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Tuesday, 11 March 2014
16:30 - 18:00 Wind turbine noise: How to avoid disturbing the neighbours
Resource Assessment  


Room: Ponent
Session description

With a new version of IEC 61400-11 published this year, it may be thought that wind turbine noise emission is well understood. However, wind farm developers still receive complaints from residents about 'amplitude modulation' (AM) noise, a subject not covered by the IEC standard. Amplitude modulation will therefore be one of the main topics in this session. This session will also explore different sound propagation models and the impact of environmental conditions on propagation. Another topic for this session is how to optimise wind turbines and wind farms for sites with noise restrictions.

Learning objectives

  • Understand why it is important to consider noise for resource assessment
  • Understand how a wind farm as a whole can be optimised to maximise performance with noise constraints
  • Understand what amplitude modulation is and why it is a concern for residents near wind farms
  • Understand the role that propagation modelling plays in the prediction of far field noise
Lead Session Chair:
Saskia Honhoff, GE Energy, Germany
Andreas Fischer DTU Technical University of Denmark, Denmark
Co-authors:
Andreas Fischer (1) F P Helge Madsen (1) Knud Kragh (1)
(1) DTU Technical University of Denmark, Roskilde, Denmark

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Mr. Fischer has been working in wind energy research for more than 5 years. He is currently scientist at the Institute for Wind Energy at the Technical University of Denmark DTU. He studied aerospace engineering at the University of Stuttgart. He conducted a Ph.D. project in the field of aeroacoustics and finished the project in November 2011. Since then he has worked at DTU as Post. Doc. and been involved in various projects related to wind energy. His research is focused on experimental aerodynamics, aeroacoustics, rotor aerodynamics and smart rotor systems.

Abstract

Analyses of the mechanisms of amplitude modulation of aero-acoustic wind turbine sound

Introduction

The work is aimed at exploring the mechanisms and causes of amplitude modulation (AM) of aero-acoustic sound from wind turbine rotors. The basic hypothesis for the study is that AM is linked to a combination of angle of attack (AOA) changes on the rotor blades and the directivity characteristics of trailing edge noise. We used two parallel approaches to detect possible scenarios of significant AOA variations and evaluate its effect on the emitted sound.

Approach

Aero-elastic computations were performed with the code HAWC2 and complemented by measurements of the inflow to the blade of a 2.3MW wind turbine with 5-hole Pitot tubes in the DANAERO experiment. The experiment comprised also the measurement of high frequency surface pressure fluctuations close to the trailing edge characterizing the source of TE noise. Further we evaluated data of an experiment conducted in the acoustic wind tunnel of Virginia Tech University to establish a relationship between surface pressure fluctuations and the emitted far field sound. The focus was on cases when the aerofoils were stalled.

Main body of abstract

The measurements on the full scale wind turbine exhibited in scenarios with high wind speed and pitch setting a difference in surface pressure level of up to 16dB per revolution in the frequency range below 100Hz. These considerable AOA changes were found to be due to a considerable wind shear measured in a nearby met mast.
The evaluation of the wind tunnel experiment revealed a direct relation between surface pressure fluctuations and far field sound. However, this relation is different to the one in attached flow conditions and the radiated sound is higher. We found as well that the noise increase in stall is related to aerodynamic stall behaviour. If the aerofoil has a sudden drop in lift after stalling, this goes hand in hand with a sudden increase in radiated sound.
Two different control strategies for reducing AOA variations over a revolution were investigated: individual pitch control and yaw control. Individual pitch control is promising to reduce AOA variations if a considerable shear is present, but for high turbulence levels the results were less good. The power loss when applying this control strategy was less than 1%. Yaw control is also able to reduce AOA variations when the turbine operates in shear, but a yaw error to the other side increased AOA variations significantly.

Conclusion

The conclusion of our study is that AOA variations can cause amplitude modulation. AOA variations occur for example if the turbine operates in shear. The high frequency surface pressure measurements of the DANAERO experiment revealed a high amplitude modulation. We assume that such a modulation of the sound source causes considerable modulation in the far field based on the simultaneous measurement of surface pressure and far field sound in an aero-acoustic wind tunnel. Amplitude modulation can be mitigated with individual pitch control or yaw control.


Learning objectives
Understand the mechanism of amplitude modulation of wind turbine sound
Learn the influence of rotor and blade design on amplitude modulation
Learn how amplitude modulation can be mitigated by control