Full-scale vs. scaled aerodynamics of 5-MW offshore VAWTs under pitch motion: A numerical analysis

Earlier studies have shown that the pitch motion of the platform can enhance the power coefficient of offshore vertical axis wind turbine (VAWT), particularly in the realm of scaled model. However, there is a shortage of research employing computational fluid dynamics (CFD) to evaluate how that motion affects the aerodynamic performance of full-scale VAWT. Moreover, the differences in aerodynamic characteristics between full-scale and scaled VAWTs are not well-understood. To fill this knowledge gap, we utilize high-fidelity CFD simulations to investigate the aerodynamic performance of a 5.3-MW full-scale offshore H-type VAWT in the presence of pitch motion. Subsequently, we conduct a comparative analysis of the variations between the full-scale VAWT and its scaled counterpart. Furthermore, we explore the influence of pitch amplitude and period on the overall performance. The findings reveal that, with low solidity (i.e., 0.1) and moderate tip speed ratio (i.e., 3.5), periodic pitch motion results in a 16.42% improvement in the power coefficient for the full-scale VAWT. In contrast, the scaled counterpart experiences a significant 56.71% reduction in the power coefficient. Meanwhile, for the full-scale VAWT, selecting a larger pitch amplitude and shorter pitch period holds the potential to enhance the power coefficient; however, the durability concerns stemming from the extensive fluctuations in blade torque need to be addressed. Additionally, it is observed that pitch motion introduces turbulent structures into the wake, facilitating both expansion and recovery. Due to the scale effect (Reynolds number effect), it's crucial to recognize that conclusions drawn about the aerodynamic performance of full-scale VAWT cannot be directly extrapolated from scaled model.