EAE338
Chapter 5
Airframe Noise – High Lift Device Noise
Xin Zhang
Aeronautics and Astronautics, University of Southampton, Southampton, UK
1 Problem Definition
2 Noise Sources
3 Noise Characteristics
4 Prediction Methods
5 Noise Attentuation Methods
6 Summary
References
1
3
6
7
8
10
10
1 PROBLEM DEFINITION
1.1 High lift devices
High lift devices (HLDs) are deployed on wings during
take-off and the approach-and-landing phases of aircraft
operation to increase lift; they are retracted during cruise so
as not to affect the cruise performance. There are different
types of HLD, both mechanical (“unpowered”) and fluidic
(“powered”). Flaps and slats are common mechanical types.
Deflecting angles (δs and δf ), gaps (gs and gf ), and overlaps
(os and of ) define the geometrical settings (Figures 1 and 2).
Flaps (Figure 1c) are the inner movable parts of a wing
located on the trailing edge of the wing on both sides of aircraft. During take-off and the approach-and-landing phases of
aircraft operation, flaps can be lowered by the same extent on
both sides of the wing. Slotted and split flap configurations
are commonly found on commercial aircraft; their deployment exposes flap side edges (Figure 2). The mechanisms for
Encyclopedia of Aerospace Engineering.
Edited by Richard Blockley and Wei Shyy
c 2010 John Wiley & Sons, Ltd. ISBN: 978-0-470-75440-5
deploying the flaps are enclosed in flap track fairings, which
are movable support structures connecting the main element
of the wing to the flaps (Figure 2). In addition to flaps, ailerons
are outer movable portions of the trailing edge of the wing,
which deflect opposite to each other to provide roll control.
Deployment of ailerons provides a rolling moment about the
longitudinal axis (fuselage axis) of the aircraft. Ailerons can
also be lowered on both sides of the wing to serve as an
extension to flaps; these are then referred to as flaperons.
Slats (Figure 1b) belong to a class of mechanical devices
deployed near the leading edge of the wing. Slats are connected to the main element of the wing by slat tracks, which
are movable support structures (Figure 1b and Figure 2).
Deploying a slat exposes the slat cove and forms a gradually reducing gap between the slat and the main element. The
particular shape of the gap facilitates the acceleration of flow
between the slat and the main element. The gradual downward expansion of the gap forms a horn shape that facilitates
amplification of acoustic disturbances and possible acoustic
resonance in the gap region.
Spoilers are aerodynamic devices attached on the surface of a wing (Figure 1c). They normally take the form of
small, hinged plates of various shapes, which can be extended
upward and/or downward from the wing. Spoilers are used
to slow an aircraft, or to aid an aircraft’s descent if they are
deployed on both wings simultaneously. Spoilers can also
be used to generate a rolling motion for an aircraft, if they
are deployed on only one wing or in a different manner on
opposing wings.
1.2 Aerodynamic function
Mechanical HLDs are retracted during cruise so that the
cruise performance is not affected. The deployment of a
2 Acoustics and Noise
Figure 1. HLD geometry and pictures of slat showing slat tracks and flap side edge: (a) geometrical settings, (b) slat with slat track, and
(c) flap with spoiler.
mechanical flap is equivalent to an increase in camber.
Generally, lowering the flap leads to an increase in lift coefficient CL by a constant amount over the linear range of angle of
attack (α). This additional lift allows the aircraft to fly slower
with the flaps down. Deployment of HLDs also introduces a
drag penalty.
Deployment of a slat increases CL,max significantly and
extends the lift curve. Slats therefore supplement the effect of
trailing edge flaps. Their aerodynamic function is to improve
high angle of attack performance and prevent the appearance of leading edge separation at high α and low Reynolds
number, through more favorable pressure gradients induced
by one element of a wing on another.
1.3 Noise problem
Airframe noise is the non-propulsive components of the noise
generated by an aircraft in flight. The noise is produced by
the separated and turbulent flows interacting with airframe
components. Various components (geometries) and time and
length scales are involved in the noise generation process; the
radiated noise has a rich mixture of narrow and broadband
noise content, spanning at least three decades in frequency.
Landing gears and high lift devices, for example, slats and
flaps, are the major sources of airframe noise (Crighton,
1995).
