Bias Flow Acoustic Liner with Straight and Tapered Apertures

Experimental and computational studies are performed on slit resonators, i.e., one aperture with a high aspect ratio that spans through the center of the plate. An impedance tube experiment is conducted to investigate the macroscopic response of slit apertures. The test specimens include straight and tapered apertures. Subsequently, the effect of introducing a bias flow is investigated. The absorption performance increases when the sound pressure level increases to a level that causes a production of shed vortices. When a bias flow is introduced, the absorption coefficient reaches its maximum absorption of the incident sound wave in the region near the resonant frequency for a Mach number close to 9.71 . 2D numerical simulations are performed and validated with the experimental results. Good agreement is obtained for the majority of the simulated cases. Vortex shedding and its effect on the absorption coefficients is also investigated. INTRODUCTION Acoustic liners are widely used devices in aero-engines, and they are installed on the inner side of the nacelle to reduce fan noise in turbofan engines of commercial airplanes (conventional acoustic liners without bias flow). They are also used in the combustion chamber to reduce the acoustic instabilities caused by the combustion (acoustic liners with bias flow where the bias flow is caused by secondary air flow). An acoustic liner is typically made of a perforated metal sheet backed by a cavity. Each aperture of the perforated sheet and the cavity form a Helmholtz resonator. The resonator effectively absorbs the sound near the resonant frequency, however, its absorbing performance decreases at off-resonant frequencies. Howe [1] theoretically proposed that a low frequency (low Strouhal number) sound wave can be significantly attenuated by a jet flow by converting the acoustical energy into energy of fluctuating vorticity, which is shed from the nozzle edge. Bechert [2] proposed another theory to explain this phenomenon, and this was supported via experimental data. Bechert [2] also proposed a simple theory to predict the optimum Mach number of bias flow to obtain the perfect attenuation. On the other hand, Howe's theory to predict the sound absorption coefficient including the effects of a bias flow is well supported by an experiment by Hughes and Dowling [3]. Hence, this led to the idea that the off-resonant performance of a resonator can be improved if a jet (or a bias flow) is introduced from an aperture of an acoustic liner. Lahiri et al. [4] collected this type of experimental data and showed that the application of a bias flow through the aperture widens the frequency range of dissipation, with the penalty of reduced peak performance near the resonant frequency. Zhao and Li [5] wrote a summary on tunable acoustic liners including a liner with bias flow. In the field of numerical simulation, Mendez and Eldredge [6] performed a 3D large eddy simulation (LES), and Ji and Zhao [7] performed a 2D lattice Boltzman method (LBM) for an aperture with bias flow, and both made a comparison with Howe's theory, obtaining good agreements. Roche et al. [8] conducted 3D and 2D axisymmetric numerical simulations for a cylindrical acoustic resonator in the case without a bias flow using direct numerical simulation (DNS). Good agreement was obtained between 2D and 3D results. In a previous study (Ramdani et al. [9]), 2D simulations of a slit resonator were conducted using LES for cases with and without bias flow. Good agreement was obtained for the non-bias flow case. However, the bias flow case was not validated due to the lack of the experimental results for the simulated model. Tam et al. [10] conducted a series of experiments and DNS simulations to slit apertures with a 90° corner (straight aperture) and 45° corner (tapered aperture). The results obtained using the simulation supports the usage of computational aeroacoustics (CAA) as a design tool. Wada and Ishii [11] performed experiments for acoustic liners with a bias flow passing through the apertures of a perforated plate (circular straight perforations) and observed that the absorption range of the liner became wider and was not concentrated around the resonant frequency as in the case of the conventional liner. They compared the experimental results with Howe's extended theory proposed by Luong et al. [12], which considered the thickness of the perforated sheet and obtained good agreements. In a previous study (Tanaka et al. [13]), the macroscopic effect of the design parameters (such as the shape of the aperture and the flow velocity when a bias flow is applied through the aperture) on the impedance of an acoustic resonator was experimentally investigated via an acoustic impedance tube. The results revealed that the fully tapered aperture exhibited a wider absorption frequency range when compared to that of a straight circular aperture. However, little was known about the reason for such a behavior given the difficulty of visualizing the flow around the small apertures in the experimental setup using the impedance tube. In the present study, the acoustic performance of the liner and the flow field around the perforated plate is numerically solved using the compressible Navier Stokes equations to understand the acoustic and fluid dynamic behavior of the liner and the effect of the shape of the perforation at a microscopic level. Bias Flow Acoustic Liner with Straight and Tapered Apertures Soufiane Ramdani1, Nobuhiko Yamasaki1, Yuzo Inokuchi2, Tatsuya Ishii3 1 Department of Aeronautics and Astronautics Kyushu University 744 Motooka, Nishi-ku, Fukuoka 819-0395, JAPAN 2Civil Aviation College 3Japan Aerospace Exploration Agency International Journal of Gas Turbine, Propulsion and Power Systems June 2019, Volume 10, Number 3 Manuscript Received on October 17, 2018 Review Completed on June 27, 2019 Copyright © 2019 Gas Turbine Society of Japan