Experimental Validation of TNO Trailing Edge Noise Model and Application to Airfoil Optimization

1. Experimental Validation ofTNO Trailing Edge Noise Modeland Application to Airfoil Optimization Franck Bertagnolio, Helge Aa. Madsen, and Christian Bak Aero-Elastic Design, Wind Energy Division Risø DTU, National Laboratory for Sustainable Energy Roskilde, Denmark

2. Outline • Trailing Edge Noise • TNO Trailing Edge Noise Model • Validation against Measurements • Aeroacoustic Optimization • of a Wind Turbine Airfoil

3. Far field sound Trailing Edge Noise

4. TNO Trailing Edge Noise Model • 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)

5. 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 F22 (Isotropic turbulence spectrum, Von Karman), Fm (Gaussian), … • Integral length scale: (Lutz et al, 2007)

6. Validation: LM Glasfiber Wind Tunnel • Aerodynamic Test Facility • NACA0015 Airfoil Section • Surface Pressure Measurement Holes

7. NACA0015: Surface Pressure Spectrum Re=1.6x10^6 - No Turbulence Grid - x/C = 0.567 a=0o a=4o a=8o a=12o

8. 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

9. 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

10. 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)

11. Relaxing Geometric Constraints Initial airfoil: RISØ-B1-18 Constraints relaxation: - Preserve all constraints - ymin and ymax along chord - y,xx (Airfoil curvature)

12. Relaxing Geometric Constraints Far Field SPL A-Weighted SPL

13. Mechanism Behind SPL Reduction Boundary Layer Development along Chord Original Optimized Turbulent Kinetic Energy Profile Original Optimized Near Trailing Edge

14. 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