www.DLR.de • Chart 1 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Efficient and Airworthy Passive and Active Airframe Noise Control Strategies
Michaela HerrDLR Institute of Aerodynamics and Flow Technology, Technical Acoustics
www.DLR.de • Chart 2 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Outline
Problem definition and overview
Airframe noise reduction at the source:-
Reduction of parasitic noise sources
-
Landing gear noise reduction-
High-lift noise reduction
-
Basic principles: Edge noise reduction -
Slat noise reduction
-
Flap side-edge noise reduction
Summary of achievements and future needs
This lecture focuses on experimental and applied research in airframe noise reduction and is an attempt to subsume related international activities without claiming to be complete.
www.DLR.de • Chart 3 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Year
E
PN
dBta
rget
2000 2020-20
-15
-10
-5
0
EU
US
Problem definition EU and US strategic noise reduction targets
EU ACARE “visions 2020”: Reduce noise impact by one half
per
operation relative to 2000
technology.
NASA “pillar goals”: Reduce perceived noise impact of future aircraft by one half
relative to
1997
technology within 10 years (AST and QAT program) and by three quarters
(-20 dB) within 25 Years!
Reduction by one half “subjectively” corresponds to -10 dB, i.e. -90 % in sound power
www.DLR.de • Chart 4 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Silent Aircraft Initiative (MIT Cambridge, 2003): Develop a conceptual design for an aircraft whose noise would be almost imperceptible
outside
the perimeter of a daytime urban airport.
Problem definition EU and US strategic noise reduction targets
EU ACARE “visions 2020”: Reduce noise impact by one half
per
operation relative to 2000
technology.
NASA “pillar goals”: Reduce perceived noise impact of future aircraft by one half
relative to
1997
technology within 10 years (AST and QAT program) and by three quarters
(-20 dB) within 25 Years!
flight direction
Source: http://silentaircraft.org
Source: DLR
www.DLR.de • Chart 5 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Years
0
+ 12Base (2000) + 4 + 20
- 3
- 9
- 6
+ 8 + 16
-2020-VISION-2020
-VISION
-Based on FP4 & FP5 Projects-(1998 ---200-7-)
-2010 Solutions:-•-Generation 1 -Noise Technologies
-•-Noise -Abatement-Procedures
-2020 Solutions:-•-Generation 2
-Noise Technologies-•-Novel Architectures
-Based on FP6 & FP7 Projects-(2004 ---……-)
-Technology-Breakthrough
-ACARE GoalAve
rage
dec
ibel
s pe
r airc
raft
oper
atio
n
-2020
ACARE goal
Based on FP4 & FP5 Projects (1998 – 2007)
Based on FP6 & FP7 Projects (2004 – …)
Technology breakthrough
2020 VISION
2010 Solutions:•
Refinement of existing architectures
•
Generation 1 noisetechnologies
•
Noise abatementprocedures
2020 Solutions:•
Novel architectures•
Generation 2 noisetechnologies
Earliest product entry into service
Problem definition ACARE roadmap
www.DLR.de • Chart 6 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Years
0
+ 12Base (2000) + 4 + 20
- 3
- 9
- 6
+ 8 + 16
-2020-VISION-2020
-VISION
-Based on FP4 & FP5 Projects-(1998 ---200-7-)
-2010 Solutions:-•-Generation 1 -Noise Technologies
-•-Noise -Abatement-Procedures
-2020 Solutions:-•-Generation 2
-Noise Technologies-•-Novel Architectures
-Based on FP6 & FP7 Projects-(2004 ---……-)
-Technology-Breakthrough
-ACARE GoalAve
rage
dec
ibel
s pe
r airc
raft
oper
atio
n
-2020
ACARE goal
Based on FP4 & FP5 Projects (1998 – 2007)
Based on FP6 & FP7 Projects (2004 – …)
Technology breakthrough
2020 VISION
2010 Solutions:•
Refinement of existing architectures
•
Generation 1 noisetechnologies
•
Noise abatementprocedures
2020 Solutions:•
Novel architectures•
Generation 2 noisetechnologies
Earliest product entry into service
Problem definition ACARE roadmap
Noise reduction technology targets• Engine components low-noise design• Improved nacelle and nozzle liners• Airframe components low-noise design
(Landing gear, high-lift devices)
Technology enablers• Extensive application of computational
aeroacoustics
(CAA)• …
www.DLR.de • Chart 7 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
slats
flaps
LDGair
frame
engine
total
EPN
L, E
PNdB airframe 4 EPNdB
slats
flaps
LDGair
frame
engine
total
EPN
L, E
PNdB airframe 4 EPNdB
Problem definition
Reminder: aircraft noise source ranking at approach (typical tube + wing configuration)
Source: AIRBUS Operations S.A.S.