HLD noise sources include slats, flap side edges, slat
and flap tracks, and trailing edges (Figure 3). Among them,
slats and flap side edges are the two main sources of noise
(Figure 4). Airframe noise is a significant part of approach
noise for most of modern commercial aircraft. The introduction of high bypass ratio engines and improved integration
since the 1980s means airframe noise now plays a similarly
important role as engine noise in defining the overall aircraft noise during approach. The airframe also contributes a
non-negligible portion of total noise during take-off (cutback)
conditions.
The first investigations into airframe noise took place in the
1970s and 1980s. Flight and wind tunnel measurements have
been performed on both landing gears and HLDs, which provide global insight into source mechanisms and the relative
importance of specific noise sources. Based on experimental
results and analytical studies, trends and scaling laws have
been formulated to produce empirical/semi-empirical noise
prediction tools from landing gears and HLDs. One of the
most intensively studied cases, both experimentally and analytically, is the basic mechanism of turbulent boundary layer
Airframe Noise – High Lift Device Noise 3
The very challenging noise reduction targets and the large
number of contributing components imply that all noise
sources have to be reduced significantly. The approach noise
source reduction challenge has to be addressed by reducing airframe noise to the same amount as engine noise. This
means that dominant airframe noise sources, which are due
to the deployment of both high lift devices and landing gears,
must be considered. Some noise reduction methods are discussed in Section 5. Many fields of investigation need to be
pursued simultaneously, that is, reduce the noise source by
including new configurations that enable favorable installation effects and improved aircraft procedures around an
airport to confine the noisy area to lie within the airport
boundary.
2 NOISE SOURCES
2.1 Slat
Figure 2. High lift devices on Airbus A300.
trailing edge noise from a two-dimensional airfoil (see the
review by Crighton, 1995). Although useful for extrapolating
to different geometries and flow conditions, these prediction models do not allow for the acoustic assessment of new
designs or the development of low noise concepts. However
with the adoption of new noise reduction methods and appearance of new aircraft configurations for “silent” flight, it is
conceivable that trailing edge noise will assume an increasingly important role. Trailing edge noise is briefly reviewed
in Section 2.4.
Since the 1990s, environmental concern and more stringent regulations have provided a new impetus for airframe
noise research. New measurement techniques, for example,
phased microphone arrays (Sijtsma, 2007) and flight test programs (Piet et al., 2005), can provide the means to gain
insight into the noise generation physics (see Section 2), to
develop scaling laws (see Section 3), and to provide databases
to establish empirical/semi-empirical prediction tools and
validate numerical models (see Section 4). Sub-component
sources can be studied and flow/noise physics explored. The
characteristics and ranking of various airframe noise sources
can be also determined for different types of aircraft.
The slat noise source is distributed along the spanwise direction of the wing (Figure 5). Although the local source strength
is generally lower than that of the flap side edge, whose source
is concentrated around the edge, the overall contribution of
the slats is comparable to the flaps, since slat noise sources are
distributed over a larger area. When a slat is deployed, it forms
a gradually downward expanding gap region between the slat
and the main element; furthermore, the need to retract during
cruise means a cove is required that is exposed during deployment. The trailing edge of the slat is also of finite thickness,
which contributes tonal noise due to vortex shedding. The
HLD noise in the low-to-middle frequency range (<2 kHz)
is dominated by slat noise. Source identification methods
such as phased microphone arrays can help to identify noise
sources either in model tests or in flight tests.
The flow field around the slat produces complex features
(Ma et al., 2008). Flow separates at the slat cusp or at the edge
of the cusp extension (i.e., slat cover) to form an unsteady
shear layer. The instability of the shear layer leads to the
linear amplification and ultimately to nonlinear saturation of
disturbances. The saturation leads to the roll-up and formation of discrete vortices in the shear layer (Figure 6). These
vortices (or vortical structures) grow in size along the shear
layer and pass through the narrow gap between the slat and the
main element. The large vortical structures can impinge upon
the cove surface; a low-speed recirculation flow is formed in
the cove region. This region is bounded by the unsteady shear
layer and the flow experiences significant fluctuations, which
drive the broadband noise sources.
