Aircraft noise prediction

The boundary element method (BEM) in the context of acoustics or Helmholtz problems is reviewed in this paper. The basis of the BEM is initially developed for Laplace’s equation. The boundary integral equation formulations for the standard interior and exterior acoustic problems are stated and the boundary element methods are derived through collocation. It is shown how interior modal analysis can be carried out via the boundary element method. Further extensions in the BEM in acoustics are also reviewed, including half-space problems and modelling the acoustic field surrounding thin screens. Current research in linking the boundary element method to other methods in order to solve coupled vibro-acoustic and aero-acoustic problems and methods for solving inverse problems via the BEM are surveyed. Applications of the BEM in each area of acoustics are referenced. The computational complexity of the problem is considered and methods for improving its general efficiency are reviewed. The significant maintenance issues of the standard exterior acoustic solution are considered, in particular the weighting parameter in combined formulations such as Burton and Miller’s equation. The commonality of the integral operators across formulations and hence the potential for development of a software library approach is emphasised.

The Boundary Element Method in Acoustics: A Survey

Experimental characterization of the turbulent boundary layer over a porous trailing edge for noise abatement

https://backend.orbit.dtu.dk/ws/files/256022566/1_s2.0_S0376042121000427_main.pdf

Turbulent boundary layer trailing-edge noise: Theory, computation, experiment, and application

A semi-empirical model to determine the wall pressure frequency spectrum beneath a twodimensional, pressure gradient turbulent boundary-layer is presented. The model is derived based on the experimental wall pressure data of various research groups. The experimental database includes both the equilibrium flat plate and non-equilibrium airfoil boundary-layer flow cases and covers a large range of Reynolds numbers, 1.0 × 103 < Reδ2 < 3.0 × 104. The enhanced model is a combination of the modified Chase-Howe, Goody and Rozenberg models, and is a simple function of the ratio of pressure and timescales of the outer to inner part of the boundary-layer. The key advantage of the present model is that it incorporates the Reynolds number, the boundary-layer loading as well as pressure gradient effects through an amplitude scaling function and timescale ratio, and compares well to the experimental data. Spectral features of the detailed measurement data and various scaling behavior of the wall pressure spectrum are e...

A semi-empirical surface pressure spectrum model for airfoil trailing-edge noise prediction

Systematic testing of the microstructural and aeroacoustic properties of porous metals applicable as low-noise trailing-edge (TE) treatments has been initiated within the Col-
laborative Research Center SFB 880|Fundamentals of High-Lift for Future Civil Aircraft. Generic TE noise experiments were performed at Re = 0.8x10^6 to 1.2x10^6 in DLR's
open-jet AWB facility. Complementary flow measurements in the closed test section MUB wind-tunnel of the TU Braunschweig served to quantify the induced aerodynamic effects. The presented database forms part of an ongoing cumulative effort, combining experimental and numerical methods, to gain a deeper understanding of the prevalent TE noise reduction mechanisms. For the large variety of porous materials tested herein a clear dependence of the achieved broadband noise reduction (reaching 2-6 dB at maximum) on the flow resistivity was identified. Basic design recommendations for material resistivity and
pore sizes, the latter to minimize high-frequency self-noise contributions, were deduced for low-noise TE applications. An acoustic nearfield pressure release across the porous region, adversely coupled with a loss in lift performance for porous TE replacements, appears as the major noise-reduction requirement.

/pdf/specifcation-of-porous-materials-for-low-noise-trailing-edge-2ovl4td5tq.pdf

Specifcation of Porous Materials for Low-Noise Trailing-Edge Applications

The ultimate goal of the present study is the elaboration of an empirical noise prediction
model applicable in the early development stage of low-noise aircraft. To this end,
a series of aeroacoustic tests were performed to investigate the characteristics of flap
side-edge noise. Two generic models, an isolated flap and a high-lift wing were selected
for the experiments. Measurements of sound pressure levels as well as sound directivity
were done using dedicated microphone arrays. Moreover, the acoustic investigation
was complemented by flow measurements done with a 7-hole probe. Further analysis
of the data revealed the effectiveness of the flap tip cross-flow velocity in scaling noise
spectra. The main characteristics of the noise directivity were also determined. These
results allow the formulation of a preliminary empirical prediction scheme.

