The hydrodynamic and structural loading characteristics of straight and helical-bladed vertical axis tidal and current flow turbines

Vertical axis cross flow tidal and current flow turbines are proposed to generate electrical energy from the ocean's kinetic energy, however little is known of their performance and loading characteristics. In this work, fixed pitch straight and helical-bladed vertical axis turbines were investigated using numerical simulation models to perform examinations of the influence of geometrical layout and rotational rate on power output, torque fluctuations, mounting forces, structural stress, and deflection magnitudes. These studies were conducted to establish the relative merits of various turbine configurations, including variations in strut section, strut-blade mounting tab design, strut location, as well as varying degrees of blade helicity and section angle inclination. To establish performance and loading parameters, transient Computational Fluid Dynamics (CFD) and Double Multiple Streamtube (DMS) hydrodynamic simulation models were developed, which were then coupled with structural models using beam theory and Finite Element Analysis (FEA) techniques. Extensive verification and validation of the hydrodynamic results against Experimental Fluid Dynamics (EFD) results from literature for turbines of differing geometrical layouts was performed to ensure simulation accuracy. Simulation results show that numerical models can accurately simulate performance when compared to EFD results if suitable modelling techniques are utilised. The results show that significant differences in performance characteristics such as power output and mount- ing force magnitudes occur for turbines with differing geometrical layouts. Straight-bladed turbines were found to generate higher power output, torque fluctuation levels, mounting forces, structural stress, and defection magnitudes than helical turbines of the same frontal area, as a result of the blade inclination of the helical turbine to the inflow. The influence of strut design on power output was also significant, with low-drag struts located at the blade tips generating the highest power output when compared to turbines with high-drag struts located at the quarter-span location. For both straight and helical turbines the highest stress magnitudes were found at the blade-strut joints. Overall these results demonstrate that straight-bladed turbines are better suited for harnessing tidal and current flow ocean energy than helical-bladed turbines, as they generate higher power outputs whilst not incurring any significantly adverse structural penalties. These results are significant as previous numerical simulation and EFD works have concentrated on each configuration individually, with little known of their respective merits. The numerical models developed as part of this work are capable of accurately capturing the complex behaviour of vertical axis turbines for differing geometrical layouts, allowing for future design investigations to be conducted without the need for EFD. The development of these models, and the inclusion of suggested simulation guidelines in this work, has created a useful design toolbox for future use that is suitable for turbine optimisation studies as well as coupling with fatigue evaluations to ensure turbine longevity.