(data from SILENCE®
project)
www.DLR.de • Chart 8 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Problem definition
This lecture focuses at efforts which directly address the noise reduction at the source, i.e. retrofit solutions for the existing aircraft fleet
These are generally complex manipulations of flow/surface interactions i.e.-
manipulations of the sound generation mechanism itself or
-
of relevant input parameters in the source area (local flow velocities, TKE)
This is in contrast to acoustic absorbers that reduce the noise once generated!
www.DLR.de • Chart 9 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Used methodology
Technology readiness levels and related scaling issues
96 82 3 54 71
RefineValidateAdaptUnderstandDiscoverMaturity Phase
Production system flight proven by successful operation
Production system flight qualified by demonstration
Complete system prototype validation in operational environment
Acoustic system prototype validation in a relevant environment
Component validation in relevant environment
Component laboratory validation
Analytical and experimental proof-of -
concept
Technology concept and/or application formulated
Basic principles observed and reported
Description(source: SILENCE®
project)
TRL
Noise certification In service
large-scale WTT (DNW-LLF)
CAA
Large scale testing
small-scale WTT
Sample/ small scale testing
Technology programs Commercial programs
Activity
www.DLR.de • Chart 10 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
full-scale WTT
Full scale testing
full-scaleflight test
Used methodology
Technology readiness levels and related scaling issues
96 82 3 54 71
RefineValidateAdaptUnderstandDiscoverMaturity Phase
Production system flight proven by successful operation
Production system flight qualified by demonstration
Complete system prototype validation in operational environment
Acoustic system prototype validation in a relevant environment
Component validation in relevant environment
Component laboratory validation
Analytical and experimental proof-of -
concept
Technology concept and/or application formulated
Basic principles observed and reported
Description(source: SILENCE®
project)
TRL
Noise certification In service
large-scale WTT (DNW-LLF)
CAA
Large scale testing
small-scale WTT
Sample/ small scale testing
Technology programs Commercial programs
Activity
1/13.2 scale1/13.2 scale 1/3.3 scale1/3.3 scale
www.DLR.de • Chart 11 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Airframe noise reduction at the source
Reminder: classical airframe noise sources+ tracks, slat side-edges
wheel bay (cavities)landing gears
flap side-edge
wing tip
slotted slatgear-wake/flap interaction
tracks
jet/flap interaction
spoiler
1/7.5 scaled model in DNW-LLF
1/7.5 scaled model in DNW-LLF
www.DLR.de • Chart 12 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Airframe noise reduction at the source
Reminder: classical airframe noise sources+ tracks, slat side-edges
wheel bay (cavities)landing gears
flap side-edge
wing tip
slotted slatgear-wake/flap interaction
tracks
jet/flap interaction
In addition to these classical sources, on real aircraft structures numerousparasitic sources of excess noise can be detected.
spoiler
1/7.5 scaled model in DNW-LLF
1/7.5 scaled model in DNW-LLF
www.DLR.de • Chart 13 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Reduction of excess noise sources
Noise due to design details at original components, generally not resolved at small-scale WT models or in CAA simulations
flap side-edge cavities
MD-11
Fuel overpressure or anti-ice vents
pin holes at landing gears
MD-11 MLG
Slat track cutouts
+ slat-side edge cavities
www.DLR.de • Chart 14 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Reduction of excess noise sources
Such components (tonal or broadband) could be often easily avoided. Overall source ranking unclear; only limited measurement information available
Noise due to design details at original components, generally not resolved at small-scale WT models or in CAA simulations
flap side-edge cavities
MD-11
Fuel overpressure or anti-ice vents
pin holes at landing gears
MD-11 MLG
Slat track cutouts
+ slat-side edge cavities
www.DLR.de • Chart 15 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Baseline: Narrowband array analysis, R = 268 m
0 500 1000 1500 2000Frequency [Hz]
Sou
nd P
ress
ure
Leve
l [d
B]
All cavities sealedBaseline
10 dB
Cruise Configuration
Frequency [Hz]
Soun
d Pr
essu
re L
evel
Baseline“Low noise”
v = 105 m/sf = 14 Hz
Fan-Tones
Wing Cavity Tones
0 500 1000 1500 2000Frequency [Hz]
Sou
nd P
ress
ure
Leve
l [d
B]
All cavities sealedBaseline
10 dB
Cruise Configuration
Frequency [Hz]
Soun
d Pr
essu
re L
evel
Baseline“Low noise”Baseline“Low noise”
v = 105 m/sf = 14 Hz
Fan-Tones
Wing Cavity Tones
Tonal parasitic noise: -
Landing gear: elimination of ‘pipe’
resonance by pin-hole covers, but
such means are not popular due to water condensation problems.-
Wing: elimination of shear-layer induced Helmholtz resonance by vortex generators flightworthy solution
Reduction of excess noise sources
Loudest sound sources during approach to landing until full flaps are extended
www.DLR.de • Chart 16 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Baseline: Narrowband array analysis, R = 268 m
0 500 1000 1500 2000Frequency [Hz]
Sou
nd P
ress
ure
Leve
l [d
B]
All cavities sealedBaseline
10 dB
Cruise Configuration
Frequency [Hz]
Soun
d Pr
essu
re L
evel
Baseline“Low noise”
v = 105 m/sf = 14 Hz
Fan-Tones
Wing Cavity Tones
0 500 1000 1500 2000Frequency [Hz]
Sou
nd P
ress
ure
Leve
l [d
B]
All cavities sealedBaseline
10 dB
Cruise Configuration
Frequency [Hz]
Soun
d Pr
essu
re L
evel
Baseline“Low noise”Baseline“Low noise”
v = 105 m/sf = 14 Hz
Fan-Tones
Wing Cavity Tones
Tonal parasitic noise: -
Landing gear: elimination of ‘pipe’
resonance by pin-hole covers, but
such means are not popular due to water condensation problems.-
Wing: elimination of shear-layer induced Helmholtz resonance by vortex generators flightworthy solution
Reduction of excess noise sources
Loudest sound sources during approach to landing until full flaps are extended
Up to 6 dB(A) during approach at noise monitoring positions; but: not at certification point when flaps are fully extended
During departure at noise monitoring points 0.6 dB(A), on take-off certification point 0.2 dB(A)
www.DLR.de • Chart 17 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Broadband parasitic noise: reduction potential by sealing
Frequency, kHz0.1 1.0 10.0
10 dB
BaselineSealed cavitiesA
-wei
ghte
d Le
vel
Frequency, kHz
A-w
eigh
ted
Leve
l
0.1 1.0 10.0
10 dB
BaselineClean Flap-edge
full-scale WTT
A320 Slat-track cut-outs A320 Flap edge cavity
Reduction of excess noise sources
www.DLR.de • Chart 18 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Foam fillerTape sealing
Broadband parasitic noise: reduction potential by sealing
full-scale WTT
A320 Slat-track cut-outs A320 Flap edge cavity
Reduction of excess noise sources
Lufthansa initial flight test showed ~2 dB broadband noise reduction (for frequencies from 0.5 to 1.5 kHz).
