Flow separation impacts on the hydrodynamic performance analysis of a marine current turbine using CFD

Although there are different strategies to control the operation of marine turbines, the so-called ‘stall-regulated strategy’ is one of the most widely used and mature control strategies. Since the stall phenomenon is closely related to flow separation around the turbine blades the treatment of this separation requires great care during the design and performance analysis of turbines when using computational fluid dynamics (CFD). This study investigates appropriate methodologies and approaches to simulate the hydrodynamic performance of horizontal marine turbines with a specific emphasis on the flow separation phenomena. The well-known viscous flow solver ANSYS-CFX was employed as the main CFD code to predict the power extraction coefficient of these turbines. The investigations were carried out by using both numerical and experimental methods applied on tidal stream turbine models tested in the Emerson Cavitation Tunnel of Newcastle University, UK and the circulating water channel of Harbin Institute of Technology, China. The measured power extraction coefficients generally agreed well with the numerically predicted ones except for one of the models with the lower pitch angle which displayed large discrepancies over the entire operating range. The detailed flow analyses from the CFD studies with this turbine and other model at higher pitch angles revealed that large-scale detached vortices developed downstream of the model with the lower pitch angle may have contributed to this large discrepancy. The study therefore draws attention to the importance of the combined use of the CFD and model test-based approaches in the design and performance analysis of marine turbines.