The finite thickness trailing edge leads to alternating
shedding of small vortices, which is a possible source of
4 Acoustics and Noise
Figure 3. Illustration of main noise sources associated with high lift devices.
high-frequency tonal noise. The trailing edge plays an important role in defining both the high frequency (through trailing
edge vortex shedding) and low-middle frequency (with large
vortical structures passing through the gap) noise content.
Trailing edge noise sources and acoustic resonance between
the slat and the main element generate tonal noise, although
this feature is often observed in model tests rather than in
flight. Due to the unsteady flow field around the slat, the
noise generation mechanisms represent a complex aeroacoustic problem. Multiple sources result, contributing to the
generally broadband nature of the slat noise. It also presents a
severe challenge to predict the far-field radiated noise levels
Figure 4. Airframe noise sources localization of an Airbus A340
in flyover configuration with high lift devices for 3150 Hz; onethird octave bands for an emission angle of 90◦ . Reproduced with
permission from Piet et al., (2005).
Figure 5. Example of HLD noise sources on a 4.7% scale MD-11
model; flow from left to right. Reproduced with permission from
Guo, Yamamoto, and Stoker (2003).
Airframe Noise – High Lift Device Noise 5
Figure 6. LES computation of large vortical structures and slat
trailing edge vortices.
and directivity (see section 4). The same observations also
apply to the flap noise problem.
2.2 Flap side edge
The flap side edge is associated with a source of intensive
HLD noise, which dominates the middle to high frequency
range (Figure 5). The flow field at the flap side edge is dominated by two streamwise vortices that merge to form a single
larger vortex. Near the leading edge of the flap, a vortex is
formed as the boundary layer from the pressure surface separates and forms a shear layer. The roll-up of this shear layer
forms the primary vortex (Figure 7). On the suction surface
of the flap, a smaller secondary vortex forms. The vortices
grow in size and strength along the chord of the flap. Eventually they merge together to form a single large vortex. This
vortex eventually separates from the flap surface at moderate
to high flap deflection angles. If the adverse pressure gradient is sufficiently large at high flap deflecting angles, vortex
breakdown can occur. The potential acoustic sources at the
flap side edge (Khorrami and Singer, 1999) are free shear
layers and their rollup, formation of multiple vortices, vortex merging, convection of turbulent boundary layers past a
sharp edge, and vortex breakdown.
Several models exist to describe flap side edge noise
(Hardin, 1980; Howe, 1982; Sen, 1997). The Hardin’s model
1980 suggests that the turbulence in the boundary layer that
is swept around the side edge, passing very close to it, is
responsible for noise production. The magnitude of sound
radiation is related to the magnitude of vorticity and its distance from the sharp edge. Howe’s model (1982) is based
on flow through a slot between a flap and the non-deflected
part of the main element. The gap between the flap side edge
and the non-deflected main element is the main influence on
the intensity of the radiated sound. Howe’s theory gives the
far-field sound pressure generated by turbulence fluctuations
near the slot in terms of quantities represented by an acoustic
Green’s function. Sen’s analysis (1997) is based on oscillation of the edge vortex as a noise mechanism. The frequency
is dependent on the airfoil circulation, edge thickness, and
mean distance from the edge. When the vortices remain close
to each other and undergo small motions, they only feel the
effects of the wall weakly and tend to move in a mutually
cancelling manner. This results in low acoustic production.
The model suggests that to lower acoustic production at the
flap side edge, the vortices should be moved far from the wall.
2.3 Slat track and flap track fairing
Noise levels from slat tracks have been found to be higher
compared to those from a clean configuration by about 8 dB
(Dobrzynski et al., 1998). The noise mechanism of the slat
track is associated with the fact that the slat tracks are installed
perpendicular to the wing leading edge, and thus causes flow
separation. The slat track cut-outs in the wings are also a
possible source of overall slat noise. Flight tests show the
flap track fairings act as sources of noise as well.
2.4 Trailing edge
Figure 7. Surface streaklines show roll-up of vortices on flap side
edge.