www.DLR.de • Chart 19 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Full scale (component and flyover) testing at revealed sources of excessnoise:
-
Tone noise from flow over holes in wing surface or hollow pins atlanding gear components
-
Broadband noise from flow over cavities in wing leading-edge and flap side-edge
These contributions could be easily avoided in the early design phase(technical solutions trivial), retrofits are costly transfer to the existing fleet rather unlikely
Note: Quantification of source ranking is only available for selected A/C (limited full-scale test data including source localization); these sourcescould dominate the overall component noise which limits currentprediction capability
From the researcher’s standpoint the classical sources are much more interesting reduction is more challenging
Reduction of excess noise sources Synopsis
www.DLR.de • Chart 20 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear noise reduction
wheel bay (cavities)landing gears
www.DLR.de • Chart 21 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear (LG) noise reduction
For current short and long range aircraft (not for typical business jets) the undercarriages are the primary sources of airframe noise.
Possible
noise reduction efforts:-
Avoidance of flow separation of the various bluff-body/rim elements and hence, prevention of wake/solid-body interaction (single elements of individual LG but also interaction noise between the LGs of an A/C)
www.DLR.de • Chart 22 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear (LG) noise reduction
For current short and long range aircraft (not for typical business jets) the undercarriages are the primary sources of airframe noise.
Possible
noise reduction efforts:-
Avoidance of flow separation of the various bluff-body/rim elements and hence, prevention of wake/solid-body interaction (single elements of individual LG but also interaction noise between the LGs of an A/C)
-
Lowering the complexity and/or number of individual LG components-
Reduction of the local flow speeds at the installation position of protruding parts
Reminder: <p2> ~ u∞6,
i.e. a 3-dB noise reduction could be achieved by reducing u∞
by only 11%!
www.DLR.de • Chart 23 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear (LG) noise reduction
For current short and long range aircraft (not for typical business jets) the undercarriages are the primary sources of airframe noise.
Possible
noise reduction efforts:-
Avoidance of flow separation of the various bluff-body/rim elements and hence, prevention of wake/solid-body interaction (single elements of individual LG but also interaction noise between the LGs of an A/C)
-
Lowering the complexity and/or number of individual LG components-
Reduction of the local flow speeds at the installation position of protruding parts
Low-noise LG solutions:-
Add-on LG noise reduction technologies streamlined fairings
-
New architectures-
Optimized LG installation
www.DLR.de • Chart 24 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
LG noise reduction Maximum noise reduction potential
10 dB
BaselineLow Noise
Low Medium HighFrequency
1/3-
Oct
. Ban
d Le
vel
Straightforward approach: unrealistic complete fairing installed infundamental LG study (Dobrzynski, 1995)
full-scale WTT (DNW-LLF)
www.DLR.de • Chart 25 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
LG noise reduction Maximum noise reduction potential
10 dB
BaselineLow Noise
Low Medium HighFrequency
1/3-
Oct
. Ban
d Le
vel
Straightforward approach: unrealistic complete fairing installed infundamental LG study (Dobrzynski, 1995)
full-scale WTT (DNW-LLF)
Aim: Provide noise reduction of similar order for realistic treatment Huge basic noise reduction potential of > 10 dB for not practicable solution!
