Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 422

Figure 7-11. Effect of yaw error on the noise spectra of a large-scale HAWT with a
downwind rotor. [Shepherd, Willshire, and Hubbard 1988]
be expected because of the reduced aerodynamic loading associated with a yaw error. At
higher frequencies yaw errors cause some small increases in the sound pressure levels.
Noise from Aileron Control Surfaces and Vortex Generators
Some experimental HAWT blades have contained ailerons for speed and power control.
Data for two different aileron configurations are given in Shepherd and Hubbard [1984].
The unusually high noise levels observed in these tests are believed to result from the
excitation of internal cavity resonances in the blades by the external flow. Well-designed
aileron systems with sealed bulkheads would not have this problem.
In some situations, small tabs or vortex generators are installed on the low-pressure
surfaces of both HAWT and VAWT blades to delay local stall and generally improve
aerodynamic performance. Studies to evaluate the effects of vortex generators on noise
radiations show these effects to be insignificant [Hubbard and Shepherd 1984].
Machinery Noises
Most of the acoustic noise associated with the wind turbines studied to date has been
aerodynamic in origin. Potential sources of mechanical noise, such as gears, bearings, and
accessories, have not been important for large-scale HAWTs. Narrow-band analyses of
noise from two large-scale HAWTs (Figures 7-4 and -5) show some cooling-fan noise and
some identifiable components at the shaft speed of the generator (30 Hz) and harmonics
of this speed. Because these mechanical components generally radiate in the crosswind
direction and are not normally heard, they are of only secondary importance.
For some of the smaller HAWTs and some VAWTs, however, gear noise can be an
important component of the total acoustic radiation. Some straightforward approaches to
controlling gear noise are to include noise and vibration limits in the design specifications
and to apply noise insulation around the gearbox.