Download - Almost 40years Airframe Noise Research
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Folie 1 > AIAA/CEAS-2008-AFN > Dobrzynski
Almost 40 Years of Airframe Noise Research What did we achieve?
Werner DobrzynskiDLR, Institute of Aerodynamics and Flow Technology
14th Aeroacoustics Conference, 5-7 May 2008 / Vancouver
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Outline
Problem definition and historical overview
Noise sources and mechanisms Noise reduction technologies
Summary of achievements and future needs
This informative talk focuses on experimental and applied researchin airframe noise and is an attempt to subsume related activities
worldwide without claiming to be complete.
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Why are we Interested in Airframe Noise ?
With the introduction of high-bypass ratio engines around 1970(for fuel saving !) a significant reduction in jet noise was achieved
As a result airframe noise became relevant in the approach phase,where engines are throttled down
Substantial research efforts into airframe noise started after 1970
Airframe noise is generated through the interaction of turbulentflows with solid bodies, e.g. landing gears and lifting surfaces.
1th generation engines 2nd generation engines
B707 A380
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Overall Aircraft Noise Ranking at Approach
APPROACH (long range aircraft) APPROACH (short range aircraft)APPROACH (long range aircraft) APPROACH (short range aircraft)
Aircraft Noise Source Breakdown:
Source: Airbus
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History of Noise Reduction Technologies
Based on theoretical work in aeroacoustics in the 60ies, alreadyfrom 1975 on most of the basic noise reduction technologieswere invented:
Trailing edge/ slat noise porous edge extensions (Bohn, 1976) perforated edge extensions andedge serrations (Grosche, 1979 and
Fink, et. al 1980)
Flap noise porous leading edge inserts (Fink, et. al 1980) Side-edge noise porous edge replacements (Fink, et. al 1980,
and later Revell, et. al 1997)
More current developments are:
Trailing edge/ slat noise TE-brushes slat cove cover; -filler; -liner
Flap side-edge noise side-edge fences, -brushes, -porous inserts moldline technology (for hinged flaps)
Landing gear noise application of streamlined fairings
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Major Sources of Airframe Noise
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Tone noise from fuel vents oranti-ice vents
0 500 1000 1500 2000Frequency [Hz]
Sound
Pressure
Level
[dB]
All cavities sealed
Baseline
10 dB
Cruise Configuration
Frequency [Hz]
SoundPressureLevel
BaselineLow noise
v = 105 m/s
f = 14 Hz
Fan-Tones
Wing Cavity Tones
0 500 1000 1500 2000Frequency [Hz]
Sound
Pressure
Level
[dB]
All cavities sealed
Baseline
10 dB
Cruise Configuration
Frequency [Hz]
SoundPressureLevel
BaselineLow noiseBaselineLow noise
v = 105 m/s
f = 14 Hz
Fan-Tones
Wing Cavity Tones
Parasitic Airframe Noise Sources Wing Systems
Frequency
0.1 1.0 10.0
10 dB
BaselineSealed cavities
A-weigh
tedLevel
A320 Slat-track cut-outs:
Frequency
A-weightedLevel
0.1 1.0 10.0
10 dB
BaselineClean Flap-edge
A320 Flap edge cavity
Example: MD-11
Fuel overpressure vent
Broadband noise fromslat track cut-outs
Broadband noise fromflap side-edge cavities
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Airframe noise sources of minor importance:
Wing tip noise (seldom identified in source location plotsdue to other prominent sources)
Noise from infinitely extended surfaces (e.g. fuselage) is expectedto be more than 10 dB below current high-lift noise levels(i.e. below clean aircraft trailing edge noise), this is
boundary layer noise including roughness effects
noise from surface vibration (low radiation efficiency due toimpedance mismatch between wall material and air)
Free wake turbulence (quadrupole noise is insignificant for M < 0.2)
Tonal vortex shedding noise from smooth circular landing gear struts(seldom observed for high flight Reynolds numbers)
Second Order Airframe Noise Sources
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Landing Gear Noise Problem Definition
10 dB
BaselineLow Noise
Low Medium High
Frequency
1/3
-oct.BandL
evel
More than 10 dB reductiondemonstrated for not practicalcomplete aerodynamic fairing
