Experimental Validation ofTNO Trailing Edge Noise Modeland Application to Airfoil Optimization

Franck Bertagnolio, Helge Aa. Madsen, and Christian Bak

Aero-Elastic Design, Wind Energy DivisionRisø DTU, National Laboratory for Sustainable EnergyRoskilde, Denmark

EWEC 2009, Marseille, 16-19 March 20092 Risø-DTU, Technical University of Denmark

Outline

• Trailing Edge Noise

• TNO Trailing Edge Noise Model

• Validation against Measurements

• Aeroacoustic Optimizationof a Wind Turbine Airfoil

EWEC 2009, Marseille, 16-19 March 20093 Risø-DTU, Technical University of Denmark

Trailing Edge Noise

2y3y

1y

1 2( )U y

( , )P k

1 2 3, ,u u u Far field sound ( )PS

EWEC 2009, Marseille, 16-19 March 20094 Risø-DTU, Technical University of Denmark

TNO Trailing Edge Noise Model

2

22 21 10 2 2 2 22 1 22 2 0

1 3 2

( , ) 4 ( ) ( , ) e dkym c

k UP L y u U k y

k k y

k k

Parchen (1998) combines a diffraction problem solution with knowledge of the turbulent fluctuations in the boundary layer

• Airfoil Surface Pressure Spectrum (Blake,1986) Lighthill analogy in spectral domain

Solution for the Mean shear-Turbulence interaction:

• Far Field Noise (Ffwocs Williams and Hall, 1970 ; Chandiramani, 1974; Chase, 1975; Howe, 1978; Brooks and Hodgson, 1981)

1 120 1

( ) ( , )d4span

P

LS P k k

R c k

EWEC 2009, Marseille, 16-19 March 20095 Risø-DTU, Technical University of Denmark

Model Practical Implementation

Input data originates either from XFOIL or EllipSys2D

Boundary layer quantities required as input are:

• Directly obtained from the codes (U1 (y2), Shear, kt (CFD), …)

• Classical turbulence theory results for 22 (Isotropic

turbulence spectrum, Von Karman), m (Gaussian), …

• Integral length scale: (Lutz et al, 2007)

3/ 2t

2 2( ) 0.387k

L y

EWEC 2009, Marseille, 16-19 March 20096 Risø-DTU, Technical University of Denmark

Validation: LM Glasfiber Wind Tunnel

Aerodynamic Test Facility

NACA0015 Airfoil Section

Surface Pressure Measurement Holes

EWEC 2009, Marseille, 16-19 March 20097 Risø-DTU, Technical University of Denmark

NACA0015: Surface Pressure Spectrum

Re=1.6x10^6 - No Turbulence Grid - x/C = 0.567=0o =4o

=8o =12o

EWEC 2009, Marseille, 16-19 March 20098 Risø-DTU, Technical University of Denmark

Validation: NACA0012 [Brooks and Hodgson]Anaechoic Wind Tunnel Facility at NASA Langley (1981)Re=1.6M, 2.9M – Aoa=0o

Surface Pressure

Far Field SPL

EWEC 2009, Marseille, 16-19 March 20099 Risø-DTU, Technical University of Denmark

Discussion on Validation

• Quantitative model results might be erroneous (Difference in conventions?? )

• but it may be that TNO model fails to accurately predict measurements(However, no fundamental differences between experiments NACA0015-LM / NACA0012-NASA…??)

• TNO model correctly captures tendencies observed

in measurements

• TNO model (using XFOIL) is a good candidate for aeroacoustic optimization

EWEC 2009, Marseille, 16-19 March 200910 Risø-DTU, Technical University of Denmark

Airfoil Optimization

• Goal: Reduce trailing edge noise

• AirfoilOpt: SIMPLEX algorithm (gradient based method) Cost function minimization subject to non-linear constraints Both cost function and constraints can involve:

1) Aerodynamic characteristics (XFOIL)2) Geometric characteristics

TNO model implemented in the code

• Noise Optimization Procedure: Maximum SPL from TNO model used as cost function Various constraints to preserve aerodynamic and geometrical

characteristics of original airfoil (PARAMETER STUDY, see paper)

EWEC 2009, Marseille, 16-19 March 200911 Risø-DTU, Technical University of Denmark

Relaxing Geometric Constraints

Initial airfoil: RISØ-B1-18Constraints relaxation: - Preserve all constraints - ymin and ymax along chord

- y,xx (Airfoil curvature)

EWEC 2009, Marseille, 16-19 March 200912 Risø-DTU, Technical University of Denmark

Relaxing Geometric Constraints

Far Field SPL A-Weighted SPL

EWEC 2009, Marseille, 16-19 March 200913 Risø-DTU, Technical University of Denmark

Mechanism Behind SPL Reduction Boundary Layer Development along Chord

Turbulent Kinetic Energy Profile

Near TrailingEdge

Original

Optimized

Original Optimized

EWEC 2009, Marseille, 16-19 March 200914 Risø-DTU, Technical University of Denmark

Conclusions

• TNO model validation gave mixed results (Convention problem / Model error ???)

• Airfoil noise optimization: SPL ~ -1 to 2dB

• Noise reduction reached through:reducing camber flattening of suction side

Reduction of TKE at trailing edge