A numerical investigation into the aerodynamic effects of tubercles in wind turbine blades
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Wind turbine performance is clearly affected by complicated environmental effects such as atmospheric turbulence, ground boundary layer, and variation of free-stream wind direction and amplitude. Since the main goal of a wind turbine is energy production, the irregular nature of the wind is considered the main obstacle to a constant power output. Sinusoidal modifications (i.e., tubercles) placed on the leading edge of wind turbine blades seem to be a promising solution to this problem, since they generate vortices able to delay flow separation and improve the aerodynamic performance in the post-stall regime. The main objective of the present study is to give insights into the application of tubercles applied on the leading edge of wind turbine blades, specifically the NREL Phase VI wind turbine, such that performance enhancement can be achieved. Tubercles are sinusoidal bumps located at the leading edge of humpback whale flippers, which are able to improve flow attachment by acting like flow control devices similar to vortex generators. This discovery was the starting point for the development of several projects in the application of tubercles in different areas. In the present work, tubercles have been applied to the NREL Phase VI wind turbine blade to study their effects on blade aerodynamics and wind turbine performance. In particular, tubercle effects on shaft torque and annual energy production (AEP) have been analyzed; more specifically, tubercle amplitude, wavelength, and spanwise location have been considered as design variables. Moreover, since the physical phenomenon behind tubercles is still not fully clear, a physical analysis has been conducted to understand their working principles and to compare the new findings with previous works. Since past research on wind turbine application considers random values of tubercle geometric parameters (amplitude and wavelength), in the present work a more systematic study has been made by using a design of experiments (DoE) for the generation of tubercle configurations to test by a three-dimensional computational fluid dynamics (CFD) analysis. In particular, the thesis research has been developed in three main phases. Firstly, amplitude and wavelength have been considered as two design variables for a Latin hypercube DoE, and 20 blades have been generated. Then, since it has been observed that tubercles on whale flippers are unevenly distributed and placed closer to the tip, only the tubercle spanwise location has been varied, keeping fixed amplitude and wavelength. Finally, all three design variables listed above (i.e., amplitude, wavelength, and spanwise location) have been considered together in a 57-case hybrid DoE (Latin hypercube + full factorial). All the blade geometries have been simulated by a CFD analysis, which was embedded in a high-performance computing simulation framework made of a geometry creation code, a mesher, and a CFD solver. Results in terms of shaft torque and AEP have been compared with the baseline turbine underlying the importance of tubercles especially in the off-design conditions, when the blade is fully stalled and characterized by a strong spanwise flow, which is partially blocked by the streamwise vortices generated by tubercles. The CFD results have been also used as training points for a surrogate model generation, which helps to identify the regions in the design space where the performance improvement is relevant. In particular, tubercles seem to be beneficial in the design condition when they are placed closer to the blade tip; in the off-design regime, they can be extended over the entire blade with particular attention to the second half, which is the most influential in the power generation. Values of tubercle amplitude and wavelength that positively affect the performance have been also identified in a limited region of the design space, which varies depending on the wind speed considered.