Power augmentation of Darrieus-type turbines by means of novel solutions and multi-fidelity simulations

Vertical-axis wind turbines (VAWTs) represent a valuable alternative to horizontal- axis ones for non-conventional installations like highly turbulent environments and off- shore floating. Due to their inherently complex aerodynamics, nonetheless, characterized by a continuous oscillation of the angle of attack on the blade, often above the static stall limit, their development has fallen behind. The nature of this technological gap is twofold. On the one hand, new strategies for increasing their performance and stabilizing their operation must be developed. In this perspective, one can work both on improving the airfoils' performance with passive flow control devices and on controlling the angle of attack (e.g., with active blade pitching). On the other hand, analysis tools with a fidelity higher than the ubiquitous Blade Element Momentum (BEM) method are needed. Blade- resolved Computational Fluid Dynamics (CFD) has shown its potential for this application, but its elevated computational cost makes it suitable only for analyses of few cases. In between the two, interest is being devoted at developing hybrid approaches, able to conjugate the accuracy of CFD and the calculation cost reduction coming from a lumped-parameter modeling of airfoil aerodynamics. Among the different methodologies available, the Actuator Line Method (ALM) is particularly promising. Several shortcomings of this approach have not been solved by the scientific community yet, in particular: the spreading of aerodynamic forces in the domain, the sampling of the angle of attack from the resolved flow field, and a robust dynamic stall modelling. Moving from this background, this thesis presents a comprehensive approach to the power augmentation of vertical-axis rotos. Two strategies have been investigated, i.e., Gurney Flaps and active blade pitching. To this end, high-fidelity, blade-resolved CFD simulations were sided by a new generation ALM tool, here developed within the commercial solver ANSYS® FLUENT®. In the effort of tailoring the ALM to this type of machines, different features have been implemented and discussed in the present study, including a novel strategy for sampling of the angle of attack from the resolved flow field, a sensitivity analysis on the force spreading within the domain and several sub-models to account for secondary aerodynamic effects. Particular attention has been given to dynamic stall and to tip effects modelling. Validation on selected test cases, for which high-fidelity blade forces and wake field data were available from wind tunnel tests and blade-resolved simulations, has proved the reliability of the developed ALM tool. Effectiveness of the proposed power augmentation strategies has been demonstrated also via their application to a hydrokinetic rotor (HVAT - hydrokinetic vertical-axis turbine), designed in collaboration with an industrial partner. Both ALM and blade-resolved CFD simulations showed a simultaneous increase in the turbine aerodynamic efficiency and a reduction in fatigue loading.