At flight Reynolds numbers, turbulent boundary layers exist
on the surfaces of a wing. The scattering of turbulence structures at the trailing edge generates sound radiation. For
6 Acoustics and Noise
two-dimensional sources that are located within a wavelength
of the trailing edge, the far-field radiation scales with the
fifth power law of velocity. The far-field intensity directivity
varies with θ as sin2 (θ/2) where θ is the angle measured from
downstream (Ffowcs Willams and Hall, 1970). These basic
dependences are independent of the nature of the unsteady
flow near the edge.
Amiet’s model (1976) is a useful tool to predict the trailing edge noise radiation. The theory is valid for compressible
flow and assumes a convective pressure spectrum on the
wing surface, which propagates past the trailing edge, producing a radiating pressure field of similar magnitude. The
induced loading of the airfoil is calculated by standard gustinteraction methods. The mean Mach number effects are
included exactly in Amiet’s work. One of the key assumptions
is that the turbulent flow is “frozen”, that is, the turbulence
spectrum remains unchanged over the chord. Amiet’s theory
is derived in the coordinate system, [x, y, z], where x, y, and
z are the streamwise, wall normal, and the spanwise directions, respectively, non-dimensionalized with the semi-chord
b. Amiet’s classical result of the far-field spectrum, S(x, 0, z,
ω) for an observer in the y = 0 plane is obtained as
S(x, 0, z, ω) =
ωbz
2πc0 σ 2
2
ly (ω)d| £ |2 Sqq (ω, 0)
(1)
where £ is the directivity factor that can be evaluated analytically according
√ to Amiet (1976, equation (5)), c0 the sound
speed, σ(= (x2 + (1 − M 2 )z2 ) the scaled radius from the
trailing edge, ω the frequency, d the spanwise width of airfoil,
ly (ω) the spanwise correlation length of wall turbulence, and
Sqq (ω, 0) the spanwise cross-spectrum of surface pressure.
3 NOISE CHARACTERISTICS
3.1 Spectral and scaling laws
The aerodynamic field associated with HLDs contains a variety of flow physics and therefore a broad range of spatial and
temporal scales. The HLD noise is generally broadband in
nature, spanning three decades due to the multiple sources.
The existence of multiple sources gives rise to a rather uniform far-field directivity (Figure 8) and follows different
scaling laws. Far-field radiation of two-dimensional sources
would follow the fifth power law of velocity (Crighton, 1975).
Around the tracks and flap side edge, however, a number of
possible noise sources exist. These are generally broadband
in nature and therefore the acoustic wavelengths vary significantly. If the sources are located close to edges of the devices,
that is, within one wavelength, the noise radiation is domi-
Figure 8. A340 airframe noise sources directivity in flight.
Reproduced with permission from Chow, Mau and Remy (2002).
nated by diffraction (Ffowcs Willams and Hall, 1970). In this
case, the far-field noise radiation scales with the fifth power
of velocity. If, however, the sources are located away from
the edges by more than one wavelength, the noise radiation
would follow the sixth power law of velocity, that is, a dipole
radiation (Curle, 1955).
The far-field slat noise spectrum is mainly broadband with
one or more narrow peaks (Dobrzynski et al., 1998). Slotted slats represent the major source of aerodynamic noise
followed by noise radiated from flap side edges. Slat noise
peaks at rather low frequencies (between 0.2 and 0.4 kHz at
full scale). Flap side edge noise is more prominent at mid frequencies, say around 1–2 kHz. The slat broadband spectrum
has a maximum at a Strouhal number of St = 1–4 (where the
Strouhal number is based on the freestream velocity and the
slat chord). A high frequency (St ∼ 10) peak may also appear
in model tests (Choudhari et al., 2002). The source of this
high-frequency noise is the vortex shedding at the trailing
edge of the slat. The low-to-mid frequency broadband noise
is attributed to the amplified perturbations in the free shear
layer. The flap noise dominates at a Strouhal number centered
around approximately 12.5 based on flap chord (Choudhari
et al., 2002). Since vortex merging and breakdown are lowfrequency phenomena, it has been assumed that shear layer
instabilities are responsible for the bulk of the concentrated
audible noise generation.
Empirical/semi-empirical methods exist to predict farfield noise, which are based on power laws (see section 4.1).