www.DLR.de • Chart 26 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear noise reduction Airworthiness requirements 1/2 Operational aspects:
-
Runway loads restrictions define number and spacing of wheels-
Gear locations are defined by lateral aircraft stability and rotation before liftoff
-
Brake heating dissipation requirement (fairings which would delay cooling increase the turnaround time on airport)
Security aspects:-
Emergency extension requirement:
-
Mechanical free-fall system which disengages the uplocks and allows the landing gear to fall due to gravity
-
Mechanical gear downlock affects MLG side-stay design-
Tire burst requirements affects location of hydraulical/electrical dressings and enforces redundancy, equipment located in the landing gear bay should be protected
www.DLR.de • Chart 27 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear noise reduction Airworthiness requirements 2/2 Cost aspects:
-
Effects on the airframe should be minimized (minimum bay size for stowing)
-
System complexity must be as low as possible (articulated components should be avoided)
-
Viability of fairing materials and maintenance access: Potential
add-on fairings must not obstruct quick routine inspection and should be easily maintainable (contamination)
www.DLR.de • Chart 28 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Source: RAIN project
Farfield wall-mounted microphones
Landing gear noise reduction Noise reduction potential by realistic fairings
Airbus 320 NLG and MLG Fairings
-NLG -MLGNLG MLGSource: SILENCE®
projectSource: Herkes, et. al. (AIAA 2006-2720)
Flight testing of LG fairings: all devices manufactured w.r.t. airworthiness requirements
NASA QTD 2 Boeing 777 MLG
fairings
www.DLR.de • Chart 29 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Source: RAIN project
Farfield wall-mounted microphones
Landing gear noise reduction Noise reduction potential by realistic fairings
Airbus 320 NLG and MLG Fairings
-NLG -MLGNLG MLGSource: SILENCE®
projectSource: Herkes, et. al. (AIAA 2006-2720)
Flight testing of LG fairings: all devices manufactured w.r.t. airworthiness requirements
NASA QTD 2 Boeing 777 MLG
fairings
- 2 EPNdB achieved on total A340 landing gear source noise, - 0.4 EPNdB on A/C level (approach certification noise; LG + HLD)
www.DLR.de • Chart 30 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Leg door filler
Landing gear noise reduction Noise reduction potential by realistic fairings
Upper side-stay fairing
Undertray
Wheel caps
Articulation link fairing
Drag stay fairings
Tow bar/axle fairing
Hub caps
Centre steering fairing
MLGNLG
Design details SILENCE®
A340 flight test
www.DLR.de • Chart 31 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear noise reduction Technologies under development: elastic fairings Ravetta, et. al. (AIAA 2007-3466): Initial study at ¼
scale B777 gear
www.DLR.de • Chart 32 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear noise reduction Technologies under development: elastic fairings Ravetta, et. al. (AIAA 2007-3466): Initial study at ¼
scale B777 gear
Cloth fairings promise an additional 2-dB noise reduction compared to solid fairings (solid fairings cause flow deflection that might lead to noise increase in adjacent areas with increased flow velocity).
Challenge: Flow resistance should be low enough to reduce deflection effect, but high enough to limit the wake flow velocity (mesh fairings induce high- frequency excess noise that has to be shifted to low-weighted frequencies)…
www.DLR.de • Chart 33 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear noise reduction Noise reduction potential by new architectures Current design efforts involve iterative loops based on experimental
experience, supported by CFD calculations General remarks:
-
Belly mounted landing gears are beneficial (2-3 dB quieter than wing mounted equivalent) parts of the legs and side stay are hidden in the bay/belly fairing
-
Bogie aligned with airflow during approach is beneficial to typical ‘toes up’
position
‘toes up’
flowflow
www.DLR.de • Chart 34 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear noise reduction Noise reduction potential by new architectures Example: SILENCE®
DNW-LLF test results at full-scale mock-up
NLG baseline
1 10 100Strouhal Number fm·s/v
60
70
80
L m -
60·lo
g(v/
v ref)
(dB
) = 90°A340 original MLGRAIN add-on fairingsSILENCER WP2.3.2
A340 original MLGRAIN add-on fairingsSILENCER WP2.3.2 NLG
~ - 7 dB(A)
1 10 100Strouhal Number fm·s/v
60
70
80L m
- 60
·log(
v/v r
ef)
(dB
) = 90°
MLG
~ - 5 dB(A)
MLG baseline
www.DLR.de • Chart 35 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear noise reduction Noise reduction potential by new architectures Example: SILENCE®
DNW-LLF test results at full-scale mock-up
NLG baseline
1 10 100Strouhal Number fm·s/v
60
70
80
L m -
60·lo
g(v/
v ref)
(dB
) = 90°A340 original MLGRAIN add-on fairingsSILENCER WP2.3.2
A340 original MLGRAIN add-on fairingsSILENCER WP2.3.2 NLG
~ - 7 dB(A)
1 10 100Strouhal Number fm·s/v
60
70
80L m
- 60
·log(
v/v r
ef)
(dB
) = 90°
MLG
~ - 5 dB(A)
MLG baseline Corresponding in-flight prediction: -4.1 EPNdB on LG level
www.DLR.de • Chart 36 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Landing gear noise reduction Synopsis Mid-
to long-term LG noise reduction potential on component level:
-
~ 5 EPNdB to be realized by realistic fairing solutions(estimate based on WTT, flight test: ~2 EPNdB); high TRLmain implementation issue: weight, heat dissipation, maintenance
access, system complexity-
~ 5 EPNdB to be realized by future architectures; medium TRLmain implementation issue: structural and system integration
LG noise reduction efforts are on a good track w.r.t the 2020 goal; further efforts are needed to eliminate the remaining drawbacks and to further increase the achieved noise reduction potential
Note: Active flow control devices (blowing/suction, plasma actuation) are atvery low TRL: basic studies promise ~1 EPNdB LG noise reduction
on top
