experimental study of broadband trailing edge noise of a ...experimental study of broadband trailing...

PhD Defense

Experimental study of broadband trailing edge noise

of a linear cascade and its reduction

with passive devices

Arthur Finez

LMFA/Ecole Centrale de Lyon

Thursday 10th May 2012

A. Finez (LMFA/ECL) PhD Defense 10/05/2012 1 / 37

Introduction

IntroductionNoise reduction

Why aircraft noise is to be reduced ?

Orly airport noise disturbance map.: LDEN>70, : LDEN>65, : LDEN>55. [ACNUSA]

Noise problems nearby airports.

Introduction

IntroductionNoise reduction

Why aircraft noise is to be reduced ?

Orly airport noise disturbance map.: LDEN>70, : LDEN>65, : LDEN>55. [ACNUSA]

Noise problems nearby airports.

Fan noise contribution

Engine of an A380 aircraft

Take-off and landing :⋍ 40%.

Introduction

IntroductionDiscriminating fan noise sources

Fan noise composition

Tonal noise, significantly reduced in the past decades,

Broadband noise, still to be reduced.

Many broadband noise sources near the rotor

Figure: Axial flow compressor rotorflow phenomena [AGARD-AG-328].

Introduction

- Turbulence - Leading Edge (LE)interaction noise,

Introduction

- Turbulent Boundary Layer (TBL) -Trailing Edge (TE) interaction noise,

Introduction

- Blade Tip noise,

Introduction

- Blade Tip noise,

- Turbulence - Shock Surfaceinteraction noise,

Introduction

- Blade Tip noise,

- Stall noise ...

Introduction

- Blade Tip noise,

- Stall noise ...Figure: Axial flow compressor rotorflow phenomena [AGARD-AG-328].

Introduction

IntroductionCascade Effect

Blades are not isolated in a fan !

- Reflections,

- Resonances,

- Duct propagation ...

Introduction

- Reflections,

- Resonances,

Literature : few studies on cascade trailing edge noise

- Analytical modeling : Howe 92 ; Glegg 98 ;

- Experiments : Parker 66, Sabah & Roger 01.7

Introduction

- Reflections,

- Resonances,

Literature : few studies on cascade trailing edge noise

- Analytical modeling : Howe 92 ; Glegg 98 ;

- Experiments : Parker 66, Sabah & Roger 01.7

Need for cascade noise experiment & model validation.

Introduction

IntroductionObjectives

This study aims at

- Designing an aeroacoustic cascade set-up,

- Measuring cascade TBL-TE noise with various U1, α1...

- Assessing the extent of the cascade effect,

- Validating existing cascade noise models,

- Reducing cascade noise.

Introduction

Outline

1 Cascade ExperimentSet-upAerodynamic LoadingCascade Acoustics

2 Analytical Model ValidationInput Data for ModelsAmiet’s Isolated Airfoil Noise ModelGlegg’s Cascade Noise Model

3 Cascade Noise ReductionSet-upAcoustic ResultsPIV resultsNoise reduction mechanisms

Cascade Experiment Set-up

Outline

ExperimentSet-up of a unique linear cascade

Linear cascade of 7 NACA 6512-10 airfoils, studied by Emery 1.

Chord c 100 mm σ = c/s 1.43Pitch s 70 mmUpstream velocity U1 80 m/s M 0.23Span L 200 mm Re 5.3 × 105

1. Systematic two-dimensional cascade tests of NACA 65-series compressor blades at lowspeeds, NACA report 1368, 1957

ExperimentImprovements from Sabah’s set-up

2001 : Sabah’s PhD thesis

- Aim : measurement of cascade leading edge and trailing edge noise,

- High background noise level.

Some improvements

- Side panels,

- Brushes at the end of woodboards,

- No boundary layer exhaust.

Some improvements

- Side panels,

- Brushes at the end of woodboards,

- No boundary layer exhaust.

Downstream far-field acousticspectra

100 1000 1000030

frequency, Hz

Sabah ;Kevlar walls ;Kevlar walls without BL exhaust ;Metal walls without BL exhaust

⇒ downstream measurements only.

Cascade Experiment Aerodynamic Loading

Outline

Aerodynamic LoadingValidating the cascade design point

Reference point near maximum efficiency

α1 = 15◦, β1 = 35◦, U1 = 80 m/s. (σ = 1.43)

0 20 40 60 80 100−0.6

−0.4

−0.2

x/c, %

Control on center blade.Measurement,Bock RANS computation,Emery & al.,(α1 = 14.1◦ , β1 = 30◦, σ = 1.5)Sabah.

α1 = 15◦, β1 = 35◦, U1 = 80 m/s. (σ = 1.43)

0 20 40 60 80 100−0.6

−0.4

−0.2

x/c, %

Control on center blade.Measurement,Bock RANS computation,Emery & al.,(α1 = 14.1◦ , β1 = 30◦, σ = 1.5)Sabah.