These power laws are empirically fits for the low-to-mid frequencies (around the spectral peak). Although empirical and
hence approximate, they can give reasonable estimates. There
are, however, difficulties in their use. Various power laws exist
based on different aircraft types and datasets. Generally for
slats the velocity power law scales with 5.6 and for flaps 5.3
Airframe Noise – High Lift Device Noise 7
(Dobrzynski et al., 1998; Guo and Joshi, 2003). The fifth
power law of velocity in the low-to-mid frequency range is
in agreement with the fact that slat trailing edge noise is the
most dominant noise generation mechanism. Different values, however, have been reported. For slats, the value varies
from 4.5 based on Strouhal scaling (Pott-Pollenske et al.,
2006) to 8; the value is thus frequency dependent. The noise
generated by all of the combined HLDs followed a scaling
law of V05.5 (Chow, Mau and Remy, 2002).
3.2 Directivity
For the HLD noise radiation and an observer on the ground,
the maximum radiation angle is in the aft quadrant; the maximum radiation angle of the flap noise is in the forward quadrant (Chow, Mau and Remy, 2002; Guo and Joshi, 2003). Here
the aft quadrant refers to an observation angle range of 90◦ –
180◦ defined from the flight direction and forward quadrant
0◦ –90◦ . Slat noise is significant at low-to-mid frequencies.
Due to the broadband nature of the slat noise sources, the
directivity of the radiated sound is weak, exhibiting a gradual fall-off from the peak radiation direction. From patterns
observed in flyover noise of an aircraft with HLDs deployed,
flap noise peaks in the forward arc at high frequencies.
From flyover measurements, the trailing edge flap noise
varies with the square of the sine of the flap deflection angle.
Frequency scales with Strouhal number relative to flap chord.
The directivity of the trailing edge flap noise is that of a lift
dipole normal to the flight direction.
4 PREDICTION METHODS
4.1 Semi-empirical or component-based methods
The complex nature of airframe noise means current “whole
aircraft” prediction methods are generally based on prediction of various component noise fields. This necessitates
a good database of a wide range of model tests and fullscale aircraft. Early work on component-based methods was
described by Fink (1977) and was incorporated into the
ANOPP (Aircraft Noise Prediction Program). ANOPP is a
semi-empirical code released by NASA Langley that incorporates publicly available noise prediction schemes. The code
is continuously enhanced with the latest developments. The
latest HLD development is that based on Guo, Yamamoto
and Stoker (2003). Users of ANOPP should be aware of the
technology level and constraints used in its synthesis.
Fink’s methods start with a definition of “clean” airframe
noise, which is assumed to be entirely associated with trailing edge noise of wings and horizontal tails. In this method,
contributions of various components (slats, flaps, landing
gears, etc.) are added to the cruise configuration noise levels.
No interactions are included. The assumption of weak interaction can only be viewed as a first approximation. Fink’s
assumption of HLD component noise is based on trailing
edge noise. For example, trailing edge flap noise is modeled
as a single lifting dipole field and slat noise is based on an
extension to clean wing noise. These assumptions are now
known to be incorrect.
Recent developments of phased microphone array and
physical understanding have resulted in improvements in
terms of incorporating individual sub-components and the
possibility of including flow quantities as well as individual
geometrical parameters. According to Guo, Yamamoto and
Stoker (2003) the noise spectrum is given as
S = S0 F1 (St)D(ϕ)M
b1
CLb2
b3
l
(sin α)b4 (sin δ)b5
r
(2)
where S0 is a constant. The frequency dependency of the noise
is given by the normalized spectrum F1 (St) in terms of the
Strouhal number
St =
fl
V0
(3)
where l is the length of the component, for example, thickness. The directivity is given by the directivity factor D(ϕ),
where ϕ is the directivity angle in the flyover plane, measured from the flight path. The spectrum is assumed to be
proportional to some powers of all of the other parameters,
which include the flow Mach number M = V0 /c, with c the
constant sound speed, the angle of attack α , the sectional lift
coefficient of the component CL , the deflecting angle of the
component δ, and the length of the component normalized
by the far-field microphone distance r. For each particular
component, flow and geometric parameters unique to that
component are to be added to the general expression. For
example, the strengths of the side edge vortex and the velocity of the spanwise crossflow are added for the flap side edge
noise sources. Similarly for the slat noise sources, the vortex strengths in the cove region, the velocity of the flow in
the gap between the slat trailing edge and the main wing,
and the width of the gap are added. The dependencies of the
noise spectra on these parameters are assumed to be of the
simple form of a power law. The indices of the power laws
(b1 , b2 , b3 , · · ·) are aircraft dependent.
