Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 415

number of blades) and integer multiples of this frequency. Of lesser importance for most
configurations are mechanical noise components from operating bearings, gears, and
accessories.
An example of a spectrum of wind turbine noise is shown in Figure 7-2. These data,
which were measured 36 m downwind of a vertical-axis wind turbine (VAWT), show the
decrease of sound pressure levels with increasing frequency (a general characteristic of wind
turbines). All sound pressure levels presented in this chapter are based on root-mean-square
(RMS) values of pressure; they are referenced to 2 x 10 -5 Pa and are averaged over 30 to
180 seconds, depending on the frequency bandwidth. The spectrum generally contains
broadband random noise of aerodynamic origin, although discrete components identified as
mechanical noise from the gearbox are also evident. The blade passage frequency is readily
apparent in the time history illustrated in Figure 7-2, as is the random nature of the emitted
sounds.
Figure 7-2. Typical narrow-band noise spectrum of a wind turbine, measured 36 m
from a VAWT generating 185 kW at a wind speed of 16.5 m/s (bandwidth = 2.5 Hz)
The many analytical and experimental acoustical studies conducted on horizontal-axis
wind turbines (HAWTs) indicate that for given geometrical and operational characteristics
(such as power output, rotor area, and tip speed) HAWTs with downwind rotors (downwind
of the tower) will generate more noise than will those with upwind rotors. This is because an
additional noise source in downwind rotors is introduced when the rotating blades interact
with the aerodynamic wake of the supporting tower.
Because little information on the acoustics of VAWTs is currently available, it is dif-
ficult to directly compare the noise-generation characteristics of HAWTs and VAWTs. Ex-
ample VAWT spectra, levels, and directivity data are contained in Kelley, Hemphill, and
Sengupta [1981] and Wehrey et al. [1987]. The blades of a VAWT interact with the
aerodynamic wake of the rotor's central column in a manner similar to the way that a
downwind HAWT rotor interacts with its tower wake, but at a greater distance relative to
the column diameter. Thus, the magnitude of the noise from a VAWT caused by this
interaction is expected to be less than that of an equivalent downwind HAWT rotor and
greater then that of an upwind HAWT rotor. There is currently no detailed information
available describing other aerodynamic noise sources associated with VAWTs. Thus, to
gain an understanding of the acoustics of this type of turbine, additional studies are needed.