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

振动学界普遍认为，旋转缸-轴系统在高速运转超过临界转速时，会发生剧烈的振动。非定常振动发生在缸-轴系统的转速、淹没水深和固有频率等非常有限的条件下。1)建立了一套实验装置，用于测量旋转的圆柱体受恒振幅正弦振动时所受的非定常流体力。我们同时测量了非定常流体力和旋转圆柱体的位移，并计算了非定常流体力对圆柱体系统产生的单位时间能量。对实验参数的组合：转速、振荡频率、振幅和下水深度进行了能量评估。2)发现非定常流体力是相角导致气缸位移的，导致在非常有限的转速和振荡频率范围内能量为正值。3)气缸轴系统在上述规定的相同条件下运行时发生不稳定振动。4)拍摄了旋转气缸发生不稳定振动时在气缸周围产生的旋波，并对视频图像进行了分析。研究发现，当不稳定振动发生时，旋转圆柱体周围产生的波动与圆柱体-轴系系统的固有振动具有相同的频率。5)研究表明，旋转圆柱体-轴系发生的流致不稳定振动与圆柱体周围产生的流体波有很强的相关性。然而，理论研究仍然需要澄清不稳定振动的机制，这必须以本报告所示的实验结果为基础。

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.