Almost 40years Airframe Noise Research

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    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 !