A component-based method also exists for a slat
(see Pott-Pollenske et al. (2006) for a complete description). The expression for far-field pressure is approximately
done in the 1/3-octave band log(f) scale. An equivalent sound
8 Acoustics and Noise
pressure spectrum can be expressed as
2
l
S = F2 (St)D(α, ϕ, ψ)M sin ψ
r
5
3
(4)
where l is the length of the wetted trailing edge and ψ is the
sweep angle. The frequency content is given by the normalized spectrum F2 (St) (equations (6) and (7) in Pott-Pollenske
et al. (2006)) and allows for database and deflecting angle corrections. D(α , ϕ, ψ) is a directivity factor based on equation
(10) in Pott-Pollenske et al. (2006), which includes radiation
angles in both polar and azimuthal directions. The Strouhal
number St is based on the chord of the slat. The above equation is based on flight and model test data.
The accuracy of a component-based method is constrained
by the accuracy and breadth of the database employed, and
therefore by the aircraft types in the database. It could be
misleading if applied to other types. Current research on
physics-based methods could lead to better and more flexible
methods in the future.
4.2 Computational methods
For HLD noise, the physics of interest are characterized by the
co-existence of a multitude of sound generation and propagation mechanisms and disparate spatial and temporal scales.
The physics are described by the Navier–Stokes (N-S) equations. A full solution of the N-S equations is not feasible at
present for engineering applications. The current approach
is therefore to seek efficient methods to accurately predict
the noise sources and subsequent far-field acoustic properties by solving various (reduced) forms of the governing
equations. Generally, an HLD noise problem can be tackled
on three fronts: noise generation, propagation, and far-field
radiation. Each of the three can be treated by a different
set of suitable governing equations with different simplifications but retaining the major physics. The noise generation
occurs in the immediate surrounding area of an HLD, where
aerodynamics and acoustics cannot be separated and influence each other. For source modeling, methods include
direct numerical simulation (DNS), large eddy simulation
(LES), detached eddy simulation (DES), unsteady Reynolds
averaged Navier–Stokes (URANS), and steady Reynolds
averaged Navier–Stokes (RANS) coupled with a stochastic noise generation and radiation (SNGR) approach. In the
sound propagation area, the aerodynamic field influences the
acoustic wave propagation without feedback. Solutions of
this problem include installation effects. For sound propagation, noise propagates in non-uniform mean flow (weak
coupling). The problem can be treated by solution of the
Euler equations, linearized Euler equations, or acoustic perturbation equations. For the far-field noise radiation problem,
solutions can be found by using Lighthill’s acoustic analogy,
which is an exact rearrangement of the N-S equations under
certain conditions. A widely used acoustic analogy approach
is an integral solution of the Ffowcs-Williams and Hawkings
(FW-H) equation (Ffowcs Williams and Hawkings, 1969).
The source and propagation problems are often solved
using a high-order computational aero-acoustics (CAA)
code. CAA is concerned with the accurate numerical prediction of aerodynamically generated noise as well as its
propagation and far-field characteristics. The inherently
unsteady nature of aero-acoustic phenomena, the disparity
in magnitude between mean and acoustic flow quantities,
and the high frequencies often encountered place stringent
demands on the numerical treatment. The trend within the
field of CAA has been to employ high-order accurate numerical schemes that have in some manner been optimized for
wave propagation to reduce the required number of grid
points per wavelength while still ensuring tolerable levels
of numerical error.
A schematic of the the CAA approach is shown in
Figure 9. Examples of HLD CAA computation can be found
in Choudhari and Khorrami (2007) and Ma et al. (2008).
5 NOISE ATTENTUATION METHODS
5.1 Sound absorption: acoustic liner
The slat noise spectra are generally broadband in nature with
a broad hump between St = 1–4. High-frequency tones can
also exist. The trailing edge of a slat is generally recognized as
an important area for both broadband and narrow band noise
generation. Acoustic absorptive treatment on the surface of
the main element could suppress potential image sources at
the trailing edge, while treatment in the slat gap could attenuate high-frequency sound (Ma et al., 2006; Ma et al., 2008).
