Investigation and Multi-fidelity Modelling of Darrieus Hydrokinetic Turbines

Hydrokinetic energy has gained attention recently due to its low environmental impact and cost-effectiveness. In particular, Darrieus-type Hydrokinetic Turbines (HKTs) are well suited for rivers and small watercourses because of their compact design, simple blade structure, and ease of operation, with no need for pitching or yawing adjustments. These turbines also demonstrate efficiency improvements when arranged in closely spaced arrays. However, accurately modelling their performance remains challenging due to complex hydrodynamics, unsteady flow behaviour, and interactions with the surrounding environment, including channel walls and the free surface. While analytical models fail to capture these complexities, high-fidelity simulations are computationally prohibitive. This thesis addresses these gaps by advancing the understanding of Darrieus turbine interactions within arrays and with hydraulic environments while enhancing modelling capabilities for real-world scenarios. The thesis is based on a collection of six peer- reviewed journal articles addressing these challenges. Numerical models with varying fidelity were developed using Computational Fluid Dynamics (CFD) in Ansys Fluent©. Initially, two- dimensional blade-resolved unsteady RANS simulations were employed to investigate mechanisms driving efficiency gains in turbine arrays. Results revealed that mutual interaction zones suppress inflow deviation, increasing blade angles of attack and enhancing performance. The Actuator Line Method (ALM) was then tuned as a medium-fidelity tool for modelling Darrieus rotors without resolving blade boundary layers. ALM accurately captured twin-rotor interactions at medium and high tip-speed ratios (TSRs), predicting blade loads, torque, and wake structures. A comprehensive three-dimensional model was subsequently developed by coupling ALM with the Volume of Fluid (VOF) method to simulate free surface effects. Additionally, a more computationally efficient model using the Actuator Cylinder (AC) concept was created for quasi- steady-state analysis. To validate the computational model, an experimental campaign was conducted by the industrial partner, HE-PowerGreen. Field measurements were obtained for the flow field characteristics and the performance of an array composed of four closely spaced turbines. Both the CFD-ALM-VOF and CFD-AC-VOF models were validated against the experimental data, showing good agreement in flow velocity profiles, turbine performance, and free surface variations. The ALM shows accurate matching with the experimental measurements in terms of both flow field and array performance. On the other hand, the AC model provides reasonably accurate turbine performance predictions and captures array impacts on the channel and flow field, though it overestimates blockage effects and underestimates turbine-to-turbine interactions compared to the more precise ALM. Despite this, the AC model achieves a favourable trade-off between accuracy and computational efficiency, reducing computational time by an order of magnitude. Both models emphasised the critical role of two-way interactions between turbines and the hydraulic environment, including variations in water level and inflow velocity. The findings of this thesis advance the understanding of Darrieus turbine behaviour in arrays and complex hydraulic conditions. The developed models enhance predictive capabilities, offering valuable tools for academia and industry to optimise hydrokinetic energy systems.