1 2 3 4 5 6 7

−0.4

−0.2

numero de pale

Control on other blades.At 50% chord,

Suction Side,Pressure Side.

Cascade Experiment Cascade Acoustics

Outline

Velocity DependenceSpecific frequency scaling

Raw PSD

100 1000 1000020

frequency, Hz

100m/s

Scaled PSD

100 1000 1000020

frequency, Hz

z)Far-field PSD at r=2 m

Scaling to U1 =80 m/s using PSD ∝ U 61 ,

Frequency axis unchanged ⇒ He= fL

c0rather than St= f δ∗

Acoustic ResonancesNear-field / far-field coherence

Remote Surface Unsteady Pressure Probes

Near the center blade trailing edge, midspan plane

- To measure entry data of analytical models (boundary layer statistics),

- To investigate near-field/far-field coherence.

1000 10000−0.1

frequency, Hz

θ < 0◦,θ > 0◦.

ParkerResonances : mode β

plate nodes

1000 10000−0.1

frequency, Hz

θ < 0◦,θ > 0◦.

ParkerResonances : mode α

plate nodes

1000 10000−0.1

frequency, Hz

θ < 0◦,θ > 0◦.

ParkerResonances : mode γ

Acoustic InterferenceVisible in far-field measurements

Blade-to-blade reflections

100 1000 1000020

frequency, Hz

100 1000 1000020

frequency, Hz

100 1000 1000020

frequency, Hz

δ1 δ2

|1 − eik(δ1+δ2)|

100 1000 1000020

frequency, Hz

δ1 δ2

|1 − eik(δ1+δ2)|

100 1000 1000020

frequency, Hz

δ1 δ2

|1 − eik(δ1+δ2)|

Most obvious cascade effects have been observed

To go further, need for analytical modeling : computation of the cascadetransfer function.

Analytical Model Validation Input Data for Models

Outline

Input Data Measurement : BL Statistics

# Φpp(ω)

100 1000 1000060

frequency, Hz

Cascade,

Isolated airfoil,

On center blade,U1 ≃ 70 m/s,α1 = 35◦ ,β = 45◦ .

# Φpp(ω)

100 1000 1000060

frequency, Hz

Cascade,

Isolated airfoil,

# lz =∫ +∞

0γ(ω, η) cos(Kzη)dη

100 1000 100000

frequency, Hz

η (mm) : 1, 2.5, 3.5, 5.

# Φpp(ω)

100 1000 1000060

frequency, Hz

Cascade,

Isolated airfoil,

0 50 100 150 200 250 3000

St= 1 × f/U1

U1, m/s60,70,80,100.

# lz =∫ +∞

0γ(ω, η) cos(Kzη)dη

100 1000 100000

frequency, Hz

η (mm) : 1, 2.5, 3.5, 5.

Analytical Model Validation Amiet’s Isolated Airfoil Noise Model

Outline

Analytical Model Validation Amiet’s Isolated Airfoil Noise Model

Analytical ModelsAmiet’s isolated airfoil TE noise model

Isolated airfoil formulation :

S (n)pp (x , y, z , ω) = 2L

4πc0S20

)2 ∣

Φpp(ω)lz

S0= 0, ω

7 independant airfoils, observer in the midspan plane, infinite span, shear layer refraction.

n=4 θ4

Measurement,Prediction,

200 1000 1000020

frequency, Hz

Analytical Model Validation Glegg’s Cascade Noise Model

Outline

Glegg’s Analytical ModelOriginal formulation

Linear cascade model- Fan azimuthal periodicity ⇒ periodic sources in linear cascade

- Small numbers of propagating modes,

- BL information in Q,

- 3D : K = (Kx ,Kz ).

ps =−2πi

m=+∞∑

m=−∞

QHm(Kx)e−iωt+iKz z K −

m e−iγ−

m (x−yd/h)−2iπmy/Bh

(2π)2i(γ−

m + Kx)J(m)+ (−Kx)J

(m)−

(γ−

- 3D : K = (Kx ,Kz ).

ps =−2πi

m=+∞∑

m=−∞

m e−iγ−

(2π)2i(γ−

(m)−

(γ−

0 1 2 3−1

−0.5

−0.05

ℜ(ps), Pa

ω = 1 ⇒ 1 propagative mode.

- 3D : K = (Kx ,Kz ).

ps =−2πi

m=+∞∑

m=−∞

m e−iγ−

(2π)2i(γ−

(m)−

(γ−

0 1 2 3−1

−0.5

−0.05

ℜ(ps), Pa

ω = 1 ⇒ 1 propagative mode.

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

ℜ(ps), Pa

ω = 3 ⇒ 5 propagative modes.

Glegg’s Analytical ModelChoice of Periodicity Parameter B

Choosing B with the aim of isolating a single source contribution.

