No abstract

Recent studies conducted by NASA have identified over the rotor liners as one of the most promising avenues for aircraft noise reduction. However, conventional noise absorption technologies cannot withstand the harsh pressure and temperature conditions near the engine core. While open celled metal foams are an alternative material choice, their open porous structure results in comparatively lower absorption characteristics. In this talk, we summarize the results from our recent efforts on improving the acoustical properties of metal foams for aircraft noise reduction applications. We show that compressing the foams substantially improves their absorption coefficient because of the reduction in the effective pore size. Unfortunately, this improvement comes with an undesirable weight penalty. To overcome this, we propose an optimized layered configuration to create a step-wise relative density gradient that provides comparable absorption characteristics while being over 20% lighter than the benchmark compressed foam. Our results show that metal foam liners with spatially gradient property configurations can provide broadband engine noise reduction while minimizing the resulting weight penalty.

Additive manufacturing allows the fabrication of acoustical materials with previously unrealizable micro- and macrostructural complexities. However, the still nascent understanding of various geometrical defects occurring during the additive process remains a barrier to accurately predicting the acoustical behavior of such complex absorbers. In this study, we present the results from our efforts on numerically modeling the absorption behavior of periodic porous absorbers fabricated using the stereolithography (SLA) technique using the hybrid micro-macro multiphysics approach. Specifically, we focus on understanding the role played by the expansion or shrinkage of the solid ligaments during the SLA process on the transport parameters of the final printed samples. First, the periodic absorbers are modeled using COMSOL multiphysics, where the transport properties are derived using the Johnson-Champoux-Allard-Lafarge-Pride (JCALP) semiempirical model. Results from the parametric study guide the design and fabrication of test articles that closely match the initial design requirements. Finally, the fabricated samples are tested using an impedance tube, and the obtained absorption properties are compared to the a priori numerical predictions. Results indicate that accounting for fabrication defects within the numerical modeling schema can provide reliable sound absorption predictions for additively manufactured porous absorbers.

Spatial property gradients can significantly enhance the noise reduction potential of porous structures. However, predicting the acoustic behavior of structures with gradient microstructures continues to remain a challenge. For porous structures with controlled periodic microstructures, researchers have recently demonstrated the use of a multiscale asymptotic method to extract the acoustic transport properties necessary for use in semi-empirical predictive models. Here, we propose the adaptation of this method to enable the modeling of porous absorbers with stepwise property gradients. The unit cell of the chosen microstructure is modeled in COMSOL Multiphysics and the acoustic transport properties are extracted using the multiscale method. The extracted properties are then used to formulate the transfer matrix of each unit cell. The global sound absorption and transmission loss behavior of the stepwise gradient structures are further predicted by combining the appropriate local transfer matrices. Our results show that the integration of the unit cell and transfer matrix methods provides a robust way of predicting the acoustic behavior of stepwise gradient porous structures with various combinations of layer thicknesses and geometries. The method provides a computationally efficient method to model such structures and can further the development of porous structures with application-specific acoustical properties.

Recently, we have demonstrated the feasibility of using extrusion-based additive manufacturing methods to fabricate fibrous sound absorbers [1]. Here, we investigate the feasibility of predicting the sound absorption behavior of such structures by extracting their geometry-based transport properties using the hybrid, multiphysics modeling approach. The acoustical transport properties are calculated by modeling the fibrous unit cell and solving three boundary value problems over the representative elementary volume of the periodic fluid domain. Our results show that the absorption predictions obtained using the as-designed unit cell significantly differ from the experimental measurements obtained using conventional two-microphone impedance tube tests. Further investigations conducted using an optical microscope reveal that while the printed fiber diameter remains uniform over its central portion, the fiber diameter decreases drastically near the fiber root. Finally, we show that incorporating these geometrical differences within the model improves computational predictions and accounts for the deviations between the numerical and experimental absorption coefficients. [1] Johnston W, Sharma B. Additive manufacturing of fibrous sound absorbers. Additive Manufacturing. 2021 May 1;41:101984.