Development of an empirical prediction model for flap side-edge noise

n this paper, the relation between noise generated at flap side-edges (FSE) and mean
flow characteristics in the vicinity of the FSE solid surface is investigated. Of interest is
(1) the identification of mean flow features near the flap tip which are important to FSE
noise generation, (2) the extraction of quantitative parameters relating flap settings to
the emitted noise levels and, finally, (3) the formulation of a prediction scheme based
on these observations. The results of the investigation demonstrate that FSE noise
can be properly represented by the summed contributions of two independent source
mechanisms. Moreover, this new formulation necessitates only a limited knowledge of
the mean flow parameters in the vicinity of the FSE to model the noise emission. It
is assumed that unsteady vorticity fluctuations, originating from the unstable pressure
side shear layer, sweeping over the forward half of the upper tip edge, lead to midto-
high frequency noise production. Flow unsteadiness in the proximity of the suction
side surface near the FSE aft half, are assumed to be the main contributor in the
low-to-mid frequency range. A prediction scheme is formulated and validated for a
wide range of experimental measurements at small scale wing configurations. This new
formulation only necessitates knowledge of the flap chord length and lift coefficient to
provide accurate estimates of the far field noise radiation.

/pdf/flow-field-measurements-to-characterize-flapside-edge-noise-4sjeui3ay7.pdf

Flow Field Measurements to Characterize FlapSide-Edge Noise Generation

A laser-generated non-intrusive impulsive sound source is used to perform shield-
ing experiments in the acoustic wind tunnel Braunschweig at a generic 2D wing with
NACA0012 profile. The sound source was already shown in previous reports to have a
monopole-like character with a nearly omni-directional sound radiation pattern, both
in stagnant medium and in a flow field. In the current report, emphasis is first put
on the analysis of the data post-processing steps required for the obtention of correct
spectral and time domain data for the laser sound source. In particular, the subject
of microphone corrections is reconsidered in details. A missing free-field correction of
the results in,1 is corrected herein to reveal the correct time domain evolution of the
generated pressure pulses. Further, non-linear propagation effects are considered. It is
found that sound attenuation due to atmospheric absorption does have a non-negligible
broadband impact on the far-field source noise levels, increasing in importance with mi-
crophone distance. Other non-linear effects related to high-amplitude wave propagation
i.e. wave steepening and higher rates of attenuation with far-field distance, could not
be observed herein. Noise shielding results are presented for two Mach number values
and three angles of attack. Except for a source location in the proximity of the trailing
edge, the flow is found to have no measurable influence on the shielding characteristics
of the NACA0012 wing for the experimental setup considered. For a source near the
trailing edge, a reduced noise shielding is measured upstream of the trailing edge. It
is postulated, that noise is generated through the interaction of the source sound wave
with the trailing edge, and that it is amplified in the forward arc according to the mean
flow field velocity.

Analysis of the Noise Shielding Characteristics of a NACA0012 2D Wing

A laser-generated sound source is used to generate a nearly omni-directional sound
field. This sound field is investigated experimentally to reveal its characteristics as a
function of the surrounding medium mean velocity. It is found that the sound source
approximately has monopole character when generated in a stagnant medium. Increas-
ing the surrounding flow velocity leads to a positive Doppler-shift combined with a
convective amplification for an observer downstream of the source while the reverse
is observed upstream of the source. The laser sound source behaves therefore as a
convected point heat source in a uniform flow. A solution of the convected wave equa-
tion for a moving point heat source in a uniform subsonic flow is derived, providing
a closed-form description of the source and appropriate scaling parameters for exper-
imental results. The derived theoretical model enables the determination of the heat
release function resulting from the laser energy deposition in the medium and associ-
ated to the measured far-field pressure pulses. An implementation of the heat release
function in CAA codes is straigthforward, making numerical replication of shielding
tests with the laser sound source possible.

Karl-Stéphane Rossignol

Papers

Specifcation of Porous Materials for Low-Noise Trailing-Edge Applications

Development of an empirical prediction model for flap side-edge noise

Flow Field Measurements to Characterize FlapSide-Edge Noise Generation

Analysis of the Noise Shielding Characteristics of a NACA0012 2D Wing

On the Relevance of Convection Effects for a Laser-Generated Sound Source