Numerical investigation of slat noise attenuation using acoustic liners

Abstract

Noise generated by high-lift devices such as slats on a wing is a major contributor to the overall airframe noise during the landing approach of a commercial aircraft. In this work the concept of attenuating slat noise using absorptive acoustic liners in the slat/main element gap is explored using a time domain computational aeroacoustic (CAA) technique. The aims of the development and application of the computational method are to reveal the mechanism of the slat noise generation and demonstrate the feasibility of controlling the slat noise using acoustic liners. A model scale three-element high-lift airfoil comprising a main element, a leading edge slat and trailing edge flap geometry is employed in the investigation. Numerical simulations are performed to investigate the generation and far field propagation of the slat noise. A numerical approach is developed that combines near field flow computation with an integral radiation model to predict the far field acoustic signal. A time domain impedance boundary condition (TDIBC) is implemented to simulate the effect of the liner material directly.
Both the high frequency tonal noise and low frequency broadband noise generated from the slat are investigated.
For the high frequency tonal noise, an unsteady Reynolds-averaged Navier-Stokes (URANS) simulation using high-order spatial and temporal schemes for the wing without acoustic liners shows the presence of vortex shedding and associated high frequency acoustic sources behind the slat trailing edge. To evaluate the mitigating performance of liners on the generated noise and find an optimized liner impedance value, an exercise is conducted on a range of liner impedance values by solving the linearized Euler equations (LEE) for a modeled acoustic source located at the trailing edge of the slat to find a optimized one. Using the optimized impedance value, URANS computations for the wing with liner treatment are conducted. The results show that acoustic liners on the slat cove and on the main element can provide useful attenuation of the high frequency slat trailing edge noise.
For the low frequency broadband noise, the noise sources are calculated by both the pseudo-laminar zonal method and the stochastic noise generation and radiation (SNGR) approach. The pseudo-laminar zonal calculation is basically an URANS calculation with the two-equation shear stress transport (SST) κ - ω model to model the effect of turbulence and a region in the slat cove is set as laminar zone. In the SNGR approach broadband sources of noise are modeled using stochastic noise generation method from a numerical solution of the steady Reynolds-averaged Navier-Stokes (RANS) equations using the κ - ω closure and then the acoustic field is calculated by solving the acoustic perturbation equations (APE) using high- order spatial and temporal schemes. By comparing the results of pseudo-laminar method and that of SNGR approach, the SNGR method has been shown to be a potentially useful method to model the generation of broadband slat noise and to investigate the attenuation of slat gap acoustic liners, for which the interest is in changes of noise level rather than the absolute value. The broadband noise attenuation effect of the acoustic liner treatment is studied by applying a broadband TDIBC to the acoustic field obtained by the SNGR method. The far-field directivity is obtained through an integral surface solution of Ffowcs Williams and Hawkings (FW-H) equation. Predictions for a non-optimized acoustic liner show a moderate amount of attenuation.
To accurately simulate the broadband noise generation and radiation, a LES using a high-order spatial scheme and implicit temporal integration is conducted for the high-lift configuration with slat deployed and the calculated results show the characteristic of the unsteady flow and the mechanisms of the broadband noise generation. The recorded noise sources are then used to drive the APE to simulate the noise propagation and the attenuation by acoustic liners. The source driven APE results agree well with that of LES in term of far field directivity and sound pressure level. Similar to the SNGR simulation, a moderate amount of attenuation is achieved by the acoustic liner treatment.

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ORCID for Jae Wook Kim: orcid.org/0000-0003-0476-2574

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