Unlike other noise control techniques, such as serrated tapes
at the slat trailing edge or slat cove fillers, the acoustic liner
treatment would not modify the slat shape and would thus
be expected to have minimal effect on the wing pressure distributions and the resulting lift. The most efficient treatment
would be to apply liners on both the slat cove and the main
element. The main element treatment provides useful attenuation by influencing the diffraction around the wing. Care
must be taken to allow for a complete concealment of the
liners when the slat is retracted during cruise. For the narrow
band high-frequency tones, a reduction of more than 4 dB has
been reported. For broadband noise, a reduction of around
2 dB seems possible.
Airframe Noise – High Lift Device Noise 9
Flow and noise
source computation
RANS +
SNGR
URANS
LES
DES
DNS
Noise sources
Noise propagation
Far-field radiation
Euler, LEE or APE
Lighthill’s
acoustic
analogy
Integral surface
solutions, e.g.
FW–H
Figure 9. Schematic of hybrid approach for HLD noise computation.
5.2 Active flow control: blowing
Active flow control, for example, steady and unsteady blowing, can be used to alter the vortical structures associated
with the slat and the flap side edge flow fields and therefore to attenuate noise radiation. The source of noise at the
flap side edge can be attributed to the oscillation of the vortical structures. This leads to pressure fluctuations near the
rigid surface and thus to sound radiation. One approach is
to displace or to destroy the vortical structures by blowing
air into it (Koop and Ehrenfried 2004). Blowing reduces the
amplitude of surface pressure fluctuations and therefore the
level of the radiated sound. Blowing air changes the circumferential velocity profile of the vortex and thus the dynamic
interaction between the shear layer and the vortex instability.
It also has the effect of displacing the vortical structures away
from the solid surface, which reduces the sound pressure level
and thus the level of the radiated sound. A reduction in the
far-field sound pressure level of 3–4 dB above 1.25 kHz is
reported.
5.3 Edge treatment: porous material and brushes
The spanwise pressure discontinuity introduced by the flap
side edge plays an important role in defining the vortices
around the edge and the flow through the gap. A porous flap
side edge alleviates the pressure discontinuity at the flap side
edge and reduces the magnitude of vorticity in the shear layer
wrapped around the side edge vortex. This leads to a weaker
side edge vortex. A porous side edge also displaces the vortex
further away from the solid surface by allowing a finite mass
flux through it, thereby reducing its strength as an acoustic
source. Porous material can also be used on the slat trailing
edge. The method reduces the high-frequency local vortex
shedding and therefore eliminates a possible source of highfrequency noise.
Brushes can also be applied to the flap side edge to influence the vortex flow as well as slat trailing edge to influence
discrete vortex shedding. For a flap side edge with brushes, a
source strength reduction of approximately 5 dB is possible.
5.4 Local geometry modification: cove filler, fence,
and serration
To stabilize the slat cove flow, a cove cover can be used, which
extends the cusp following the direction of the shear layer. To
stabilize the slat cove flow as well as to eliminate one possible
channel of acoustic feedback, a slat cove filler can be used.
Although it is efficient in attenuating noise, it is difficult to
manufacture a device to allow the slat with a cove cover to
retract. Use of a cove filler could have an impact on CL,max
as it constrains the flow through the gap. It is possible that
the aerodynamic effect is case dependent and optimization
work is needed to retain the aerodynamic efficiency. For a
combined porous flap side edge and slat cove filler, a noise
reduction of more than 2 dB at some angles has been reported
(Chow, Mau and Remy, 2002).
Flap side edge fences have been used to reduce noise.
Dobrzynski, Gehlhar and Buchholz (2001) showed that on
a full-scale A320 wing airbus, a flap fence could achieve a
noise reduction. The rationale underlying the use of a flap
side fence is to increase the distance between the edge vortex
system and the top surface of the flap. Various forms of fences
have been used, including tip fences extending above and
below the flap surface, with a reported peak noise reduction
of 9 dB in the mid-range frequencies. Lower tip fences that
extend approximately one flap thickness below the flap lower
surface have also been explored with reported overhead noise
reduction of 3–4 dB for frequencies between 4 and 10 kHz.
