Mitigation of broadband duct flow noise using liner with gradient surface resonant compliance

Broadband noise mitigation in flow ducts remains a crucial area of study, especially in the low-frequency regime where conventional liner technologies such as dissipative liners, micro-perforated panels, and Helmholtz resonators are often ineffective. To address this limitation, the use of interior duct surface resonant compliance, which leverages aeroacoustic-structural interactions to mitigate low-frequency noise in duct flow, is found to be particularly promising. We investigate a novel approach using multiple compliant liner units, each comprising elastic panels backed by air-filled cavities, strategically flush-mounted on the duct walls. By strategically tuning the fluid-loaded resonant frequency of each elastic panel to introduce gradient surface resonant compliance, we create overlapping stopbands that enhance low-frequency noise mitigation. In this study, a robust numerical methodology based on the perturbation evolution method is utilized. A weak broadband acoustic excitation is introduced to simulate a realistic aeroacoustic flow duct environment. A detailed parametric study is carried out to compare three compliant liner system configurations: (1) baseline with uniform surface resonance distribution, (2) increasing resonance distribution, and (3) decreasing resonance distribution along the flow direction. The study confirms that the baseline compliant configuration yields remarkable reductions in broadband noise. The gradient-resonance compliant configurations further improve the performance, achieving enhanced low-frequency noise mitigation and increased overall sound transmission loss. The findings of the study demonstrate that strategically varying the fluid-loaded resonant frequencies of elastic panels enhances the structural resonant characteristics, thereby increasing the stopband width by 18.0 % for increasing resonance distribution configuration, while a 7.0 % widened stopband was demonstrated by decreasing resonance distribution with maximum transmission of 50.0 dB. Furthermore, the compliant liner systems demonstrated a remarkably lower drag penalty (≤ 10 %) than the minimum value observed in conventional acoustic liner experiments reported in the literature.