Experimental and numerical studies of low-frequency duct acoustic liner using metamaterial technology

Abstract

Low-frequency noise absorption has been one of the greatest challenges in the fields of engineering, especially when it involves the presence of mean duct flow in applications such as ventilation systems. The currently popular approach involves passive dissipative mitigation devices that absorb the energy of by-passing noise. Usually, the mitigation devices are made of porous or fibrous materials, and as such their characteristic size would have to be comparable to the wavelength of noise for achieving a satisfactory absorbing effect. This renders the devices impractically bulky and heavy in low frequencies noise control. Coupled with the usual high flow resisting nature of the devices, the current approach is far from ideal for applications with strict flow speed requirements or tight spatial constraints. Recent researches on metamaterials focus on their application to acoustics and provide a possible alternative solution to the problem of controlling low-frequency duct acoustics. Nevertheless, the effectiveness of metamaterials when exposed to duct airflow has never been studied. This project aims at studying the capability of AMM technology under a grazing duct flow in a controlled environment, using both experimental and numerical approaches. For the experimental studies, duct silencers consisting of metamaterial liner units are fabricated. The design of the liners is loosely based on the membrane-type metamaterial concept of Mei et al. (2012). The acoustic performance of duct liners is evaluated from recorded time traces of acoustic pressure. Spectra of the measured performance are expressed in terms of acoustic coefficients and transmission loss which involve a selection of different design parameters of the liners and velocity settings of the duct flow. Insights and characterization of the liner acoustic performance by the varying parameters will be given. For studying the local mitigation process of the liners which is beyond the capability of the experiment, a numerical study using a time marching Direct Aeroacoustic Simulation (DAS) approach has been conducted. The DAS resolves the aeroacoustic-structural interactions of the metamaterial liner under an unsteady compressible grazing flow. An attempt is made to expand the scope of the structural model employed in the DAS to incorporate multiple constituents on the liner with different material properties. Validation of the updated model is conducted by experimental means. The numerical study qualitatively 5actions among the multiple repeating unit cells within the liner are also investigated.