Vortex Flow Around an In-Line Forced Oscillating Circular Cylinder

Abstract

In this study, the flow features of vortex shedding from a circular cylinder oscillating along the direction of the flow were observed by visualizing water flow experiment at the ranges of the frequency ratio f/fK=0-6.2, amplitude ratio 2a/d=0.0625-1.0, and Reynolds number Re=570-1700. The variations of mean vortex shedding frequency were investigated and the existence range of the lock-in was shown. It was obtained that although the cylinder oscillation frequency is lower than the natural Karman vortex's frequency; the lock-in phenomenon arises. It was found that the distribution of the lock-in form in the lock-in region was shown and that it became the tendency where the range with the frequency which becomes the lock-in form of fVK/f=1/1 with the increase of amplitude ratio expands. The flow patterns were classified into five kinds by the configuration of vortex shedding and the direction of vortex shedding. The structure of the vortex flow which depends on the frequency ratio was clarified by observing flow from both to the axial direction and the direction of span. In the case of lock-in state fVK/f=1/1, two kinds of vortex structure were obtained. One of which was three-dimensional characteristic on the side of low oscillation frequency, and the other was two-dimensional characteristic on the side of high oscillation frequency. In the case of lock-in state fVKf=1/2, even if the lock-in had occurred, it was shown that the phase difference sometimes occurred to the direction of the span in vortex shedding.