| Summary: | The objectives of this thesis were (i) to investigate the main sources and paths of noise on modern utility
size wind turbines; (ii) to explore methods of reducing the noise; (iii) to assess our current ability to
accurately predict and measure wind turbine noise. The accomplishment of these objectives would enable
quieter wind turbines to be developed and allow them to be located near residential dwellings with greater
confidence that the noise would not be a nuisance.
A comprehensive review of the current literature was carried out and the findings were used as a basis for
the investigative work conducted. It was found that wind turbine noise could be classed as either
aerodynamically produced noise or mechanically produced noise. Aerodynamically produced noise on
wind turbines arises mainly from the interaction of the flow over the blade with the surrounding air.
Mechanically produced noise arises from a number of sources such as the gearbox, generator and
hydraulic pumps. The noise can be radiated directly from the noisy component (airborne) and / or
transferred through the structure of the turbine and radiated elsewhere (structure-borne) such as the tower.
The prototype Windflow 500 wind turbine near to Christchurch was used for the majority of the
investigative work carried out, and to assess the predictions made. The main radiators of noise from the
turbine were identified as the blades (86 – 90% of the total sound power), the tower (initially 8 – 12% but
later reduced to ~4% of the total sound power), and the nacelle cladding (1% of the total sound power).
A prominent tone in the sound power spectrum from the turbine was observed in the 315 Hz 1/3 octave
band. This was shown to be predominantly caused by gear meshing in the second stage of the gearbox at
311 Hz. The presence of the tone was significant because under commonly used standards a tonal penalty
would be applied to the measured sound pressure level from the turbine to account for the extra
annoyance caused by the tone. This in turn would mean that any potential wind farms would need to be
sited further from residential dwellings than would otherwise be necessary in order to comply with noise
regulations. Investigations were carried out that addressed the noise radiated from each of the main contributors
outlined above. The sound power level radiated from the tower was found to be effectively reduced by
attaching rubber tiles at strategic locations inside the tower. Noise radiated from the nacelle was reduced
with a combination of acoustic insulation and acoustic absorption inside the nacelle. An investigation
into the gearbox noise was also carried out. Attempts to reduce the tonal noise caused by gear meshing
were made with little success but the investigation provided a good basis upon which to conduct further
work.
Preliminary investigations into both structure-borne and aerodynamically generated blade noise were
carried out. The structure-borne blade noise investigation showed that the blades readily vibrated at a
range of frequencies, the result being that structurally transmitted noise radiated from the blades was
likely to be present at high levels. Research showed that the structure-borne noise radiated from the
blades could be significantly reduced by partially filling the internal cavity of the blades with foam. The
investigation of aerodynamically produced noise was carried out on a section of Windflow 500 blade in
the low noise wind tunnel at the University of Canterbury. The tests showed that the blade generated
noise at a range of frequencies including those in the 315 Hz 1/3 octave band. This suggested that the
tonal noise measured from the blades was not only due to structurally transmitted noise from the gearbox
but was also contributed to by aerodynamically produced noise. It was found that the noise from the
blade section could be reduced by up to 4.5 dB at certain frequencies by attaching serrated strips to the
trailing edge of the aerofoil.
Empirical equations for prediction of wind turbine sound power levels were evaluated and found to be in
good agreement with measured data. It was found that accurate spectral predictions of the sound power
level were much more difficult. However given spectral data for a turbine, it was found that accurate
predictions of the noise propagation from the turbine could be made, taking into account meteorological
effects and the effect of complex topography. It was found that the CONCAWE propagation model was
well suited to the prediction of noise propagation from wind turbines because of its superior handling of
meteorological effects. In an investigation carried out which modelled the Gebbies Pass site of the Windflow 500 it was found that the CONCAWE model could predict sound pressure levels from the
turbine to within 2 dB at distances of up to 1400 m.
Further work in the area of wind turbine noise should be focused on the reduction of blade noise. This is
especially relevant to the Windflow 500 since blade noise was found to be by far the largest contributor to
total noise radiated from the turbine. Acoustic treatments elsewhere would therefore produce only small
reductions in the total sound power emitted by the turbine.
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