流れ方向強制振動円柱まわりの渦流れ

摘要

In this study, the How 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 = 0.062 5～1.0, and Reynolds number Re=570～ 1 700. 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 f_(VK)/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 f_(VK)/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 f_(VK)/f=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.%流れの中に置かれた柱状物体がその背後に形成されrnる振動流れと相互干渉して自励振動を起こす現象は，rn工学的に大変重要な問題である．それゆえ，弾性支持rnされた柱状物体の渦励振による挙動と流れの観測や柱rn状物体を強制的に振動させた時の物体からの渦放出のrnようすなどを調査することが多くの研究者によって行rnわれ，数多くの知見が蓄積されている．