Leading-edge vortex and aerodynamic performance scaling in a simplified vertical-axis wind turbine

Numerical analysis is conducted to investigate the aerodynamic performance and characteristics of flow around a simplified vertical-axis wind turbine (VAWT) by varying the tip-speed ratio and number of blades. The tip-speed ratios considered are lambda = R omega / U 0 = 0.8 - 2.4, and the numbers of blades are n = 2 - 5 at the Reynolds number of R e = U 0 D / nu = 80 000, where D ( = 2 R ) and omega are the turbine diameter and rotation rate, respectively, U0 is the free-stream velocity, and nu is the kinematic viscosity. The primary flow feature observed around the VAWT is the formation and evolution of leading-edge vortices (LEVs) at lower tip-speed ratios of lambda = 0.8 - 1.2, which have a notable impact on the power coefficient in the upwind region. At high tip-speed ratios ( lambda > 1.2), the LEV is not generated due to fast blade rotating speeds. Depending on the tip-speed ratio and solidity ( sigma = n c / pi D, where c represents the blade chord length), these LEVs are generated at different azimuthal angles and exhibit varying strengths. A modified tip-speed ratio, lambda ' = lambda / pi ( 1 - sigma ), proposed in the present study allows the flow structures with different lambda's and n's to exhibit similarity when they are represented with lambda '. Thus, the time-averaged power coefficient (i.e., aerodynamic performance; C over bar P W) is a function of lambda ' (rather than lambda and n) in the range of sigma = 0.2 - 0.5 considered, and its maximum occurs at lambda ' = 0.45 - 0.5 regardless of the number of blades, providing the optimal tip-speed ratio of lambda opt = gamma pi ( 1 - sigma ), where gamma = 0.45 - 0.5. Finally, we show that C over bar P W / ( sigma lambda 3 ) is a function of lambda '. Published under an exclusive license by AIP Publishing

Automated summary

Numerical analysis is conducted to investigate the aerodynamic performance and characteristics of flow around a simplified vertical-axis wind turbine (VAWT). The primary flow feature observed is the formation and evolution of leading-edge vortices (LEVs) at lower tip-speed ratios, which have a notable impact on the power coefficient. A modified tip-speed ratio allows for the representation of flow structures with different tip-speed ratios and number of blades. The time-averaged power coefficient is a function of the modified tip-speed ratio, and its maximum value occurs at a specific range of the modified tip-speed ratio.