Computational analysis of ducted wind turbines noise

Abstract

As the suitable locations for large wind farms in the mainland are limited, wind energy generation in the urban environment is gaining interest. The Diffuser Augmented Wind Turbine (DAWT) is a promising concept where the turbine is embedded into an airfoil-shaped diffuser (also named shroud or duct), which has the function of increasing the mass flow across the rotor. Such device is
designed to overcome the low wind speed present in urban locations.
Noise regulations represent a limit to the installation of wind turbines in urban areas. To the author’s knowledge, the far-field noise generated by DAWTs has been investigated only by Hashem et al. [31], who reported noise increase up to 20 dB compared to unaugmented turbines. However, the noise generation mechanisms are not clear yet. Furthermore, only partial data [15] on the
fluid-dynamic behaviour of DAWT when operating in yawed inflow conditions are present, with no information on the noise pattern.
The current study investigates both the fluid-dynamic and acoustic fields for a ducted wind turbine using a computational approach. The commercial DonQi turbine is taken as reference. The fluid-dynamic flow field is computed using the Lattice-Boltzmann Method solver Exa PowerFLOW. The Ffowcs-Williams and Hawkings (FW-H) analogy is employed to compute the far-field noise.
Three cases are investigated: the ducted turbine at 0 deg and 7.5 deg yaw angles and the unducted turbine at 0 deg yaw angle.
The fluid-dynamic analysis reveals that the presence of the diffuser accelerates the flow in the tip region, resulting in a significant increase of the thrust and the power produced by the turbine. In correspondence, the tip vortices present a higher intensity. These vortices interact with the boundary layer of the diffuser and form long vortical structures convected beyond the diffuser trailing
edge. The presence of a yaw angle creates a non-axisymmetrical velocity pattern at the rotor disk, resulting in a power drop of 10.8%. However, no stall on the blades is detected. Contrarily to the observations by Cresswell et al., a separation region is formed on the diffuser suction side.
Regarding the far-field noise, the directivity pattern for the unducted turbine presents a zone of low noise above the rotor. The addition of a diffuser causes a noise increase by approximately 5 dB upwind and downwind and by up to 15 dB in the low noise region, resulting in a more uniform noise distribution. This effect is ascribed to the higher flow speed in the tip region and to the diffraction
of acoustic waves by the diffuser. Increasing the yaw angle to 7.5± is found not to have a relevant impact on the far-field noise.