Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 177

Figure 17: Instantaneous snapshot of the acoustic pressure fl uctuation fi eld (top)
and the radiated sound pressure levels (bottom).
Figure 18: Sound pressure levels emitted by two wind turbines.
wind turbine. A similar approach has been used in [31] where the acoustic sources
have been imposed in two instances to model the noise emitted by two wind tur-
bines. The isocontours of the sound pressure level are shown in Fig. 18. The fl ow
computations in [30, 31] are based on a fi nite volume approach with unstructured
sliding grids leading to very large computational efforts. Szasz and Fuchs [32] used
a fl ow solver based on the fi nite difference approach with Cartesian structured grids
to compute the acoustic sources. The solid boundaries have been accounted for using
the immersed boundary method, the acoustic solver was the same as in [30, 31]. This
approach to compute the fl ow fi eld turned out to be very effi cient and it has been
recently developed further to compute the fl ow around several wind turbines. In this
way the effect of the wind turbine wakes on the noise generation of downstream
wind turbine can be accounted for. Figure 19 shows the vortical structures around
four wind turbines visualized by the l 2 method.
6.4 Noise propagation models
As it was mentioned above, the solution of the compressible Navier-Stokes
equations includes both noise generation and propagation. For large problems,