Wind-induced noise in bridge members and its mechanism

为了弄清平板叶栅扶手的风致声特性，进行了一系列风洞试验。当风斜向板时，可以观察到纯粹的响亮声音。声频与风速的关系以及流动可视化实验结果表明，产生声音的原因如下：从板块迎风边缘分离出来的气流撞击同一板块的另一个迎风边缘，产生较大的压力波动。观察到，随着风速的增大，声频呈阶梯状增加，并与平板叶栅所围空间的声振荡共振频率相对应。频率的逐步增加表明在声音的产生和声学谐振振荡之间存在反馈。为了研究利用周期声和实验方法抑制结构流激振动的可能性，对圆柱涡激振动进行了数值模拟。采用基于ALE公式的有限元法模拟了涡激振动，并采用预测-校正法进行了时间积分。实验和数值结果表明，转换波频率的激励对改变圆柱周围的流动特性和涡激振动特性最为有效。这些变化似乎是由于在柱后早期涡形成过程中对涡生长的促进所致。

To clarify the characteristics of the wind-induced sound from handrails of flat plate cascades, a series of wind tunnel experiments is conducted. Pure loud sound is observed when the wind is oblique to the plates. The relation between the sound frequency and the wind speed as well as the result from the flow visualization experiment suggests the cause of the sound as follows ; the flow separated from the windward edge of the plate impinges the other windward edge of the same plate, generating large pressure fluctations. It is observed that the sound frequencies increase in a step-wise manner as the increase of the wind speed, and they are shown to correspond to the resonant frequencies of acoustic oscillation of the space enclosed by the flat plate cascades. The step-wise increase of the frequencies indicates that there is a feedback between the sound generation and the acoustic resonant oscillation.In order to investigate the possibility of suppressing flow-induced vibration of structures by applying periodic sound as well as experiments to the flow around the structures, numerical simulations of the vortex-induced vibration of a circular cylinder are carried out. A FEM based on the ALE formulation was employed to simulate the vortex-induced vibration, with the time integration being carried out by the predictor-corrector method. The experimental and numerical results revealed that the excitation with the transition wave frequency is the most effective in changing the flow characteristics around the cylinder and the characteristics of the vortex-induced vibration. These changes appear to be caused by the promotion of the vortex growth in the early vortex formation behind the cylinder.