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Associated Benefits

According to the simulations conducted by BST, the overall performance of wind turbines is increased due to applying riblets. They reduce turbulence and improve the performance of the blades by reducing the fluctuation velocity and the boundary layer thickness.

Two main problems with wind turbines are energy losses and increasing noise with rising wind speed. When modifying the blade surface with riblets, they not only counteract those problems by increasing power performance and reducing the noise, but also stable aerodynamics, increase lift, reduce drag and have a positive influence on the stall behaviour.

Since a wind turbine sees not only a variety of wind speeds but also different angles of attack, all considered riblet configurations need to be evaluated for different AoA not only in simulation, but also in a low speed wind tunnel for validation.

In Figure 2, the drag polars of different riblet configurations are being displayed. While they all show a similar curve, the configuration II-V01 (blue curve) seems to display the best results with the highest lift coefficient for the corresponding drag coefficient. The lift coefficients over drag coefficient change curves display a similar result (see Figure 3).
Figure 1 Scheme of the different flow conditions on a wind turbine combined with the possible riblet application areas
Figure 2: Drag polars for different riblet configurations
Figure 3: Lift value over drag change for different riblet configurations
Riblets not only reduce the boundary layer thickness, but also the velocity fluctuations. Additonally they can change the boundary layer profile. All this effects increase the glide ratio, and therefore lead up to 2% performance improvement over the annual power production. Also a more stable aerodynamic behaviour is to be expected through a stall delay due to delayed separation. This leads to less structural stimulation and an increase of the annual power production of up to 5%. However, local dynamic stalling is still present due to fast changing AoA due to turbulence or tower interaction.

The last additional benefit lies in a noise reduction. The primary noise production is of aerodynamic nature, leading to a couple of main sources: In the following picture, the influence due to turbulent boundary layer thickness and trailing edge vortex shedding and their possible noise reduction are highlighted. Commonly, the rotational speed and AoA are lowered as well as the blade pitch at certain wind speeds to lower sound pressure levels.
Figure 4:  Different noise sources on a wind turbine with the possible reduction due to riblets
In the next picture, the potential improvements of sound pressure level due to stalling, the tip vortex and the rotor after wake are shown. The real sound pressure level will overall be decreased by 5 dB.
Figure 5: Different noise sources on a wind turbine with the possible reduction due to riblets
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