of low-noise design; main implementation issue: weight, structural and system integration, air/energy supply, complexity vs. passive devices
www.DLR.de • Chart 37 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
High-lift noise reduction
Basic principles: edge noise reduction
+ slat side-edgesflap side edge
wing tip
slat trailing-edge
u
www.DLR.de • Chart 38 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
High-lift noise reduction
Basic principles: edge noise reduction
+ slat side-edgesflap side edge
wing tip
slat trailing-edge
u
gear/flap interaction
jet/flap interaction
www.DLR.de • Chart 39 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Basic principles: trailing-edge (TE) noise reduction
First airframe noise reduction approaches go back to the 1970ies,mainly dedicated to edge noise reduction
Possible TE noise reduction mechanisms are:-
Modification of the edge enhancement factor by matching of the edge boundary conditions to free air (‘impedance adjustment’)/ or modification of the ‘scattering center’
(geometric break up of the
edge contour)-
Acoustic absorption
-
Hydrodynamic absorption
These could be realized by means of-
Serrated edges (note: <p²> ~ cos³)
-
Porous material application-
Slotted and brush-type edges
u
Reminder: Not a mechanism but also successful: changes of the governingparameters, e.g. reduction of the incoming flow velocity because <p²> ~u∞
5
www.DLR.de • Chart 40 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Basic principles: trailing-edge (TE) noise reduction
First airframe noise reduction approaches go back to the 1970ies,mainly dedicated to edge noise reduction
Possible TE noise reduction mechanisms are:-
Modification of the edge enhancement factor by matching of the edge boundary conditions to free air (‘impedance adjustment’)/ or modification of the ‘scattering center’
(geometric break up of the
edge contour)-
Acoustic absorption
-
Hydrodynamic absorption
These could be realized by means of-
Serrated edges (note: <p²> ~ cos³)
-
Porous material application-
Slotted and brush-type edges
u
NACA0012 in AWB
http://www.earthlife.net/birds/images/anatomy/owl-feather.jpg
www.DLR.de • Chart 41 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
fm, kHz
L p(1/
3),d
B
5 10 15 2030
40
50
60
70
-0.2 0.04.0 2.38.0 4.5
12.0 6.7
g, a,deg deg
u = 60 m/sSREF1BRUSH_1
NACA0012 in AWB
Parametric AWB study on TE brushes
Basic principles: TE noise reduction Brush-type TE extensions
u
www.DLR.de • Chart 42 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
fm, kHz
L p(1/
3),d
B
5 10 15 2030
40
50
60
70
-0.2 0.04.0 2.38.0 4.5
12.0 6.7
g, a,deg deg
u = 60 m/sSREF1BRUSH_1
NACA0012 in AWB
Parametric AWB study on trailing edge brushes
Basic principles: TE noise reduction Brush-type TE extensions
u
Basic design criteria (fiber diameter, slit width, length) and scaling laws available Largest noise reduction potential among all previously tested TE devices
Note: Non-flexible brushes made of steel needles provided a comparable noisereduction effect like flexible devices!
www.DLR.de • Chart 43 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Basic principles: TE noise reduction Material airworthiness requirements No deviations from basic geometrical definition Operational temperature range UV-stability/ chemical resistance/ aeronautical fluids compatibility/
humidity –
wet ageing/ ice accretion/ corrosion –
salt spray test Sand and dust contamination Mechanical/ abrasion resistance/ stiffnessWeight and balance requirements System integration aspects No impact on aerodynamical performance Noise requirements
Material selection according to nonlinear resistance behavior: -
Transparent to the acoustically relevant wall-normal velocity fluctuations but
-
Impermeable to typical mean flow velocities (no mean leakage flow through TE region)
NACA0012 in AWB
u
open porosity
www.DLR.de • Chart 44 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Basic principles: TE noise reduction Material airworthiness requirements No deviations from basic geometrical definition Operational temperature range UV-stability/ chemical resistance/ aeronautical fluids compatibility/
humidity –
wet ageing/ ice accretion/ corrosion –
salt spray test Sand and dust contamination Mechanical/ abrasion resistance/ stiffnessWeight and balance requirements System integration aspects No impact on aerodynamical performance Noise requirements
Material selection according to nonlinear resistance behavior: -
Transparent to the acoustically relevant wall-normal velocity fluctuations but
-
Impermeable to typical mean flow velocities (no mean leakage flow through TE region)
NACA0012 in AWB
u
open porosity
Critical hurdle: coming up with a design that does not create extra drag at cruise
www.DLR.de • Chart 45 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
mean flow
airfoil interior
open porosity
35 mm
Basic principles: TE noise reduction Porous material application Parametric small-scale
AWB study on porous trailing edges
NACA0012 in AWB
u
www.DLR.de • Chart 46 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Basic principles: TE noise reduction Porous material application Noise reduction potential
NACA0012 in AWBfm0/u
L p(1/
3)no
rm,d
B
0.1 0.3 0.5100
110
120
130
140
SREF1BM75_1BM50_1CLOTH_1
(0 = 1 mm)
u =40 m/s50 m/s60 m/s
a= 6.7 deg
fm0/u
L p(1/
3)no
rm,d
B
0.1 0.3 0.5100
110
120
130
140
SREF1BM75_1BM50_1CLOTH_1
(0 = 1 mm)
u =40 m/s50 m/s60 m/s
a= 0 deg
u
www.DLR.de • Chart 47 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Basic principles: TE noise reduction Porous material application Noise reduction potential
NACA0012 in AWBfm0/u
L p(1/
3)no
rm,d
B
0.1 0.3 0.5100
110
120
130
140
SREF1BM75_1BM50_1CLOTH_1
(0 = 1 mm)
u =40 m/s50 m/s60 m/s
a= 6.7 deg
fm0/u
L p(1/
3)no
rm,d
B
0.1 0.3 0.5100
110
120
130
140
SREF1BM75_1BM50_1CLOTH_1
(0 = 1 mm)
u =40 m/s50 m/s60 m/s
a= 0 deg
Materials provide noise reduction (dependence on a-o-a and flow resistance) Two of the shown materials have passed preliminary airworthiness checks Kickback: adverse a-o-a dependence Not by far the noise reduction as achieved for brushes!
