Over-the-rotor liner and soft vans have received much attention as new noise reduction means, and large-thickness perforated plate is a feasible structural form for realizing these two noise reduction means. Therefore, the acoustic impedance characteristics of large-thickness perforated plates are investigated. Based on the impedance tube and flow tube test platforms, the acoustic impedance characteristics of large-thickness perforated plates are experimentally investigated in the absence and presence of grazing flow, and the applicability of the Beihang model to the prediction of acoustic impedance for large thickness perforated plates is explored. Under no grazing flow conditions, the test acoustic impedance of a solid perforated plate with a thickness of 8 mm shows good linear characteristics, and the Beihang model is able to accurately predict the acoustic impedance of the large-thickness perforated plate. Meanwhile, a honeycomb-cavity sandwich perforated plate structure is proposed for the case where the static blade has a hollow structure. Measurement results show that the acoustic impedance of this structure is mainly provided by the perforated plates on both sides, while the honeycomb-cavity in the middle provides only a small portion of the acoustic impedance. Under grazing flow conditions, the applicability of the Beihang model to the prediction of acoustic impedance for large-thickness perforated plates is investigated in terms of both acoustic impedance prediction accuracy and transmission loss. When the grazing flow velocities are 10 m/s and 20 m/s, the Beihang model can still accurately predict the acoustic impedance of large-thickness perforated plates. As the grazing flow velocity increases, the effect of plate thickness on vortex shedding in small holes begins to become apparent. When the grazing flow velocities reach 40 m/s and 60 m/s, the prediction error of the Beihang model based on the fitting of vortex shedding strength of the thin plate on the acoustic impedance of the large-thickness perforated plate starts to increase. However, in terms of the transmission loss, the transmission loss curves predicted by the Beihang model show a similar trend with that of the experimentally measured transmission loss curve. It can be seen that the Beihang model still provides valuable guiding for the engineering application of large-thickness perforated plates.

In this paper, a model is established with application of the spectral-wave guide method, which has higher accuracy and can serve as a rapid calculation tool for sound transmission calculations. Based on this calculation model, some numerical results of circumferentially non-uniform lined annular/circular ducts are carried out, and some physical mechanisms can be discovered. The numerical results show that periodical impedance distributions along the circumferential direction will lead to discontinuous scattered modes with regular spacing; and mirror-symmetric structure liner will converge the energy of opposite modes. Relying on this mechanism, the potential of acoustic scattering can be further developed by suppressing lower or enhancing higher order modes with expressly designed segmented liner configurations. In particular, the intrinsic mechanism of mode redistribution brought about by the non-uniform liner can be subtly utilized to attenuate broadband noise. The present work indeed shows that circumferentially non-uniform liner is conducive to the reduction of the practical broadband sound source. Furthermore, the effects of non-uniform flow are considered in the model, then distinction of noise attenuation and scattered modes energy in different flows is shown. A possible mechanism is proposed that refraction effects in complex flows lead to the distinction. These works show that the current model has profound potential and availability for the research and designs of circumferentially non-uniform liner.