Investigation to the instability Mechanism of Flow Induced Vibration in a Rotating Cylinder-Shaft System Partially Submerged in Water

在振动领域中，旋转的缸轴系统在超过临界转速的高转速下运行时，会发生剧烈的振动。不稳定振动发生在转速、沉水深度和缸轴系统固有频率有限的条件下。1)搭建了实验装置，测量旋转气缸被强制恒幅正弦振荡时的非定常流体力。同时测量了非定常流体力和旋转气缸的位移，计算了非定常流体力对气缸系统单位时间产生的能量。对实验参数组合的能量进行了评估；旋转速度、振荡频率、振幅、下潜水深。2)发现非定常流体力呈相位角，导致气缸位移，导致能量在非常有限的转速和振荡频率范围内为正。3)在上述条件下运行时，气缸-轴系统存在不稳定振动。4)我们拍摄了旋转的气缸以及气缸发生不稳定振动时在气缸周围产生的旋涡波的视频图像，并对时间序列视频图像进行了分析。研究发现，当不稳定振动发生时，旋转圆柱周围产生的波与圆柱-轴系统的固有波具有相同的频率。5)研究认为，旋转缸-轴系统中发生的流致不稳定振动与缸周围产生的流体波密切相关。然而，要弄清控制不稳定振动的机理，还需要进行理论研究，这必须以本报告的实验结果为基础。

It is well known in vibration field that the rotating cylinder-shaft system undergoes a violent vibration when it is operated at a high speed of rotation over the critical speed. The unstable vibration occurs under a very limited conditions imposed to the rotation speed, submerging water depth, and the natural frequency of the cylinder-shaft system.1) The experimental apparatus was constructed to measure the unsteady fluid force acting on the rotating cylinder when it was forced to oscillate sinusoidally with a constant amplitude. We measured the unsteady fluid force and the displacement of the rotating cylinder simultaneously and evaluated the energy per unit time produced by the unsteady fluid force on the cylinder system. The energy was evaluated for the combinations of experimental parameters; rotation speed, oscillation frequency, amplitude, and submerging water depth.2) We found the unsteady fluid force was of phase-angle lead to the cylinder displacement which resulted in the energy positive for a very limited range of rotation speed and oscillation frequency.3) The cylinder-shaft system underwent an unstable vibration when it was operated under the same condition specified by the above.4) We took video picture of the rotating cylinder and swirling wave generated around the cylinder when it undertook the unstable vibration and analysed the time series video images. It was found the wave generated around the rotating cylinder has the same frequency as the natural one of the cylinder-shaft system when the unstable vibration occurred.5) The research concludes that the flow-induced unstable vibration occurred in the rotating cylinder-shaft system is strongly associated with the fluid wave generated around the cylinder. However the theoretical investigation is still needed to clarify the mechanism governing the unstable vibration which must be based on the experimental results shown in this report.