u
www.DLR.de • Chart 48 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Basic principles: TE noise reduction Slotted TEs Transfer solution: slotted TEs (anisotropic porosities)
SREF1Slotted TE_1
fm, kHz
L p(1/
3),d
B
5 10 15 2030
40
50
60
70
-0.2 0.04.0 2.38.0 4.5
12.0 6.7
g, a,deg deg
u = 60 m/s
NACA0012 in AWB
u
first proof of concept
www.DLR.de • Chart 49 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Basic principles: TE noise reduction Slotted TEs Transfer solution: slotted TEs (anisotropic porosities)
SREF1Slotted TE_1
fm, kHz
L p(1/
3),d
B
5 10 15 2030
40
50
60
70
-0.2 0.04.0 2.38.0 4.5
12.0 6.7
g, a,deg deg
u = 60 m/s
Significant noise reduction achieved; further optimization potential? CAA Related CFD Study has shown that detrimental effect on HLD aerodynamic performance vanishes for slotted configuration. (Ortmann & Wild, Journal of Aircraft, 44(4), 2007)
NACA0012 in AWB
u
first proof of concept
www.DLR.de • Chart 50 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Basic principles: TE noise reduction Slotted TEs CAA study on slotted TEs at a NACA0012:
-
Isolation of the edge enhancement contribution to noise/noise reduction
slotted solid
4 dB noise
reduction
u
Source: Fassmann et al., DLR
www.DLR.de • Chart 51 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
According to CFD results: modification of inflow turbulence (hydrodynamical absorption) not major mechanism
Currently under investigation: effect of acoustic absorption
Basic principles: TE noise reduction Slotted TEs CAA study on slotted TEs at a NACA0012:
-
Isolation of the edge enhancement contribution to noise/noise reduction
slotted solid
4 dB noise
reduction
u
Source: Fassmann et al., DLR
www.DLR.de • Chart 52 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Basic principles: TE noise reduction Synopsis A large noise reduction potential can be provided by flow-permeable TE
modifications
Main implementation issues: structural and system integration (retraction), material airworthiness and fixture, transfer of the gainedknowledge (design rules and scaling laws for brushes) from generic testconfigurations to realistic HLD components (all relevant parameters included in empirical descriptions?)
Evaluation of airworthy & aeroacoustically efficient materials is subject ofongoing projects, in particular the further development of devices withanisotropic porosities
www.DLR.de • Chart 53 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
High-lift noise reduction
Slat noise reduction
slat
800 Hz
www.DLR.de • Chart 54 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Slat noise reduction
Slat noise dominates the total high-lift noise spectrum (this is knownsince 1995)
Possible noise reduction mechanisms are:-
Manipulations of the various slat noise generation mechanisms (e.g. of the TE noise/ flow impingement noise sources, …) with the following governing parameters:
-
TKE of slat cove shear layer flow-
TE local flow speed/ mean pressure gradient
-
TE/ reattachment location boundary conditions-
Provision of local sound attenuation (acoustic absorption)
www.DLR.de • Chart 55 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Add-on treatments-
Flow-permeable TE treatment: adaptation of aforementioned TE noise reduction technologies successfully tested at 2D profiles
-
Cusp treatment/alternative cusp designs cove filler etc.-
Slat cove liner technology
Optimized slat gap/overlap settings and derivative technologies-
Adaptive slat
-
Very long chord slat Alternative high-lift system architectures (extreme case: slot-/slatless
configurations, droop nose devices) new A/C architectures
Slat noise reduction Technologies under development
Source: LEISA project, DLR
www.DLR.de • Chart 56 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Slat noise reduction Technologies under development
3-dB noise reduction on A/C level
Slat cove cover
Source: Khorrami
et al., NASA
Slat cusp sealarray measurement
Survey on add-on treatmentsA320 full-scale wing
TE brush
With additional slat TE-brushReference (with flap-edge brush)
www.DLR.de • Chart 57 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Slat noise reduction Technologies under development
3-dB noise reduction on A/C level
Slat cove cover
Source: Khorrami
et al., NASA
Slat cusp sealarray measurement
Survey on add-on treatmentsA320 full-scale wing
TE brush
With additional slat TE-brushReference (with flap-edge brush)
Degradation of aerodynamical performance might be a ‘show-stopper’ for many ideas; certification requirement: CLmax determines lowest selectable approach speed, i.e. CLmax degradation might counterbalance achieved noise benefit…
www.DLR.de • Chart 58 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Slat noise reduction Airworthiness requirements Operational aspects:
-
Maximum lift determines landing speed-
Sufficient lift for moderate
angles-of-attack to prevent tail-strike for
take-off-
Low noise treatments must not affect cruise performance if not operational (retraction)
Security aspects:-
Reliability
-
No sudden lift/ moment changes through activation of control device Cost aspects:
-
Weight -
Structural constraints (slat tracks affect front spar position, etc.)
