Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 161

(a) Trailing edge noise (b) Separation stall noise

(d) Boundary-layer vortex
shedding noise

(c) Tip-vortex noise

(e) Trailing edge bluntness noise (f) Noise due to turbulent inflow

Figure 8: Noise generation mechanisms for an airfoil.

The second group includes the noise sources generated by the airfoil itself, thus
present even in a perfectly homogeneous infl ow. The trailing edge noise ( Fig. 8a )
is due to the interaction of the turbulent eddies generated in the boundary layer and
the trailing edge of the airfoil. The turbulent eddies themselves are relatively weak
sources, but their effect is amplifi ed when they are in the vicinity of a plate (in this
case the trailing edge) [8]. Separation-stall noise (Fig. 8b) occurs when the angle
of attack is too large and the fl ow separates. In such cases the separation-stall noise
dominates the other noise sources [4]. At the tip of the blade a vortex is formed due
to the pressure difference at the pressure and suction sides. The pressure fl uctua-
tions due to the presence of this vortex generate the tip vortex formation noise
( Fig. 8c ). The boundary layer vortex shedding noise (Fig. 8d) occurs when bound-
ary layer instabilities develop along the airfoil. Such instabilities might lead to
separation and the appearance of Tollmien Schlichting waves. While the trailing
edge noise has a broadband character, the boundary layer vortex shedding noise is
tonal. Trailing edge bluntness vortex shedding noise appears when the thickness of
the trailing edge exceeds a critical limit, dependent on the Reynolds number and the
shape of the airfoil (Fig. 8e). When this critical limit is exceeded a Karman-type
vortex-street develops giving rise to a tonal noise.
The third group includes noise due to atmospheric turbulence. Due to the fl uc-
tuations in the incoming air stream, the blades suffer an unsteady loading, giving