Experimental comparison of noise dissipation effects of single- and double-layer acoustic liners

Abstract

Noise damping performances of 12 perforated liners in the configurations of single- or double-layer are studied in this work. Both experimental and numerical investigations are conducted on these perforated liners with different porosities η, thus enabling the open-area-ratio (also known as porosity η) effect being studied. To simulate practical applications for example gas turbine engines and mufflers, adjustable bias and grazing flows are simultaneously applied to these liners. This enables the role of joint grazing-bias flow being evaluated. These liners’ noise mitigating performance is characterized by acoustic power absorption coefficient Δ. And Δ is measured over the frequency range from 200 to 800 Hz. Increasing the Mach number Mg of the grazing flow is found to deteriorate the liners damping performance, while increasing the bias flow results in the maximum Δmax being increased. Additionally, Δ is found to change harmonically with the noise frequency. Furthermore the local maximum of Δ is reduced with increased forcing frequency. Compared to the single-layer perforated liner, the double-layer one shows a higher Δmax and a broader effective frequency range. On the other hand, when the porosities ηi,o of the outer and inner perforated liners are about 1.1%, the noise absorbing performances are found to be dramatically reduced, especially when the forcing frequency is higher. Increasing the porosities ηi,o gives rise to Δmax being dramatically increased and so the effective frequency range. To gain insight on the noise absorbing mechanism and 10% more power absorption capacity associated with the double-layer liners than that of single-layer liner, 2D lattice Boltzmann simulations of in-duct perforated orifices are performed in time-domain. The calculated acoustic power absorption coefficients from these 2 different configurations of perforated orifices are compared. Finally, optimum design of a single-layer acoustic liner is conducted using a frequency-domain model of a lined duct. It is shown that the orifice thickness T and the porosity ηi play critical roles in determining the optimum noise damping performance.