Numerical investigation of slat noise attenuation using acoustic liners

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Description/Abstract

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 lin-
ers 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 °ap geometry is employed in the investigation. Numerical
simulations are performed to investigate the generation and far ¯eld propagation
of the slat noise. A numerical approach is developed that combines near ¯eld °ow
computation with an integral radiation model to predict the far ¯eld acoustic signal.
A time domain impedance boundary condition (TDIBC) is implemented to simulate
the e®ect 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 ¯nd 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 ¯nd 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
i
(SNGR) approach. The pseudo-laminar zonal calculation is basically an URANS
calculation with the two-equation shear stress transport (SST) · ¡ ! model to
model the e®ect 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 · ¡ ! closure and then the acoustic
¯eld 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 e®ect of the acoustic liner treatment is studied by applying a broadband
TDIBC to the acoustic ¯eld obtained by the SNGR method. The far-¯eld 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 con¯guration with slat deployed and the calculated results show
the characteristic of the unsteady °ow 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 ¯eld directivity and sound
pressure level. Similar to the SNGR simulation, a moderate amount of attenuation
is achieved by the acoustic liner treatment.