10 Acoustics and Noise
5.5 Geometry modification: continuous moldline
technology
Continuous moldline technology (CMT) uses a flexible panel
that deforms to provide a continuous surface between two
moveable parts. The aim is to prevent discrete ends, for example, flap and slat side edges, to become sources of intense
noise. When used to remove a flap side edge, CMT connects
the flap side edge to the adjacent wing surface with an elastomeric panel that deforms during flap deflection to provide a
continuous surface without abrupt changes in curvature. This
effectively eliminates the flap side edge vortex. With CMT
applied, the flap side edge source can be reduced below measurable levels. However, there is an aerodynamic penalty to
be incurred, since the application of CMT reduces the overall
lift. This is due to the reduced loading on the main element
near the flap side edge. Wind tunnel model tests of a 20%
scale wing model (Storms et al., 2000) show reductions of
5–15 dB above 2.5 kHz for a flap side edge and up to 10 dB
in the peak SPL above 5 kHz for a slat.
introduced CFD and CAA methods to study the noise source,
propagation, and radiation. These efforts need to be continued
to achieve acceptable levels of noise reduction.
The current range of noise attenuation methods based on
local treatments is effective as long as the basic designs of
slats and flaps remain the same. It is possible that a 3–5 dB
noise reduction can be obtained. However, a 10 dB reduction requires new approaches. These new approaches could
include major design changes to the existing devices, for
example, continuous moldline technology, eliminating gaps
between the slat/flap and the main element by employing
droop edges, replacing mechanical HLDs with flow control
technologies such as circulation control.
With the above in mind, a need exists to improve the current noise prediction tools. Current empirical/semi-empirical
prediction tools are database dependent and difficult to use.
They give acceptable results but cannot be relied upon to provide accurate predictions for future new aircraft types and act
as a design tool. Physics-based models need to be developed
to overcome this limitation. Modern computational methods
such as CAA will play an increasingly important role in the
endeavor for quieter aircraft.
5.6 Approach procedure: continuous descent
and steep approach
To mitigate aircraft noise, new landing procedures may be
required, for example, continuous descent from cruise altitude and a steeper approach (more than 3 deg glide slope) at
reduced speed and high lift. By flying at a higher altitude over
residential areas close to the airport, say higher than 1000 m,
the noise reduction on the ground will be significant, in accordance with the inverse square law for far-field radiation. HLD
noise scales with a power law of velocity; reducing speed
combined with a steeper glide slope would provide significant noise reduction. However, the new landing procedure
will necessitate a higher rate of deceleration near or in the airport perimeter. One way to achieve this is to deploy spoilers.
This could lead to new sources of airframe noise.
6 SUMMARY
High lift device noise became important when modern high
bypass ratio aero engines reduced the engine noise to the same
level as the airframe noise during approach-and-landing. The
HLDs play an equally, if not more, important role to landing gears in defining the overall airframe noise. Continuous
research over the past 20 years has developed new techniques
to identify major sources of noise, generated datasets of
various types of aircraft from both model and flight tests, produced empirical/semi-empirical HLD prediction tools, and
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Abstract:
High lift devices, together with landing gears, are the main sources of airframe noise during the approach-and-landing phase
of aircraft flight. Typical high lift devices include leading edge slats and trailing edge flaps. Other high-lift-related noisegenerating devices include spoilers if deployed during a steep approach operation. All the above aerodynamic devices are
retracted during the cruise phase of aircraft operation. A slat, when deployed, forms a cove region between the slat and the
central main element of the aircraft wing. Flow separation, flow recirculation, an unsteady shear layer, and slat settings together
generate noise of mainly broadband content. For a flap, the outboard flap side edge and vortex system associated with it are
the main sources of noise. The intensity of high lift device noise generally follows a power law of flow velocity. The main
sources of noise are identified and described in this chapter. Introduction is provided concerning main semi-empirical and
computational fluid dynamics methods. Noise attenuation methods are also described.
Keywords: acoustics, aircraft noise, airframe noise, high lift devices, slat, flap, acoustic control, flow control