Flow induced vibration and noise control with flow

In pursuing the noise control, a device of a tensioned membrane backed by a cavity has been successfully used to reflect the sound at low-to-medium frequency range. However, in case of high flow speeds, tension is highly increased to compensate and maintain the performance. Flow induced vibration is also observed at high flow speeds. In this study, direct measurement of the wall pressure fluctuation is firstly conducted to reveal that the instable vibration depends on the aerodynamic loading rather than the acoustic loading. Besides, it is crucial that the flow leaks through a small gap along the membrane's lateral edge which allows for the free vibration. In this regard, the aerodynamic effect is investigated in case of the axial-flow and cross-flow directions. The instability phenomenon is found at moderately high flow speeds and it tends to disappear when membrane tension is increased. These findings are beneficial for the design of the membrane typed device.;On the other hand, the membrane type configuration can be used to reduce the strength of dipole noise source directly. The noise suppression of an axial fan was successfully demonstrated both numerically and experimentally. The fan radiated sound can induce the second mode of vibration of the membrane. Then sound radiation from the membrane propagates towards upstream and downstream and is cancelled with the dipole source due to the full couplings between the vibration of the membrane and acoustic fluctuation inside the cavity. The two-dimensional numerical model is constructed to understand the coupling mechanism of the sound propagations, cancellation and the response of the membrane. From the view point of practical usage and installation of the fan, the device is then explored in a comprehensive three-dimensional model. It is found that the optimal insertion loss can achieve more than 20dB over the frequency range of interest when the tension applied is low. It is much better than that by using expansion chamber with the same expansion ratio. Besides, the experimental result agrees well with the numerical prediction, showing the effectiveness and reliability of the numerical model.