Wind-induced noise in bridge members and its mechanism

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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.