-
Systems complexity (e.g. bleed air for flow control, etc.)-
Maintenance (contamination, icing of noise red. treatments)
max)(
223.1=23.1=
Lstallapproach ACρ
mgvv
2max
)( 23.1= L
approachL
CC
u∞
~ CLmax-1/2 with assumption: <p2> ~ u∞
5: 10 % less CLmax is about 5.4 % increase in landing speed = 1.1 dB noise increase! Cost function: <p2> ~ CLmax
-5/2
www.DLR.de • Chart 59 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
cLmax
Slat noise reduction Technologies under development
SPL
−
+
overlap (ov)
gap
Source: TIMPAN project
/2
Optimized slat gap/overlap optimization (CFD/CAA)
www.DLR.de • Chart 60 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
With aeroacoustic cost function:
SPL+10log (CLmax(ref) /CLmax
)5/2
Slat noise reduction Technologies under development Optimized slat gap/overlap optimization (CFD/CAA)
−
+
overlap (ov)
gap
Source: TIMPAN project
www.DLR.de • Chart 61 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Derivative 1: Very long chord slat (VLCS):-
Reduced gap size and increased overlap
-
Reduced slat deflection angle with adapted flap deflection for constant aerodynamical performance
≈ 4 dB
Slat noise reduction Technologies under development
1/3-Octave Band [Hz]
1/3-
Oct
ave
Ban
dLe
vel[
dB]
2000 4000 6000 800060
70
80
90
100VLCS | Run 1005 | dpt3 | Ma=0.1 | Alpha=4.5°3E Ref | Run 1098 | dpt3 | Ma=0.1 | Alpha=6.0°VLCSReference
Source noise reduction (array)
Source: LEISA project, DLR
2D WTT results
www.DLR.de • Chart 62 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
SPL,
dB
alpha-5 0 5 10 15 20 25
Conventional slat
Adaptive slat, closed gap
°
Derivative 2: Adaptive slat Full elimination of slat noise for closed gap variation
Slat noise reduction Technologies under development
Rigid leading edge region
Actuator force
Rigid trailing edge
Flexible top and cove region
Rigid leading edge region
Actuator force
Rigid trailing edge
Flexible top and cove region
Quiet for small , high aerodynamic performance (but loud) where needed = tailored solution!
Tailored solution
alpha
CL_
tota
l
-5 0 5 10 15 20 252
2.2
2.4
2.6
2.8
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
Equal performances for operational AoA
Adaptive slat: low performance at high AoA
CL
°Range relevant for acoustics
Range relevant for performancecertification (the A/C will never fly here in normal operation)
Conventional slat: high performance for high AoA
Source: OPENAIR project
www.DLR.de • Chart 63 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Derivative 2: Adaptive slat Full elimination of slat noise for closed gap variation Specified airworthiness requirements
-
System must be safe: inactive system = original gap (CLmax
)-
Must work for all possible load conditions
-
Limited impact on weight, complexity, flow-
Extension time < 2 s
Slat noise reduction Technologies under development
Rigid leading edge region
Actuator force
Rigid trailing edge
Flexible top and cove region
Rigid leading edge region
Actuator force
Rigid trailing edge
Flexible top and cove region
Source: OPENAIR project
www.DLR.de • Chart 64 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
High-lift noise reduction
Flap side-edge noise reduction
(slat side-edges)flap side edges
(wing tip)
1.6 kHz
www.DLR.de • Chart 65 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Flap side-edge (FSE) noise reduction
For typical business jets
the flaps + FSEs are the primary sources of airframe noise.
Possible noise reduction approaches include:-
Reduce vortex interaction with sharp edges
-
Remove/postpone vortex roll-up process at FSE, outboard/upwards shift of FSE vortex (enable more delocalized pressure release)
-
Reduction of the FSE cross flow velocity (increase of vortex diameter while keeping vortex strength)
-
Modification of the edge boundary condition
Airworthiness requirements cf. slat
Source: Streett et al., NASA
www.DLR.de • Chart 66 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
FSE noise reduction Maximum noise reduction potential Basic side-edge noise reduction study in the AWB:
-
Extent of porous treatment vs. airworthiness
fm, kHz
L p(1/
3),d
B
5 10 15 2060
70
80
90
100
110 30_Baseline_bottom30_Foam(full)_bottom
fm, kHz
L p(1/
3),d
B
5 10 15 2060
70
80
90
100
110 30_Baseline_bottom30_Foam(full)_bottom+160mmTEtape@SS
fm, kHz
L p(1/
3),d
B
5 10 15 2060
70
80
90
100
110 30_Baseline_bottom30_Foam(full)_bottom30_Foam(PStaped)_bottom+160mmTEtape@SS
Source: OPENAIR project
‘flow barrier’
reduced porous treatment sealed SS in retracted case
Aluminum foam FSE
www.DLR.de • Chart 67 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Survey of different passive concepts:
FSE noise reduction Technologies under development
Fences provide some 2-3 dB source noise reduction
Side-edge Fences:
Baffled Flap Side-Edge
A340
Baffled Flap Side-Edge
A340
Source: Choudhari et. al., NASA
Clean Flap-edgeBrush Edge, high density, 0.055Brush Edge, low density, 0.055
Clean Flap-edgeBrush Edge, high density, 0.055Brush Edge, low density, 0.055
Full-scale A320 wing noise reduction (DNW-LLF)
Brush or porous FSESide-edge fences
www.DLR.de • Chart 68 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Survey of different passive concepts:
FSE noise reduction Technologies under development
Fences provide some 2-3 dB source noise reduction
Side-edge Fences:
Baffled Flap Side-Edge
A340
Baffled Flap Side-Edge
A340
Source: Choudhari et. al., NASA
Clean Flap-edgeBrush Edge, high density, 0.055Brush Edge, low density, 0.055
Clean Flap-edgeBrush Edge, high density, 0.055Brush Edge, low density, 0.055
Full-scale A320 wing noise reduction (DNW-LLF)
Brush or porous FSESide-edge fences
Airworthiness analysis of porous materials (foam and mesh) is subject of ongoing EC project OPENAIR.
