Assessment of noise prediction models for long-range sound propagation of wind turbines over water

Transcription

1 Assessment of noise prediction models for long-range sound propagation of wind turbines over water Lukas Mylonas, Bahri Uzunoglu Uppsala University - Gotland Campus Cramergatan 3, Visby, Sweden Date

2 Research question Can sound prediction methods used by wind farm developers accurately calculate sound pressure levels over long distances?

3 Comparison of sound propagation Engineering methods e.g. ISO models CNPE method Accuracy (relative) Poor Very good Computing time Specific meteorological conditions Fast No Slow Yes (NPL, 2014)

4 Effects influencing outdoor sound propagation over water Wind shear Refraction Temperature gradient Temperature Atmospheric absorption Humidity Turbulence Reflection Scattering Water surface

5 (International Energy Agency, 2009) Refraction Quiet Noisy Upwind Downwind

6 Multiple reflections -6dB per 2x distance Spherical spreading -3dB per 2x distance Cylindrical spreading

7 Low-level jets Low-level jets: High wind speeds at relatively low altitude Strong reflective phenomenon (Boué, 2007)

8 Swedish prediction method Distance >1000 meters Spherical propagation on land LA= LWA log (d) ΔLa Cylindrical propagation on water LA = LWA log (d) - ΔLa + 10 log (d/1000)

9 Wide angle Crank-Nicholson Parabolic Equation (1/2) The model is based on a two dimensional Helmholtz equation: r= distance q= p r, complex sound pressure k= wave number Monopole source 2 q r q z 2 + k2 q= 0 r = 0 is the monopole source with a starting function q(0,z) Each step is an extrapolation of the previous step: q(r, z) q(r + Δr, z)

10 Wide angle Crank-Nicholson Parabolic Ground surface Equation (2/2) Acoustic impedance: Z = f σ + i8.43 f σ f= frequency, σ= effective flow resistivity Upper boundary Miki model Absorbing layer in order to avoid sound waves being reflected back Thickness of 50 wavelengths Effective Sound speed Refraction is calculated with an effective sound speed ceff: c eff = c 0 T + u T 0 u = wind speed in direction of sound propagation c0 = sound speed in the atmosphere T= temperature, T0 = average temperature (Salomons, 2001)

11 Comparison of models (1/2)

12 Comparison of models: Low level Jet profile (2/2)

13 Conclusions Refraction, multiple reflections and the resulting interference of sound waves cannot be calculated with simple engineering methods. The CNPE method can include the above effects, but is complicated and time consuming to use in practice. The CNPE method can prove useful in future to address seasonal or meteorology dependant noise disturbance especially if low frequencies are involved.

14 Thank you for your attention

15 References Boué, M. (2007). Long-range propagation over the sea with application to wind turbine noise. Swedish Energy Agency. Boulanger P., Attenborough K., Taherzadeh S., Walters-Fuller T. (1998). Ground effect over hard rough surfaces. J. acoust. Soc. Am. 104(3). pp ISO. (1996). Acoustics - Attenuation of sound during propagation outdoors - Part 2 General method of calculation. Geneve: International Organisation for Standarisation. Johansson, L. (2003). Sound propagation around offshore wind turbines - Long range parabolic equation calculations for Baltic Sea conditions. Stockholm: KTH Department of Civil and Architectural Engineering Division of Building Technology. Salomons E. M. (1998). Computational atmospheric acoustics. Kluwer Academic Publisher s. Dordrecht. Søndergaard, B. (2005). Propagation of Noise from Wind Turbines on-shore and off-shore. NWCC Technical Siting meeting. Washington 1-2 December 2005: DELTA Danish Electronics WindPRO. (2012). WindPRO 2.8 User Guide 1. edition. Aalborg: EMD International A/S. NPL. (2014). Guide to Predictive Modelling for Environmental Noise Assessment. Retrieved Februar 20, 2014, from National Physical Laboratory : International Energy Agency. (2009). Implementing Agreement for Co-operation in the Research, Development and Deployment of Wind Turbine Systems Task11. 58th IEA Topical Expert Meeting - Sound Propagation Models and Validation (p. 162). Stockholm: Vattenfall AB.