Future efforts needed to realisenoise reduction of similar orderof magnitude under practicalconstraints
First full scale LG test in 1995:
A320 Main Gear inGerman-DutchWind Tunnel
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Landing Gear Noise - Source Mechanisms
Landing gears represent a 3D cluster of noise sources: Broadband noise from turbulent vortex shedding off struts
Turbulent wake-flow / solid bodyinteraction between components
Broadband noise from gear bay free shearlayer interaction with downstream bay rim
Landing gear noise scales on a Strouhal number basis
increases with velocity to the 6th power (dipole type source) and
features an almost omnidirectional radiation characteristic
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Landing Gear Low Noise Design - Constraints
Operation
Limitation of runway loads defines number of wheels and spacing
Gear locations defined by lateral stability and rotation before lift-off(local velocity at gear position for under wing installation)
Brake cooling (fairings which would delay cooling increase theturn around time on airport)
Safety
Free fall requirement (MLG leg door can not be used as spoiler)
Tyre burst (location and redundancy of dressings)
Cost
Weight
Effect on airframe (minimum bay size for stowing)
Systems complexity (articulated components)
Maintenance (fairings obstruct quick inspection, contamination)
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Landing Gear Noise Add-on Fairing Design
NLG MLGNLG MLGAirbus NLG and MLG Fairings
Flight testing oflanding gear
fairings:
- 2 EPNdBachieved
on total A340landing gearsource noiseHerkes, et. al. / AIAA 2006-2720
Farfield wall mounted microphones
A340 fairingsmanufactured
to airworthinessrequirements
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Landing Gear Noise Advanced Low Noise Design
Iterative design process for low noise gears based on experimentalexperience with support from CFD calculations:
Design withsupport fromCFD computation
Full scale mock-upwind tunnel test
Test result: 5 to 7 dB(A) noise reduction
In-flight prediction: - 4.1 EPNdB on LG-level
1 10 100
Strouhal Number fms/v
60
70
80
Lm-60log(v
/vref)
(dB)
= 90A340 original NLG
RAIN add-on fairings
SILENCER WP2.3.2
A340 original NLG
RAIN add-on fairings
SILENCER WP2.3.25 dB
1 10 100
Strouhal Number fms/v
60
70
80
Lm-60log(v
/vref)
(dB)
= 90A340 original NLG
RAIN add-on fairings
SILENCER WP2.3.2
A340 original NLG
RAIN add-on fairings
SILENCER WP2.3.25 dB5 dB
DNW-LLF Test:
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Landing Gear Noise - Prediction Tools
Empirical and Semi Empirical Tools: Fink (NASA ANNOP) (empirical tool scaled
from early DLR scale model data)
DLR (empirical tool re full scale wind tunneldata by Dobrzynski)
ISVR (semi empirical tool by Smith) Boeing (semi empirical tool by Guo)
Semi empirical tool concepts:
Superposition of noise contributionfrom individual components
Account for small details throughcomplexity or dressing factor(governs high frequency levels)
Source: M. Smith / ISVR
CFD/CAA approaches with design to noise capabilities:
LES/ DES calculations and FW-H for simplified gear geometries only(not really convincing results yet)
Still only limiteddesign to noise
capability
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High-Lift Devices Noise Problem Definition
Challenges in HLD noise testing incomparison with LG experiments:
Testing of complete wing systemsmostly performed at small scale
(i.e. 1/4 up to 1/11) due to lack ofsufficiently large anechoic facilities.
Accordingly results suffer from:
Reynolds number effects (tone noise)
Components can not always bemanufactured to scale(trailing edge thickness, track design)
Basic investigations in 2D (i.e. no sweep)
need validation in 3D (cross flow)
Wing downwash causes inaccurateaerodynamic conditions in open test
sections for measurement of farfieldnoise directivity
1/10.6 scaledA340 modelin DNW-LLF8 m by 6 m
open testsection
1/3.8 scaled777 semi spanin NASA Ames40 ft x 80 ft
closed testsection
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High-Lift Slotted Slats - Noise Mechanisms
Source: Choudhari, et.al. / NASA Langley
Potential source mechanisms:
PIV Measurements:
Vorticity field
Trailing edge (bluntness tone noise only relevant at model scale !)
