Design of Broadband Liner Based on Cremer Concept

In recent years, various novel acoustic liners have been proposed to meet the broadband noise reduction requirements of turbomachinery, such as multi-degree-of-freedom liners. However, the increasing number of adjustable geometric parameters brings about the complexity of liner design. The Cremer concept has commonly been employed to the rapid preliminary design of acoustic impedance. This paper presents a broadband liner design method, which achieves rapid design of liner geometric parameters by incorporating the impedance prediction model into the Cremer concept. Moreover, this method maximizes the match between the optimal impedance spectrum and the liner impedance spectrum, thereby achieving the desired objective of broadband sound absorption for the liner. Taking the uniform single- and double-degree-of-freedom liner as examples, three optimization strategies are evaluated in this paper: 1) the least squares method; 2) minimization of cavity depth; 3) multi-frequency optimization. Extensive optimization results show that the noise reduction bandwidth of a single-degree-of-freedom liner is related to the cavity depth, and the widest bandwidth liner can be designed by targeting two specific Helmholtz numbers. Similarly, the noise reduction bandwidth of a double-degree-of-freedom liner is related to the cavity depth near the fluid, and the widest bandwidth liner can be designed by targeting four specific Helmholtz numbers. For more complex liners, as long as an appropriate impedance model is available, this method is expected to expedite the design process and alleviate design complexity.