www.DLR.de • Chart 69 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Continuous mold-line links -
for hinged or Fowler flaps
FSE noise reduction Technologies under development
Source: NASA
also applicable to slat side-edges
local source noise reduction:Baseline vs. moldline links + slat cove filler
www.DLR.de • Chart 70 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
High-Lift Noise Reduction Synopsis Compared to landing gears available HLD noise reduction technologies
are on a much lower TRL; main implementation issues: potential effect on L/D, structural and system integration (retraction, mechanical links, materials), potential impact on stability and control.
Promising noise reduction technologies have been identified andvalidated but need further analysis.
Actually, HLD source noise reduction through add-on means for conventional slats or FSEs is still limited to < 1 EPNdB and often suffers from a degradation in maximum lift.
Note: New configuration design requires strong application of validatedCFD and CAA tools.
www.DLR.de • Chart 71 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Summary of achievements and future needs State of the art Airframe noise reduction features on recent A/C products:
-
Novel component architectures: -
High-lift systems: Implementation of droop nose devices
as low-
noise compromise to slotted slats on inboard wing of A380 (development time ~1990-2005)
Source: AIRBUS Operations S.A.S.
www.DLR.de • Chart 72 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Summary of achievements and future needs State of the art Airframe noise reduction features on recent A/C products:
-
Novel component architectures: -
High-lift systems: Implementation of droop nose devices
as low-
noise compromise to slotted slats on inboard wing of A380 (development time ~1990-2005)
-
Landing gear design: optimization of gear/wheel number and size combination, bogie tilt angle, lower exposure of dressings to high speed flows
-
Prevention of parasitic noise sources -
Hole covering/ re-design
www.DLR.de • Chart 73 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Noise consideration at overall airplane design level and some limited noise component design features
But: Most of the reported component noise reduction technologies are on a too low TRL or induce too significant penalties to be efficiently integrated on new A/C
Summary of achievements and future needs State of the art Airframe noise reduction features on recent A/C products:
-
Novel component architectures: -
High-lift systems: Implementation of droop nose devices
as low-
noise compromise to slotted slats on inboard wing of A380 (development time ~1990-2005)
-
Landing gear design: optimization of gear/wheel number and size combination, bogie tilt angle, lower exposure of dressings to high speed flows
-
Prevention of parasitic noise sources -
Hole covering/ re-design
www.DLR.de • Chart 74 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Summary of achievements and future needs Near- to mid-term realizable gains at component level
Landing gear noise reduction -
Fairings and caps (high TRL):
~5 EPNdB (based on WTT, flight test:
~2 EPNdB); main implementation issue: weight, heat dissipation, maintenance access
-
Low-noise design (medium TRL):
~5 EPNdB; main implementation issue: structural and system integration
Slotted slat noise reduction -
Low-noise design/treatment (low TRL):
??? (actually < 1 EPNdB);
main implementation issue: potential impact on L/D, retraction, structural and system integration, airworthiness of materials
Flap side-edge noise reduction -
Low-noise design/treatment (low to medium TRL):
???; main
implementation issue: potential impact on L/D, stability & control, airworthiness of materials
www.DLR.de • Chart 75 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Summary of achievements and future needs Future needs Implementation to future product (mid term) with full noise benefit will
require further design optimization-
development and validation of efficient evaluation and design tools (CFD-CAA)
-
significant integrated demonstration efforts
www.DLR.de • Chart 76 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Acknowledgments
The author wishes to thank Dr. Roland Ewert, Michael Pott-Pollenske & Anton Rudenko, DLR Dr. Alexander Büscher, Hendrik Friedel, Eléonore Ammeux, Airbus
Operations GmbH/S.A.S Dr. Johann Reichenberger, EADS Innovation Works Nicolas Réau, Dassault Aviationfor their valuable support in providing relevant picture and data material from past and ongoing projects.
Special thanks are due to my former (now retired) colleague and mentor Dr. Werner Dobrzynski, DLRfor leaving permission to use his comprehensive lecture and documentation material. A valuable set of information has been taken from the well-
documented airframe noise survey in Dobrzynski, W. (2010): “Almost 40 Years of Airframe Noise Research ―
What Did we Achieve?”, Journal of the
Acoustic Society of America, 47 (2): 353-367.
www.DLR.de • Chart 77 > VKI Lecture Airframe Noise Reduction > M.Herr > 14.03.2012
Thank you for your attention!