In this project, our central objective is to develop next-generation, thin, and lightweight acoustic liners using advanced cellular porous materials engineered to provide high sound attenuation over a wide frequency range and capable of withstanding extreme engine environments. Cellular materials offer a distinct advantage over traditional liner materials: their structural and functional properties are directly controlled by their local microstructural architecture. Thus, by engineering their microstructure, they can be tailored to provide significantly enhanced properties without parasitic mass addition. Here, our objective is to thoroughly characterize the effect of each individual microstructural parameter on the acoustic properties of open cell metal foams. The gained understanding will be leveraged to develop better predictive models for the design of foam metal liners and to inform the development of thin liners with variable through-thickness relative density.
Collaborators: Prof. Charlie Zheng (University of Kansas), Spirit Aerosystems, ERG Aerospace Corporation, NASA Langley, and NASA Glenn
Funding Agency: NASA, Spirit AeroSystems
Bio-inspired Multifunctional 3D Printed Porous Absorbers
Traditional manufacturing techniques offer limited control over the cellular architecture of porous materials and often do not allow fabrication of complex topologies. Here, we utilize 3D printing to overcome these difficulties and fabricate bulk absorbers with novel cellular architectures for use as acoustic liners. Absorbers with controlled surface topologies were fabricated using a combination of a commercial mathematical plotting software and stereolithographic 3D printing. The developed tool generates porous architectures using a field of points defined by a 3D grid and allows precise control over all microstructural parameters. The generated structures are then converted into the native STL file format required for 3D printing using commercial 3D printers. The efficacy of the modeling and fabrication methodology is verified using x-ray microtomography. The acoustic properties of the fabricated bulk absorbers were measured using a normal incidence tube setup. The effects of surface geometry, pore size, and through-thickness density gradients on the acoustic properties of the absorbers are studied and compared with equivalent-fluid model predictions.
Collaborators: Honeywell Aerospace and NASA Langley
Funding Agency: NASA, Acoustical Society of America, Kansas Board of Regents
Design and 3D Printing of Aerogel-based Ultra-lightweight Bulk Absorbers
Aerogels are a class of synthetic materials that are derived by extracting the liquid component of a gel through supercritical drying and replacing it with gas. This process results in the formation of a nanoporous solid composed of up to 99.98% air by volume. Though aerogels have found applications in thermal blankets and as energy absorbers, their use in acoustic applications have been limited due to their fragility and lack of control over their macrostructure. Here, we utilize a novel 3D printing method, developed by Dr. Dong Lin, combining freeze casting and extrusion print silica aerogel bulk absorbers. Conventional processes such as freeze casting and sol-gel have been widely applied to manufacture aerogels; however, they are incapable of controlling macrostructures. Further, it is still significantly challenging to manipulate the aerogel microstructure during 3D printing; the current technologies can only manufacture compact structures or cross-sections with random pores. Here, 3D printing of silica with interconnected and aligned pores will be fulfilled by combining freeze casting with extrusion. This freeze casting process aligns the particles or other materials along the freezing direction. This process enables us to fabricate aerogel bulk absorbers with controlled microstructures with optimized sound absorption properties.
Collaborators: Prof. Dong Lin (Kansas State University) and Aerogel Technologies LLC.
Funding Agency: NASA, Kansas Board of Regents
Acoustic metamaterials with negative mass and modulus behavior
We demonstrate a negative bulk modulus metamaterial based on the concept of expansion chambers. Addition of a neck region to an ordinary expansion chamber significantly improves its transmission loss characteristics at low frequencies and the resulting structure displays a negative bulk modulus behavior. Additionally, membrane based metamaterials are analyzed. Using FEM, the negative density behavior of a membrane carrying a center mass and of a tensioned membrane array is analyzed and the inherent similarity of the two designs is discussed. Further, the modified expansion chamber is combined with an array of stretched membranes and the resulting structure is analyzed for double negative behavior.
Collaborators: Prof. CT Sun (Purdue University)
Funding Agency: Office of Naval Research