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

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

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

Source modelling - Blade Swish

Source modelling - Blade Swish

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

Blade Swish Characteristics Caused by the directivity and convective amplification of the dominant trailing edge noise mechanism: –mid-high frequency noise ( Hz) –peaks when the blade is moving towards the observer –close to turbine dominant under downwards sweep of the blade –at large distances (>3RD) only occurs in cross-wind directions –maximum predicted variation ~5 dB peak-to-trough

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

A further definition of 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 ‘Normal’ AM ‘Other’ AM 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? Confusion

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

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

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

variable inflow conditions across the rotor could lead to localised 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  =0.3 Possible origin of OAM (Oerlemans)  =0.6  =0.3

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)

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

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

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

Examples of existing AM methods A range of methods have been proposed for measuring amplitude modulated wind farm noise: 1.direct time domain analysis of instantaneous L eq levels 2.frequency domain analysis of the L eq levels 3.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

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

Riso studies – DAN-AERO MW

Proof that OAM is a source effect?

Proof that OAM is a source effect?

Proof that OAM is a source effect?

Proof that OAM is a source effect?

Surface pressure on suction side

Surface pressure on suction side

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

Stimuli overview Parameter Modulation depth, dB(A)0, 2, 3, 4, 5, 6, 9, 12 Sound level of total stimulus L Aeq, dB(A) 25, 30, 35, 40 and 45

Setup: user interface

‘Overall’ absolute annoyance ratings

‘AM’ absolute annoyance ratings

Adaptive rating – not normalised

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

Adaptive rating – normalised (1)

Adaptive rating – normalised (2)

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)

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.

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 often likely to only be required in down-wind conditions, but …. Need to be aware of possible overlap between OAM and NAM

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 IEC 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