Wind Turbine Amplitude Modulation: Research to Improve Understanding as to its Cause & Effect

1. Wind Turbine Amplitude Modulation: Research to Improve Understanding as to its Cause & Effect Project overview and conclusions

2. Project elements G Blade surface measurements RISO Helge Madsen – DTU (formerly RISOE) Denmark Sabine von Hunnerbein – ARC University of Salford

3. Source modelling - Blade Swish Turbulent boundary layer Trailing edge Wake Airfoil • Stefan Oerlemans NLR

4. Source modelling - Blade Swish

5. Source modelling - Blade Swish

6. Source modelling - Blade Swish crosswind upwind/downwind 5 dB(A) 20 seconds Amplitude Modulated (AM) aerodynamic noise • Stefan Oerlemans NLR

7. Caused by the directivity and convective amplification of the dominant trailing edge noise mechanism: mid-high frequency noise (400-1000 Hz) peaks when the blade is moving towards the observer close to turbine dominant underdownwards sweep of the blade at large distances (>3RD) only occurs incross-winddirections maximum predicted variation~5 dB peak-to-trough Blade Swish Characteristics

8. Literature review (historical) UK • ETSU-R-97 • ETSU WTN source study • DTI/HMP LFN study • Salford AM study • Bowdler review non-UK • van den Berg (NL) • Di Napoli (Finland) • Legarth • Lundmark (Sweden) • Toora Wind Farm (Australia) • Waubra Wind Farm (Australia) • West Wind (NZ) • Lee et al (South Korea) • internet sources (USA, Canada, etc.) • AM was not just a UK issue • reports indicated something other than ‘normal’ blade swish

9. A further definition of AM ….. Confusion ‘Other’ AM ‘Normal’ AM • commonly termed ‘blade swish’ • part of normal WTN • ~5dB modulation at source • dominant crosswind effect • decreases away from source • dominated by mid frequencies (400Hz to 1000Hz) ‘swish’ • source mechanism understood • atypical, intermittent • >5dB (>10dB) amplitude at times? • audible/noticeable at large distances downwind to >1km? • more impulsive ‘thump’ • additional lower frequency content (200 Hz to 500 Hz)? ‘whooomp’ • source mechanism?

10. What is OAM? • for the purpose of the present project OAM is objectively defined as any AM whose characteristics can not be described by the known source generation mechanisms of NAM (blade swish) • potential issue that this definition precludes propagation (as opposed to source) effects

11. Possible origin of OAM (Oerlemans) Large-scale separation (deep stall) Turbulent boundary layer Boundary layer separation Trailing edge Airfoil Airfoil Wake Airfoil

12. Possible origin of OAM (Oerlemans) fixed pitch and rpm, different inflow wind speeds

13. Possible origin of OAM (Oerlemans) a=0.3 a=0.3 a=0.6 • variable inflow conditions across the rotor could lead tolocalised stall on some portions of the blade, • medium/high wind shear conditions can lead to such inflow conditions, as can other factors … • occurrence will also be dependent on blade design and control logic yaw wake topography inflow turbulence

14. New measurements SITE A: measurements at residential properties at which OAM had been reported SITE B: detailed measurements around turbines operating ‘normally’ at a site where OAM had been reported SITE C: detailed measurements around a turbine at a site with access granted to modify the control system settings (including blade pitch to induce stall)

15. Sample new measurements near field Down-wind Cross-wind far field • directivity of OAM in near-field and at 10 Rotor Diameters (~900m) • practical observations match refined Oerleman’s model

16. Additional measurements 10 dB(A) 20 seconds • SPL measured in far field of wind turbine at ~ 1000 m • OAM only identifiable for one (differently pitched) blade

17. Detecting and measuring AM Any successful metric must: • be objective • be repeatable • be robust (avoid false positives and false negatives) • work on real and not just simulated AM noise • ideally allow automated application • be relatable to subjective response

18. Examples of existing AM methods A range of methods have been proposed for measuring amplitude modulated wind farm noise: • direct time domain analysis of instantaneous Leqlevels • frequency domain analysis of the Leq levels • Fourier transform of the spectrogram Methods 2 and 3 are variations of existing techniques used in sonar and condition monitoring Methods 2 and 3 rely on the periodicity of the noise data at the BPF as a fundamental part of the detection process

19. Objective quantification of AM A metric has been identified to quantify the level of AM (NAM or OAM) at BPF present in a sample of acoustic data Sample analysis over a 3 hour period

20. Riso studies – DAN-AERO MW

21. Proof that OAM is a source effect?

22. Proof that OAM is a source effect?

23. Proof that OAM is a source effect?

24. Proof that OAM is a source effect?

25. Surface pressure on suction side

26. Surface pressure on suction side

27. Listening tests on AM • Reports in the published literature ….. • WTN more annoying than other environmental noise • Speculation in literature • Sound characteristics to blame? • Response by industry and government • Need for dose-response relation

28. Stimuli overview

29. Setup: user interface

30. ‘Overall’ absolute annoyance ratings

31. ‘AM’ absolute annoyance ratings

32. Adaptive rating – not normalised

33. Normalising adaptive rating levels 41.8 – 40 = 1.8 reference – test = normalised

34. Adaptive rating – normalised (1)

35. Adaptive rating – normalised (2)

36. Subjective response findings • Consistent ratings between participants • Average annoyance from AM signals higher than from non-modulated noise, but …. • Strongest effect of overall level on annoyance • No clear effects with increasing • modulation depth above a certain threshold • type of modulation (see report) • addition of garden noise (see report) • use of different metrics (see report)

37. Causes of AM • The principal causal source mechanism of OAM identified as partial, transient blade stall • No evidence to suggest that OAM should occur in the far field as a result of propagation effects only in the absence of a source mechanism • Potential speculative causal factors which have been suggested to date shown to have little or no systematic association to the occurrence of OAM:for example interaction between closely spaced turbines in linear arrays, large rotor to tower height ratios, etc.. • But some of these may represent potential contributory factors.

38. Can OAM be mitigated? • Partial stall as a source of OAM could be efficiently mitigated by avoiding blade stall. • This could be achieved through a number of potential solutions to be developed and tested, including: • a. Software fixes, e.g. modifying the logic of the turbine control system • b. Physical changes including innovative blade designs or cyclical pitch control • Mitigation is oftenlikely to only be required in down-wind conditions, but …. • Need to be aware of possible overlap between OAM and NAM

39. Conclusions • AM can take at least two forms which appear to have fundamentally different source generation mechanisms • the different source mechanisms result in different radiation characteristics between NAM (‘normal’ blade swish) and OAM • there is no evidence to suggest that OAM should occur in the far field in the absence of its presence at source, but …. • measurements undertaken close to the source (e.g. at IEC61400-11 compliant microphone locations) may not properly characterise AM • NAM is an inherent feature of wind turbine noise • OAM only occurs dependent on a number of interacting factors • robust objective detection and quantification methods for AM are possible