Performance predictions of a straight-bladed vertical axis turbine using double-multiple streamtube and computational fluid dynamics models

Vertical axis turbines are increasingly being utilized to generate power from our oceans. However, due to high levels of dynamic stall and flow interaction effects of the blades, struts, hubs and shaft, they exhibit complex hydrodynamic flows which need to be fully understood to increase turbine output, service life and efficiency. Double-Multiple Streamtube (DMS) and Computational Fluid Dynamics (CFD) models were used to numerically investigate the power generated and hydrodynamic properties of these turbines. Three-dimensional (3D) transient CFD simulations were performed using an Unsteady Reynolds Averaged Navier-Strokes (URANS) solver. THe DMS model developed incorporated a new correction factor to account for strut drag effects. All simulations were validated against Experimental Fluid Dynamics (EFD) testing of a three-bladed turbine at the Australian Maritime College Circulating Water Channel. The DMS model with a newly developed correction factor for strut drag demonstrated good agreement with the CFD and EFD results for turbine power predictions across the operational tip speed ratio (λ) range. The 3D CFD model of the turbine geometry including struts, hubs and shaft also provided good agreement with EFD results for turbine power. THe 3D CFD model without struts, hubs and shaft, and the DMS model without strut correction factors overpredicted turbine performance especially at high λ, as the resistive torque generated by the struts, which reduces power, was not accounted for. All simulation results demonstrate that strut drag, and the associated resistive torque, must be modelled if accurate simulations of vertical axis turbine performance are to be obtained.