Free shear layer vortex flow reattachment
Unsteadiness of vortex core
Analytical and CFD/ CAA studiesprovided insight in majorslat noise mechanisms.
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High-Lift Slat Noise Sources of Tone Noise
Tone phenomena for scale model testing:
Low frequency tone effects due to
cavity resonances (Rossiter modes ?)(to be reduced by tripping on pressure side)
High frequency tone effects due toTollmien-Schlichting instability(to be avoided by tripping on suction side)
1/7 scale model
AWB
Low frequency tones
Source: Oerlemans / NLR
Highfrequencytones
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High-Lift Devices - Slat Noise - Effect
1/10.6 scaled A340 Model in DNW-LLF: = m + = 103
Only little effect onslat noise of moderate
landing angles-of-attack,i.e. < 10
Significant noise increasefor > 13
Lift
6 20
Lift
6 20
Landing range
Slat noisedominated
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High-Lift Devices Low Noise Design Constraints
Operation
Maximum lift (determines landing speed) 10 % less CLmax is about5.4 % increase in landing speed = 1.4 dB noise increase !!
Sufficient lift for moderate angles-of-attack to prevent tail-strikefor take-off
Safety
Reliability (low noise treatments must not affect performance
if not operational) No sudden lift/ moment changes through activation of control device
Cost
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)
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High-Lift Devices Slat Noise Reduction
3 dB noise reductionthrough slat-cove-cover
Source: Khorrami, et.al. / NASA Langley
With extendedblade seal
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High-Lift Devices Slat Noise Reduction
Slat noise reduction through cove filler: Predicted with CFD/CAA
Validated by experiments of different research groups(i.e. DLR, EADS-IW, NASA and JAXA),
But noise reduction found to be extremely sensitive re filler contour.
Source: Horne, et.al. / NASA Ames
Inboard
Mid span
Outboard
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High-Lift Devices Flap Side-edge Noise Reduction
Clean Flap-edge
Brush Edge, high density, 0.055
Brush Edge, low density,
0.055
Clean Flap-edge
Brush Edge, high density, 0.055
Brush Edge, low density,
0.055
Full wingnoise reduction
Local sourcenoise reduction Moldline Techno.
for hinged orFowler flap
Source: NASA
Brush- or porous edge:
Fences provide some 2-3 dBsource noise reduction
Source:
Choudhari, et. al.,NASA Langley
Side-edge Fences:
Baffled Flap
Side-Edge
A340
Baffled Flap
Side-Edge
A340
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To answer the question: What did we achieve?
Detailed insight in noise source mechanisms but stillno sound knowledge of all noise generation parametersfor complex 3D sources
Computational Aeroacoustics (CAA) methods have developed
rapidly and promise to enable a design-to-noise of airframecomponents in the future
Landing gear source noise reduction of up to 5 EPNdBhas been demonstrated for future low noise gear designs
High-lift devices source noise reduction is still limited to lessthan 1 EPNdB (through add-on means for conventional slats)and often suffers from a degradation in maximum lift
Low noise slat designs (for constant aerod. performance !)were identified but need validation in 3D
Flap side-edge low noise modifications were identifiedand validated
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Status and Future Needs
Status of Airframe noise reduction:
Airframe noise reduction achievements still need furtherimprovement to cope with the European Vision 2020and the even more stringent US noise reduction goals
While LG noise reduction is on a good track high-lift devicesnoise reduction needs more attention in particular with respectto maintain aerodynamic performance
Future needs:
Focussed efforts to develop CAA tools for 3D application in theindustrial design chain
Flow separation control technologies must be developed for
both gear structures and slat-less high-lift devicesIn long terms new aircraft configurations are needed, whichfeature short landing gears and enhanced lift capabilitiesfor reduced approach speed.
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Thank youfor your attention !