Wind Energy

0.8
0.6
0.4
0.2
−0.2
−0.4
−0.6
−0.8
0.0
0.2 0.0
−0.2
−0.8
−0.6
−0.8
−0.6
−0.4
−0.4
−0.2
−0.2
0.0
0.0
0.2
0.2
0.4
0.4
0.6
0.6
0.8
0.2
0.0
0.8
−0.2
0.8
0.6
0.4
0.2
0.0
−0.2
−0.4
−0.6
−0.8
Amplitude
1
0.1
0.01
0.001
50 Wind Turbine Noise 273
Turbulent kinetic energy spectrum
Point 9
0.0001
0.01 0.1 1
Frequency/f BPF
10 100
Fig. 50.2. Tip and hub vortices (left); Turbulent kinetic energy spectrum (right)
In Fig. 50.2 (left) can be noticed three hub vortices, twisted one over each
other and spinning in opposite direction to the blades as it can be seen also
experimentally [2].
Several vortical structures are developing behind the turbine starting
from the mast. Point 9 is positioned in the tip vortex (x/D =0; y/D =
−0.14; z/D = 0.49) and it shows a spectrum (Fig. 50.2 (right)) which is
dominated by the blade passage frequency.
50.3.2 Acoustic Computations
Several monitoring points are placed in the domain. Y 1(x/D =2; y/D =
2.5; z/D =1.3) is placed at the tip of the rotor and Y 6(x/D =2; y/D =
3.75; z/D =1.3) behind the turbine. Z1(x/D =2; y/D =2.5; z/D =0.1)
and Z6(x/D =6.5; y/D =2.5; z/D =0.1) are placed at the foot of the
mast and lateral to the turbine respectively. The acoustic density fluctuation
ρ ′ decreases in amplitude with the distance from the rotor. The spectra of
acoustic density fluctuation in Fig. 50.3 show that in the near field, the frequency
range is broader and shrinks towards smaller values with the distance
from the rotor. Except the blade passage frequency f = 1 and its harmonics,
the spectra have a peak in the high frequency part (Fig. 50.3). This peak is
shifted towards smaller values when the distance from the wind turbine is increased.
This is an expected behavior and is caused by the production of the
turbulence originating in the shear near the rotor blades which gets dissipated
with the distance from its source.
Although the instantaneous acoustic density fluctuation field has a similar
distribution from blade to blade, some differences can be noticed. This differences
(Fig. 50.3 (below left)) are caused mainly by the influence of the mast
over the flow. As seen in the same figure, the ground reflected waves interact
with the propagating waves from the wind turbine, also the waves radiated
from each neighboring turbine are interacting with each other.