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

ℜ(ps),

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

ℜ(ps),

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

ℜ(ps),

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

B=1000

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

ℜ(ps),

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

B=1000

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

B=10000

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

ℜ(ps),

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

B=1000

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

0 1 2 3−1

−0.5

−0.06

−0.04

−0.02

B=10000

Glegg’s Analytical ModelComparison with acoustic measurements

Last Steps :

- Expressing Q in terms of Φpp, lz , Uc,

- Integrating on Kz ,

- Summing over independant blade contributions.

Glegg’s Analytical ModelComparison with acoustic measurements

Last Steps :

- Expressing Q in terms of Φpp, lz , Uc,

- Integrating on Kz ,

- Summing over independant blade contributions.

Results : Suction side θ = 40◦

Slight modulation of singleairfoil results (3 dB).

Measurement,Glegg’s model,Amiet’s model.

200 1000 10000

frequency, Hz

Analytical ModelsConclusions

Three noise models have been adapted to the test case :

- Amiet’s isolated airfoil TE noise model,

- Glegg’s cascade TE noise model,

- Howe’s cascade TE noise model (not shown).

Results show that :

- Cascade effect ≃ 3 dB,

- Experiment variation close to model discrepancies,

- Cascade CPU (10min) ≫ Isolated Airfoil CPU (1 sec).

Results show that :

- Cascade effect ≃ 3 dB,

- Experiment variation close to model discrepancies,

- Cascade CPU (10min) ≫ Isolated Airfoil CPU (1 sec).

Cascade TE noise measured and may be reduced.

Cascade Noise Reduction Set-up

Outline

IntroductionCascade TBL-TE Noise Reduction

Successful isolated airfoil noise reduction

- Many passive devices tested in the flocon eu project,

- te serrations gave best results & were down-selected.

⇒ Does the noise reduction also occur in a cascade ?

Serrations sketches

- Straight edge

- Short serrationsλc = 2 mm, 2h = 13 mm

- Long serrationsλc = 2 mm, 2h = 20 mm,

Cascade Noise Reduction Acoustic Results

Outline

Cascade Noise Reduction Acoustic Results

Serration Acoustic ResultsFar field noise reduction, suction side direction θ = 40◦

100 1000 100000

straight TElong serrationsshort serrations

100m/s

Noiseincrease

Noise reduction

frequency, Hz

- Very similar results to single airfoil noise reduction,

- Single airfoil St0=1.18 ; Cascade St0=1.21.

Cascade Noise Reduction PIV results

Outline

Near Wake DynamicsMean Velocity U x

Straight Edge Serrated Edge

−10 −5 0 5 10

x location, mm

−10 −5 0 5 10

x location, mm

Observations

- Serration wake larger and less deep,

- Serrations : through flow with a 10◦ angle.

Near Wake DynamicsFluctuating velocity u

Straight Edge Serrated Edge

−10 −5 0 5 10

x location, mm

−10 −5 0 5 10

x location, mm

Observations

- BL removed from the airfoil surface,

- Smaller turbulent area in the pressure sidewake.

Cascade Noise Reduction Noise reduction mechanisms

Outline

SerrationsInvestigation of noise reduction mechanisms

Serrations may reduce noise through :

- BL ejection,

- Spanwise decorrelation,

- Local sweep angle ? → Analytical investigations.

Serrations may reduce noise through :

- BL ejection,

- Spanwise decorrelation,

- Local sweep angle ? → Analytical investigations.

Modified Amiet model for sweep angle ϕ :

Step 1 : Airfoil response function - compressible unsteady aerodynamics

−→U

airfoil

ϕ −→x

−→y

−→z

−→K

ϕ = 0◦,

ϕ = 15◦ ,

ϕ = 60◦ ,

ϕ = 85◦.

−2 −1.5 −1 −0.5 00

Modified Amiet model for sweep angle ϕ :

Step 2 : Far-field scattering - Curle’s theory

airfoilϕ

−→x

−→x ′−→y

−→z ′ −→z

ω=10,

ω=20.

0 20 40 60 8028

ϕ,◦

Conclusions :- Serration details cannot be analytically reproduced ⇒ only a hint,

- This new model could be useful for fan noise prediction,

- Decrease of noise power in cos ϕ3.

Conclusions

Cascade Experiment

Cascade rig significantly improved,

Cascade trailing edge noise measured,

Specific cascade effects observed and analysed.

Analytical models

Input data measured in cascade,

Amiet’s model give reasonable prediction ±3 dB above 200 Hz,

Glegg’s cascade model adapted to the experiment,

It differs from Amiet’s model in level by ±3 dB,⇒ need for increasing cascade effect via σ.

Conclusions

Cascade noise reduction

Serration inserted in cascade,

Reduction at low frequencies,

No influence of cascade effect on noise reduction,

Candidate mechanism to noise reduction :

BL removal,Spanwise decorrelation,Local sweep angle,

Amiet’s model modified for sweep angle.

Conclusions

Thank you for your attention !

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