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In this study, the flow features of vortex shedding from a circular cylinder forced-oscillating in the in-line direction were investigated by use of flow visualization experiment and numerical simulation at the Reynolds number Re=620, with varied amplitude ratio and varied frequency ratio. As a result of the experiments, it was found that although the flow structure around the circular cylinder is two-dimensional in the lock-in state of simultaneous vortex shedding, the large scale three-dimensional instability is observed in the lock-in state of alternate vortex shedding through a time lag in the boundary layer separation along the cylinder span. As a result of calculations, two typical flow patterns of the lock-in were shown, and it was confirmed that the calculated flow patterns were in reasonable agreement with previous experimental results. The fluid force act on the oscillating cylinder was investigated. It was clarified that the amplitude of the lift coefficient was larger than the amplitude of the drag coefficient in the lock-in of alternate vortex shedding, and the amplitude of the drag coefficient was larger than the amplitude of the lift coefficient in the lock-in of simultaneous vortex shedding. When the amplitude ratio 2a/d grows, this tendency becomes remarkable

The appearance of two lock-in states in the vortex flow around an in-line forced oscillating circular cylinder