245 Vortex Shedding and Vortex Formation from an In-Line Forced Oscillating Circular Cylinder

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/f_k=0〜6.2, amplitude ratio 2a/d=0.0625〜1.0, and Reynolds number Re=570. The variations of mean vortex shedding frequency were investigated. It is shown that although the cylinder oscillation frequency is lower man the natural Karman vortex's frequency, the lock-in phenomenon arises. Most of the flow patterns showed a complicated aspect, so the classification of flow pattern in the lock-in region was difficult The flow patterns in die lock-in region were classified with the aspect of separated shear layer, the timing of vortex shedding from cylinder and the arrangement of vortices in the wake. Seven kinds of basic form of vortex shedding were obtained and 18 flow patterns obtained by the arrangement of vortices in the wake.