A vertical, elastically mounted cylinder undergoes two-dimensional oscillations in a free-surface water wave. The cylinder has nonpreferential stiffness and a very low mass-damping parameter. Low damping is accomplished using a mini-jet air bearing system. Patterns of instantaneous vorticity are characterized in a horizontal plane using a technique of high-image-density particle image velocimetry (PIV) involving a two-camera system to preclude the effects of cylinder blockage during image acquisition. A generic trajectory of the cylinder oscillation is generated by tuning the ratio of the natural frequency of the cylinder to the wave frequency; this trajectory is representative of those measured in previous related experiments, for which quantitative flow visualization was not performed. In this investigation, a concept of relative wave velocity provides the most insightful link between the wave motion and the quantitative patterns of vortex generation and interaction; these features are characterized in relation to the instantaneous phase of the cylinder during its trajectory. Basic mechanisms of the vorticity patterns involve: (i) generation of large-scale concentrations of vorticity; (ii) short-duration Karman shedding; (iii) migration of large-scale concentrations about the periphery of the cylinder; and (iv) collision and distension of previously generated vorticity concentrations with the surface of the cylinder. The states of vortex formation are fundamentally different on each of the branches of the generic cylinder trajectory, even though it has nearly symmetrical left and right branches. Furthermore, it is demonstrated that the vorticity concentrations decay extremely rapidly, which means that the in-plane vortex dynamics are essentially confined to the region close to the cylinder. (C) 2003 Elsevier Ltd. All rights reserved.