Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 426

Broadband Noise Components
Extensive research on propellers, helicopter rotors, compressors, and isolated airfoils
has provided a wealth of background information and experience for predicting broadband
noise for wind turbine rotors. The main noise sources have been identified, prediction
techniques have been described, and comparisons have been made with available
experimental data [George and Chou 1984; Glegg et al . 1987; Grosveld 1985]. Measurements
to date indicate three main sources of broadband noise are as follows:
-- aerodynamic loading fluctuations caused by inflow turbulence interacting
with the rotating blades
-- turbulent boundary-layer flow over the airfoil surface interacting with the
blade trailing edge
-- vortex shedding caused by the bluntness of the trailing edge
These sources of broadband noise are illustrated in Figure 7-14, along with their sound
power dependencies, definitions of critical dimensions, and flow velocities [Grosveld
1985].
Figure 7-14. Sources of wind turbine broadband noise. [Grosveld 1985]
Another possible source of broadband noise is that of tip vortex formation. Based on
the experimental data of isolated airfoils and rotors [George and Chou 1984; Brooks and
Marcolini 1986], this source is expected to be of secondary importance relative to the three
listed. However, unusual geometries, such as those associated with some tip brakes and
deflected tip control surfaces, could result in significantly more radiated noise.
Inflow Turbulence Noise
As the wind turbine blades move through the air, they encounter atmospheric turbulence
that causes variations in the local angle of attack, which in turn causes fluctuations in the
lift and drag forces. The length scales and intensities are a function of local atmospheric
and site conditions and are different at different heights above the ground [Kelley et al.