Dynamic stall characterisation and control for vertical-axis wind turbines

According to the International Energy Agency, the global net-zero emissions objective requires the installed wind power capacity to increase 11-fold between 2020 and 2050. The scientific community has recently voiced concerns about the logistic feasibility of scenarios expecting wind power to meet this target. The deployment of wind energy capacity is limited by the availability of exploitable land. Increasing the versatility of wind turbine technologies can mitigate concerns around land use and interaction.Vertical-axis wind turbines provide an attractive design that complements their ubiquitous horizontal-axis counterparts. These turbines can operate in a broad range of wind conditions and emit little noise during operation, making them ideal for urban applications. The structural soundness of vertical-axis wind turbines has also attracted industrial attention for off-shore floating applications. However, the aerodynamic complexity of vertical-axis wind turbines has hampered their industrial deployment.The blades of vertical-axis wind turbines encounter varying flow conditions throughout a single turbine rotation, even in a steady wind. When the turbine operates at a low rotational velocity compared to the incoming flow velocity, the blades perceive significant amplitude changes in the angle of attack and relative wind velocity. Varying flow conditions can give rise to flow separation and dynamic stall. Dynamic stall is characterised by the formation, growth, and shedding of large-scale vortices. Vortices are known for enhancing the aerodynamic force experienced by the surface they form on. Although this attribute appears beneficial, dynamic stall is generally not considered desirable. For wind turbine applications, the shedding of large-scale vortices leads to a significant loss in efficiency and load transients that jeopardise the turbine's structural integrity.This thesis has two main objectives: characterise the occurrence of dynamic stall on vertical-axis wind turbines and investigate control strategies to mitigate the undesirable consequences of dynamic stall. We conceived an experimental apparatus that allowed us to obtain time-resolved force and flow measurements on a wind turbine blade for a wide operating envelope. We focus on the interplay between the instantaneous aerodynamic performance of the wind turbine blade and the flow structures forming on the surface of the blade. Our findings highlight the dynamic stall dilemma for wind turbine blades. Dynamic stall vortices forming on the turbine blades yield a significant peak in power production, but post-stall conditions lead to prolonged torque-dissipating excursions of the aerodynamic force. We also distinguish and quantify the timespan of six characteristic stall stages.Lastly, we demonstrate the potential of dynamic blade pitching as a control strategy to enhance wind turbine performance. We couple the scaled-down turbine model to a genetic algorithm optimiser that performed unsupervised experiments to seek optimal pitching kinematics at on and off-design operating conditions. Optimal blade kinematics yielded a three-fold power coefficient increase at both operating conditions compared to the non-actuated turbine and a 70% reduction in structure-threatening load fluctuations at off-design conditions. Flow measurements uncover how blade pitching manipulates flow structures to achieve performance enhancement.