Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 180

7 Noise reduction strategies

In the previous sections it has been shown that there are a multitude of parameters
which infl uence the generation and propagation of wind turbine noise. In the fol-
lowings a short list of noise reduction strategies is given:

• Blade tower interaction . For downwind turbines, the rotor blades passing the

wake of the tower, the noise levels are signifi cantly larger than for upwind tur-
bines (see e.g. [19, 34]). For this reason, new wind turbines have upwind con-
fi gurations, even if technically there are additional issues associated with them
(like the bending of the blades because of the wind load towards the tower and
not away like in the downwind case).
As it was shown in Section 6.2., the emitted sound power levels are propor-

−

•

tional to the fi fth power of the blade speed. Thus, an effi cient way of reducing
the emitted noise levels is to decrease the angular speed of the rotor. This ap-
proach is currently employed, but it has the drawback, that it involves also
a reduction of the generated electric power, thus the effi ciency of the wind
turbine decreases.
The experiments of Oerlemans • et al. [16] showed that blade surface smooth-

ness affects the emitted noise levels, clean blades emitting considerably less
noise than the rough ones.
The shape of the • blade cross section and the angle of attack is crucial on the

emitted sound pressure levels. Large angles of attack and thick airfoils lead to
separation and vortex shedding with increased noise levels. Marsden et al. [ 35 ]
reported an 89% noise reduction from an airfoil by optimizing its cross section
to eliminate the low-frequency vortex-shedding.

• Trailing edge shape . Blake (cited in [4]) observed that an airfoil with fl at pres-

sure side and beveled suction side produces the least noise. Serrations on the
trailing edge have also the potential to reduce the emitted noise levels by break-
ing up the large-scale structures and reducing by this the low-frequency oscil-
lations. Nevertheless further research is needed to fi nd optimal confi gurations,
since discrepancies are observed between the theoretically predicted and
measured values [ 4 ].

• Trailing edge material. As it was discussed in Section 6.2, trailing edge noise is

mainly attributed to the amplifi cation of the turbulent vortex caused pressure
fl uctuations by the presence of the airfoil surface. By using porous or fl exible
materials there is a potential to reduce these refl ections and by this also the
emitted noise levels (see e.g. Hayden, cited in [4]).
Several experimental measurements have been carried out to evaluate the •

infl uence of blade tip shape on the emitted noise levels but no profi les could
be found which consistently reduced the noise levels for each measurement
condition.

• Masking . Bolin [13] has shown that masking the wind turbine noise by adding

'positive' noise from natural sources (trees, waves) can be an effi cient way for
the increase of the acceptance of wind turbines, especially in quiet areas, where